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RV8/8A Best Glide

CrumbAV8R

I'm New Here
All,

What Vbg speeds have you all validated through flight tests? There have been a number of engine out, followed by stall spin accidents this past year.
I'm wondering if any of those were complicated by using the wrong speed for best glide, and then running out of energy short of the intended landing spot.

There seems to be some debate about min sink speeds and maximum glide range speeds (Vbg) on this forum. Regardless of small variances due to build, shouldn't Vbg generally be close to Vy for any given airplanes. This is L/D max at climb power.

I'm asking because I've seen some POHs show Vbg for RV7s in the 80 kt range, including the checklist I inherited when I bought my RV7 from a another owner, who was not the original builder. When I tested it, however, I found 95 kts to be the number for Vbg. I sold the RV7 last year, and now own an 8A, which I purchased from the builder. I did my own tests on it, and my Vbg test results show Vbg to be 100-104 kts, depending on wgt.


A simple way to compare whether Vbg is closer to 80k or 100k is to try this in 2 successive the landing patterns to low approaches in your constant speed prop airplane:

Run 1) trim for 80 kts, flaps up. Plan a normal pattern at a safe altitude. Set MP to min per your comfort level, prop at low rpm to simulate an engine out condition. When you roll out on final, ascertain your likely touchdown point. At a safe low approach altitude, set prop to max RPM and MP to max, and execute a normal go-around. Be ready to retrim as needed for the climb out, as always.

Run 2) do everything the same way, trimming for 80 kts. However, after you roll out on final and assess your likely touchdown point at 80 kts, lower the nose and accelerate/trim to 100kts. See how your touchdown point changes. Again, at a safe altitude, execute a normal go-around.

I found that in Run 2), after accelerating to 100k, my likely touchdown point moved substantially further away the runway. Clearly, my Vbg is much closer to 100 than to 80. These results prompted me to go out and do the tests recommended by Jaros, and get a more precise measurement of Vbg.

I'm very interested in results others may find.

Safe flying!
 
best glide

Some interesting analysis and numbers from a very smart guy here:

https://www.kitplanes.com/sawtooth-climb-performance/

sawtooth-cimb_08.jpg

If I lose my engine I'm going to trim for 100KIAS since I will (hopefully) be able to remember that - it's my target max speed for entering the pattern. I fly that number just so I will remember it if I need it for an off-field landing.

I'm told that it's better to use AOA for this, since it's weight in-sensitive, but I have not yet done that analysis in my -8.
 
Just a couple of thoughts.
1. Vy is usually greater than Vbg, due to the non-ideal prop. Most of us use ‘cruise’ props (even if CS, the blade twist is optimized for cruise) which pushes Vy higher than that for a theoretically perfect plane.
2. Testing at an airport, you most likely were landing into the wind, while measuring glide performance relative to the ground. Under these conditions your best results will be obtained with a speed which is higher than the no-wind Vbg. There is no simple formula, but Vbg plus 40% of the headwind is close. OTOH, if you were below gross weight, Vbg goes down. So it’s complicated.
 
I came up with 96 kt Vbg on a flight test at 1588 lb, no flap, Hartzell CS BA. I didn't go to max pitch when I did this though. Vy came out the same in a climb test. A real test pilot with rigorous procedure might have come up with something slightly different but I figured it was good enough to use 95 kt in my POH - easier to remember 95. I will repeat someday at max pitch for Vbg.

Glide tests were 3 minutes each at right angle to prevailing winds, in the same direction, and the same starting altitude, with glide distance determined by GPS.
 
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I did 360-degree turns starting at 2000 ft over mid-field of a long runway, at idle power (gives 600 RPM on the ground, obviously higher in flight). Unable to adjust c/s pitch at low RPM with my Hartzell governor.

at 90 kt lost 1100 ft, at 100 kt lost 1300 ft.

By comparison, my V_y seems to be about 100--105 kt.

Real engine out will make V_bg somewhat slower than at idle power which is quite a bit slower than V_y, because the prop/pumping drag is higher, so you slow down, increasing induced drag somewhat and reducing prop and profile drag.

(best glide occurs where induced drag and profile drag are equal).

BTW, people associate the drag from an engine-out with the windmilling prop. It is actually from the pumping losses in the engine. Drag is less with throttle closed than open. If you windmill the prop on an electric airplane, there is almost no discernible drag.
 
Some interesting analysis and numbers from a very smart guy here:

https://www.kitplanes.com/sawtooth-climb-performance/

View attachment 8167

If I lose my engine I'm going to trim for 100KIAS since I will (hopefully) be able to remember that - it's my target max speed for entering the pattern. I fly that number just so I will remember it if I need it for an off-field landing.

I'm told that it's better to use AOA for this, since it's weight in-sensitive, but I have not yet done that analysis in my -8.


You are correct about AOA and weight not mattering as far as L/D Max is concerned. With an AOA gauge you set max L/D a
and air speeds will change directly with weight to give you best range when the engine is running or best range engine out. That is one aspect of the beauty of an AOA indicator. If your aircraft has an L/D Max of 16 to 1 for academic purposes, your airplane will glide with engine out at L/D Max for 16 feet horizontally for every foot of altitude. If you are lighter, you still glide 16 feet horizontally for 1 foot of altitude, L/D Max will be attained at a slower airspeed. If you are heavy, conversely, at L/D Max, you still get 16 feet of horizontal travel for every foot of altitude but that will occur at a higher airspeed. The weight of the aircraft only influences the speed of L/D Max, it does NOT change your glide distance. You still get 16 feet horizontally for every foot of altitude in this example. That is one aspect of the awesome ability one gains when able to fly a precise AOA.
 
Steve, isn’t this backwards? Throttle closed the engine needs to do work on the intake stroke against the low intake pressure? Let me think some more.

Bob,
with the throttle open, the intake strokes pull a lot of air in, and the exhaust strokes pump a lot of air out. That is doing a lot of work, which the engine absorbs from the prop. With the throttle closed, a lot less air is pumped around, and a lot more of it acts like a spring, just getting compressed and expanded. You get a good fraction of the work back on the expansion strokes. If you could close the throttle completely, the prop would spin faster I think.
 
360 Degree Turns

I did 360-degree turns starting at 2000 ft over mid-field of a long runway, at idle power (gives 600 RPM on the ground, obviously higher in flight). Unable to adjust c/s pitch at low RPM with my Hartzell governor.

at 90 kt lost 1100 ft, at 100 kt lost 1300 ft.

By comparison, my V_y seems to be about 100--105 kt.

Real engine out will make V_bg somewhat slower than at idle power which is quite a bit slower than V_y, because the prop/pumping drag is higher, so you slow down, increasing induced drag somewhat and reducing prop and profile drag.

(best glide occurs where induced drag and profile drag are equal).

BTW, people associate the drag from an engine-out with the windmilling prop. It is actually from the pumping losses in the engine. Drag is less with throttle closed than open. If you windmill the prop on an electric airplane, there is almost no discernible drag.

Steve,

Altitude for a 360 degree gliding turn is largely dependent on turn rate and turn radius which in turn are dependent on bank angle and speed. Like you, I've found that V_bg is too fast for minimizing altitude loss through a 360 degree turn. The higher speed causes you to fly a larger radius than slower speeds, or you need to fly a steeper bank angle to keep the turn radius tighter and this of course increases induced drag. Best glide speed is appropriate for maximizing your glide distance in straight ahead flight (adjustments for wind, notwithstanding) but it's not so good if you need to turn to get to a landing spot...

Skylor
 
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A real test pilot with rigorous procedure might have come up with something slightly different but I figured it was good enough to use 95 kt in my POH - easier to remember 95. I will repeat someday at max pitch for Vbg.

This is an issue that has been on my mind for a long time:

It's time to fly off the 40 hour phase 1. You are an average pilot with average skills. No IFR ticket. Flight Test pilots are taught how to hold to within 1 knot I believe.

You use the 40 hours to run these tests and build your POH.

My questions are:

1) How accurate do your speeds and rates have to be to get decent numbers?

2) In order to maintain those speeds (level or climb) how do you keep from having your eyes glued to the instruments, thereby not allowing you to look outside?

3) If you do scan outside the cockpit, then look back at the instruments and see if you are off, how does that affect your test even if you correct it? Was that particular test blown?

Thanks
 
I don't overthink this too much My glide speed will vary on any given day to achieve the most important factor, keeping the ROD to a minimum. 75-80 kts is good enuf yielding around 700-800 fpm (time in the air) cause in the real world you won't be sitting back with spare brain power to work the math!
 
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look outside

...
2) In order to maintain those speeds (level or climb) how do you keep from having your eyes glued to the instruments, thereby not allowing you to look outside?

...
Might not work for you where you live, but I got with ATC and told them what I was doing, and they called out traffic for me. This was particularly helpful during climb tests as it was really hard to see over the nose at some speeds.

Even focusing inside, I found it sometimes hard to hold airspeed to 1 knot. I almost decided to plunk down the $400 GRT asks for the autopilot vertical software license key, but eventually didn't. I did notice when looking at my numbers the average over the period of the tests were within 0.1 kt, so even with some up/down, it comes out pretty good.
 
I don't overthink this too much My glide speed will vary on any given day to achieve the most important factor, keeping the ROD to a minimum. 75-80 kts is good enuf yielding around 700-800 fpm (time in the air) cause in the real world you won't be sitting back with spare brain power to work the math!

Well ok but you have to work out Vx and Vy, as well as best Glide, via test and you want the numbers to be as accurate as possible. Not everything is an emergency - like pitch to best glide in an emergency.

So I still wonder how accurate one has to be and how to achieve that accuracy.

I remember doing a flight test for climb speed and the plot was NOTHING like the ones in the aformentioned Kitplanes article.
 
Thanks to all!

Great thoughts and inputs here, everyone. Thank you.

AoA certainly is "the" key gauge for precision. Most importantly, everyone is prioritizing that foremost in any engine out situation is 1) Fly the Airplane and maintain control. If you do that all the way to the ground, your survival chances are very good, particularly if right before touchdown you can also cash in most of your excess energy, extend max flaps and reduce your velocity as much as possible--but without stalling. I will continue to practice at least one engine out situation every 30 days.

I plan to do some more Vbg testing. I will post my data on this forum. Thanks again for the prompt and informed responses.
 
Steve,

Altitude for a 360 degree gliding turn is largely dependent on turn rate and turn radius which in turn are dependent on bank angle and speed. Like you, I've found that V_bg is too fast for minimizing altitude loss through a 360 degree turn. The higher speed causes you to fly a larger radius than slower speeds, or you need to fly a steeper bank angle to keep the turn radius tighter and this of course increases induced drag. Best glide speed is appropriate for maximizing your glide distance in straight ahead flight (adjustments for wind, notwithstanding) but it's not so good if you need to turn to get to a landing spot...

Skylor

Skylor,
It is true that for minimum altitude loss in a turn, you should fly at minimum sink speed for that loading (W/cos B), rather than best L/D speed. But at the elevated loading, the polar shifts to higher speeds by sqrt(1/cos B) so the speed for min sink is faster. Not sure it is enough faster to approach the 1-g best L/D speed.

B=bank angle
 
1) How accurate do your speeds and rates have to be to get decent numbers?

I had the same problem, being a probably sub-average pilot and testing in the mountains/desert that made up my test area. So lots of noise in the data. What I did to try to work around the noise was do several runs at each speed and for each run assign a quality of 0-10. I held speed as best as I could and recorded the time to descend 500 ft with a stopwatch.

I then assumed that drag is proportional to x*v^2+y/v^2 where x is the profile drag part and y is the induced drag part. This is pretty accurate at higher speeds, but gets less accurate at higher angles of attack nearing stall. I didn't worry about those inaccuracies figuring they are less than the noise in my data.

Next, I made a spreadsheet to calculate the rate of descent for the x*v^2+y/v^2 and then sum the squares the difference between my measured rate and the calculated rate multiplied by the quality (so a run with a quality of 10 is weighted ten times as much as one with a quality of 1). I then manually iterated over x and y to minimize this sum of squares, yielding the least squares fit x and y for my measured data.

Knowing that best glide occurs where profile and induced drag are the same, you can then easily solve x*v^2=y/v^2 for v to get the best glide airspeed. For my 8A this came out to 91 knots for gross weight or 83 knots for solo weight.

So that is the not so good pilot/math nerd way to do it.

For climb speeds I did a similar calculation except I needed to come up with an estimated power curve for my engine/fixed pitch prop combination. Got 113 knots for best climb (103 solo).

Regarding how accurate you need to be in an emergency, the drag curve is pretty flat around best glide so a speed error of +/- 5 knots won't make much difference in your glide angle. Using my xv^2+y/v^2 calculation from above my glide goes from 11.0 to 10.9 with that +/- 5 knots. It does get very steep as you approach stall, so just remember to keep the speed up until you've made your landing area.
 
Bootstrap Flight Test Technique

Great discussion.

I didn't see it mentioned anywhere in this thread, but another resource for gathering glide, Vx and Vy data is to use the bootstrap flight test technique (google friendly). Very straightforward with a fixed pitch propeller and a bit more complicated with a controllable prop (additional measurements and some ground prep math required).

CAS. We like to discuss airspeeds on this forum; but if the reference isn't calibrated airspeed, we are comparing apples to oranges. Unless static source pressure error is known and an airspeed correction chart or curve calculated, 80 knots indicated in one RV-8 does not equal 80 knots indicated in another RV-8. L/Dmax and ONSPEED AOA, however, are identical for both airplanes--they're designed in.

If you have a properly calibrated AOA system and ergonomic cuing, here's how you can apply it in gliding flight (without any trigonometry):

Here's an example of maneuvering at L/Dmax AOA. L/Dmax AOA provides maximum range glide: https://youtu.be/fDYLP1-NEBc

If turn performance is critical, maximum sustained turn rate (minimum turn radius) occurs ONSPEED. Maximum endurance glide also occurs ONSPEED. Here is an example of maneuvering in an ONSPEED glide: https://youtu.be/48AVDr1kwwM

Flap considerations. When you deploy flaps, the AIRSPEED for L/Dmax decreases and the AOA changes as well (the lift curve shifts to the left as you may recall from pilot training). In RV's with 23-series wings (the 3/4/6/7/8), 10-20 degrees of flap provide more lift than drag. 10 degrees is optimum, but 20 degrees is a good compromise if you have only two flap settings (i.e., manual flaps rigged IAW Van's instructions). This results in an ONSPEED sweet spot at flaps 20 in RV's with 23013 wings: no brainer combination of optimum turn and glide performance when maneuvering close to the ground.

Residual thrust considerations. At IDLE, there is likely some residual thrust, thus any testing or site picture is going to be biased. This is especially true with a fixed pitch propeller. Keep this in mind when testing. Actual Prop-stopped glide performance can only be measured when the prop is stopped. Engine off performance can only be measured with the engine off. Without residual thrust, actual glide angle can be steeper than that derived in test at IDLE. This isn't intended to mean that testing at IDLE is a waste of time, just an important consideration to apply when looking at results or developing a rule of thumb!

Gliding close to the ground. Here's an example of flying the alpha sweet spot, engine out. It's not my intention to turn this into a discussion regarding the merits or danger or attempted turn back if the engine fails after takeoff; it's just an example of applying the technique when maneuvering in reference to the ground: https://youtu.be/trnwzBYvBQg. If you are interested in a short briefing that goes with the video, you can access that here: https://youtu.be/U1T-ePy9e94. BTW, this demo is flown at IDLE, so residual thrust is present. Maximum instantaneous turn performance occurs just prior to the aerodynamic limit (stall) at speeds below maneuvering speed/corner velocity. Here's an optimum technique based on bank angle and airspeed for folks that fly without benefit of an AOA system: http://nar-associates.com/technical-flying/impossible/impossible_wide_screen.pdf

Energy. Specific energy is neutral ONSPEED. Thats a fancy way of saying that thrust and drag are balanced. In a glide, thrust is provided by gravity and the amount of "gravity" available is a function of altitude. I'm sure I just made a physicist cringe somewhere, but the point is the supply of altitude is finite. The ONSPEED "push/pull" matrix applies: no slower than ONSPEED to optimize turn performance, maintain aerodynamic margin from stall and have proper energy for transition to landing, no matter where that might be.

Fly safe,

Vac
FlyONSPEED.org
 
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Just a suggestion, poke in the glide ratio you come up with into foreflte and see how it works out. I fussed around and came up with around 9:1 on the RV-4 w/CS. Put it into foreflite and I was waaay high. Kept adjusting until 12:1 worked about right.
 
I don’t believe 10 to 20 degrees of flap will increase gliding distance. That would be highly unusual. Even a small flap extension results in a increase in power requirements to maintain level flight and constant airspeed.
 
Sailvi, you are correct. It’s just a sweet spot for maneuvering if you are trying to simultaneously optimize turn performance and overall glide performance simultaneously close to the ground. Sorry if I didn’t make that clear, no excuse. Overall it’s technique only, but it simplifies maximizing sustained turn performance and energy management for a knuckle-dragger like me when I’m s-turning around trees to land. Flaps up, L/Dmax if range is your only consideration. If you have to turn an appreciable amount, 45 deg bank near stall is the sweet spot (optimizes instantaneous turn performance), then transition back to L/Dmax. Dave explains the math in the paper I referenced.

Cheers,

Vac
 
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Just a suggestion, poke in the glide ratio you come up with into foreflte and see how it works out. I fussed around and came up with around 9:1 on the RV-4 w/CS. Put it into foreflite and I was waaay high. Kept adjusting until 12:1 worked about right.

A lot of factors are at play here. If you were at idle power with the prop pulled back (or someone with a FP prop) you likely have some residual thrust increasing the "glide" ratio. Winds will also play a large effect on your glide ratio over the ground.

Skylor
 
With a constant speed prop what is the consensus on where to set the prop to best approximate a true engine out for practice?
 
With a constant speed prop what is the consensus on where to set the prop to best approximate a true engine out for practice?

This is sort of a trick question. If you have the usual CS prop for a single, it will go to fine pitch if oil pressure is lost. But best glide will be for coarse pitch. So....if following engine failure, you still have oil and can govern the prop, you would place it in full coarse pitch. And you may be able to simulate that drag with the engine at idle and the pitch set somewhat finer that full coarse, how much, I don't know. But, if engine failure is because of, or causes, loss of all oil, then the prop will go to fine pitch, and cause more drag than can be simulated with the engine running at idle.
 
This is sort of a trick question. If you have the usual CS prop for a single, it will go to fine pitch if oil pressure is lost. But best glide will be for coarse pitch. So....if following engine failure, you still have oil and can govern the prop, you would place it in full coarse pitch. And you may be able to simulate that drag with the engine at idle and the pitch set somewhat finer that full coarse, how much, I don't know. But, if engine failure is because of, or causes, loss of all oil, then the prop will go to fine pitch, and cause more drag than can be simulated with the engine running at idle.

Stupid question: I have Hartzell CS prop. If I have engine failure and get the prop to full coarse will it stay there if I lose oil pressure?
 
Stupid question: I have Hartzell CS prop. If I have engine failure and get the prop to full coarse will it stay there if I lose oil pressure?

No. (unless something inside is broken!). In operation oil pressure moves the blades to more coarse. Lack of oil pressure allows the blades to move back to fine pitch with no additional help.
 
Power Curve

No expertise in this field - just my two cents as a Private Pilot...

Is best glide not at the peak of the power curve? (the crossing point of lowest induced drag and parasitic drag.)

As such you can determine Vg by messing around in level cruise and see what the lowest power setting is to maintain level flight. If you drop on either side of the curve you will have to add power. Then viola! - Best glide. (I'm not accounting for density altitude or weight here)

Do I have this right?
 
No expertise in this field - just my two cents as a Private Pilot...

Is best glide not at the peak of the power curve? (the crossing point of lowest induced drag and parasitic drag.)

As such you can determine Vg by messing around in level cruise and see what the lowest power setting is to maintain level flight. If you drop on either side of the curve you will have to add power. Then viola! - Best glide. (I'm not accounting for density altitude or weight here)

Do I have this right?

This would be true if you had an electric power plant that added no additional windmill drag. But the added windmill drag will result in a best power-off glide speed a fair bit slower than the minimum power point with the engine running. Also, if you try it, you will find it very hard to measure. There was a thread a while back that claimed that they could not find a 'back side of power curve" and that the power continued to fall until stall. That is certainly not true in any of my airplanes, but the minimum power speed is not much faster than stall speed.
 
not quite

Is best glide not at the peak of the power curve? (the crossing point of lowest induced drag and parasitic drag.)

As such you can determine Vg by messing around in level cruise and see what the lowest power setting is to maintain level flight. If you drop on either side of the curve you will have to add power. Then viola! - Best glide. (I'm not accounting for density altitude or weight here)

Do I have this right?

First line above is bad physics; comparing power (energy/sec) with drag (which is a force).
If you graph "power required for level flight", you will get a distorted U shaped curve. The minimum of this curve occurs at the "minimum sink" airspeed. This minimum sink rate airspeed is also what you're finding in the procedure you describe above. Best glide speed is faster - you accept a higher rate of descent but get an even faster forward speed. To find this speed from your graph of "power required for level flight", take a straight edge; place the edge along the horizontal axis, and start to rotate it upwards, holding the place where it crosses the origin fixed. Stop where it first hits the power required curve, mark the point (this will be to the right of the minimum sink point). Draw a line straight down, read off the speed. This is best glide speed. BTW, if you want a real world answer as to how far you travel over the ground in a head- or tail-wind: just move the rotation point to the right of the origin by the amount of headwind, or to the left of the origin by the amount of the tailwind, and repeat the exercise - rotate the straight edge up until it first hits the curve. You'll see that for extreme tailwinds, best glide gets very close to minimum sink speed.
Oh yea - by more or less a coincidence, no-wind best glide speed is also the speed where induced and parasitic drag are equal (in unaccelerated flight).
edit added: As Steve points out, above, the changing efficiency of the prop with airspeed, plus the increased drag of a windmilling prop, are additional complications. To do this right you should measure the power required with the engine off, and the plane in a constant descent glide. Repeat for different airspeeds. The "power required" is just mg(dh/dt), where mg is the airplane's weight, and (dh/dt) is the vertical speed.
 
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Depending on how serious you are about getting this data, there are a couple of different techniques to set zero thrust which have been used previously.

One way is to install a pitot probe downstream of the propeller, connect it and the freestream pitot to a differential pressure gauge, and set power+pitch so that the total pressure in the prop streamtube is the same as in the freestream. Several higher powered multi engine aircraft have a system like this (eg DHC Caribou calls it a "thrust gauge") to help with OEI operations.

Alternatively, many years ago an AIAA paper Zero-thrust glide testing for drag and propulsive efficiency of propeller aircraft was based on measuring the relative longitudinal position of the prop shaft with respect to the engine front face. It was also used by CAFE, and, I think, some others.
 
someone said 100 knots?
in my rv-8 with 1500 pattern altitude on crosswind with power off (my field is 8000 feet long) I cannot slow it down enough to land short, the airplane glide characteristic is such that I always end up long. I am accustomed to flying a t6 with an overhead brake and it stops on a dime. I have to slow my rv8 to 60 knots or less by midfield downwind so that I don't use the entire runway to stop. I have a fixed pitch prop on a 360 Lycoming engine. I practice engine out approaches all the time in various airplanes and the rv-8 is the slickest of them all. I can't imagine the glide ratio with the prop stopped. But 100 knots? never.
 
Bob,
with the throttle open, the intake strokes pull a lot of air in, and the exhaust strokes pump a lot of air out. That is doing a lot of work, which the engine absorbs from the prop. With the throttle closed, a lot less air is pumped around, and a lot more of it acts like a spring, just getting compressed and expanded. You get a good fraction of the work back on the expansion strokes. If you could close the throttle completely, the prop would spin faster I think.

Engine pumping work is at a maximum with a closed throttle. Examine a p-V diagram if you're not convinced.
 
Engine pumping work is at a maximum with a closed throttle. Examine a p-V diagram if you're not convinced.

Hmmm. Remember we are talking about a dead engine -- ignition off.

On an idealized p-V diagram, regardless of throttle position, the compression and power strokes absorb no net work. Just an air spring getting compressed, then expanded. The actual pressures will be different if the throttle were closed or open, but it doesn't matter. The compression work is recovered on the power stroke.

So the question is what is the net work out of the exhaust and intake strokes. At the beginning of the exhaust stroke, ambient-pressure air will fill the cylinder through the exhaust valve, which then has to get pumped back out as the piston rises, leaving a small chamber of ambient pressure air at the top of the stroke when the exhaust valve closes and the intake valve opens. That took some work.

On the intake stroke, if the throttle is closed and the intake manifold is at low pressure, that ambient pressure air can expand and exhaust into the intake manifold as the piston travels down, doing a little bit of useful work on the way, so some work has been recovered. But if the throttle is open and the intake manifold is at ambient pressure, then as the piston travels down, ambient air from the intake manifold will be sucked in to fill that volume. Seems like that takes more work.

The remaining question in my mind is what is the equilibrium intake manifold pressure with the throttle closed? If flow from the exhaust system is coming in during the exhaust stroke and then filling the intake manifold during the intake stroke, it will pump the intake manifold up to some fraction of ambient. Also there is airflow coming in past the throttle. So the intake manifold may end up at a fairly high fraction of ambient pressure. But I think as long as it is at some pressure less than ambient, there is less work being absorbed.

Easy way to test this of course: With mixture at idle cut-off and speed near best power-off glide, what is the windmill rpm with throttle open, and closed, and what is the manifold pressure with the throttle closed. Higher RPM means less work being absorbed.
 
Depending on how serious you are about getting this data, there are a couple of different techniques to set zero thrust which have been used previously.

One way is to install a pitot probe downstream of the propeller, connect it and the freestream pitot to a differential pressure gauge, and set power+pitch so that the total pressure in the prop streamtube is the same as in the freestream. Several higher powered multi engine aircraft have a system like this (eg DHC Caribou calls it a "thrust gauge") to help with OEI operations.

Alternatively, many years ago an AIAA paper Zero-thrust glide testing for drag and propulsive efficiency of propeller aircraft was based on measuring the relative longitudinal position of the prop shaft with respect to the engine front face. It was also used by CAFE, and, I think, some others.

The zero-thrust glide data is of great academic interest for evaluating the aerodynamic performance of the airframe.

But it is not at all useful for determining engine-out glide ratio, or best engine-out glide speed.
 
If you're looking for best glide for an engine out, I have found that in my 8, slowing from cruise speed, once I hit 100kts(plus or minus a few) I can land within the area that touches the plane at my given altitude. IE, if the field I want is touching my left wingtip, I can make it, same with the spinner. If the field is aft of the wing trailing edge, I use half way to the tail. Distance doesn't help me much cuz i probably don't know if the field is 4 miles or 4.5 and if its an airport, I don't want to worry about having to put in A/P in the GPS, unless I have all kinds of time. I ops check this monthly. If the wind is greater than 15 knots, it's not a guarantee. I also practice overhead simulated engine outs high over the field and different parts of the traffic pattern. 100kts works for me cuz when it happens, its a nice simple number for my simple brain. I back it up with my AOA and my aim point.
 
Hmmm. Remember we are talking about a dead engine -- ignition off.

On an idealized p-V diagram, regardless of throttle position, the compression and power strokes absorb no net work. Just an air spring getting compressed, then expanded. The actual pressures will be different if the throttle were closed or open, but it doesn't matter. The compression work is recovered on the power stroke.
.

I’m in the ‘open throttle= less drag’ camp. I agree, power and compression strokes cancel. It’s intake and exhaust strokes that matter. Open throttle we have 1 at. coming in on the intake stroke, working against 1 at on the backside of the piston, in the crankcase. Net force equals zero, no work done. On the exhaust stroke, same deal: 1 at on both sides of the piston (1 at outside), no work done. There are, of course, some pressure losses across the valve openings. Now with throttle closed, intake manifold at, say, 1/2 at, the prop has to do work on the intake stroke, with 1/2 at inside cylinder but 1 at on back side (crankcase). On the exhaust stroke you get that work back if the exhaust exit was at 1/2 at. But it’s not. In fact, air flows ‘backwards’ thru the exhaust into the cylinder, until it’s at 1 at and, just as with full throttle, you get no energy back on the exhaust stroke.
I agree, easy to test, in an engine-out glide - over an airport, just in case...
 
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Well ok but you have to work out Vx and Vy, as well as best Glide, via test and you want the numbers to be as accurate as possible. Not everything is an emergency - like pitch to best glide in an emergency.

So I still wonder how accurate one has to be and how to achieve that accuracy.

I remember doing a flight test for climb speed and the plot was NOTHING like the ones in the aformentioned Kitplanes article.

Ok so yesterday I wen tout to attempt some Sawtooth climbs as per the Nigel Speedy Kitplanes article.

I found that holding accurate speed during high speed climbs - e.g. 170mph and 160 was easy enough, As i got to the 150, 140 range it was harder

130 and 120 it was almost impossible to get stable on speed before the start

of the test band altitude.

I didn't combine a dive test after a climb test and perhaps that's the issue. I would just reduce power, get back to the starting altitude, try to burn off the speed before starting the climb.

Also, I had the directional AP on to eliminate as much work load as I could.
All I had to do was adjust pitch.

But I'm going to go out and try again.
 
Ok so yesterday I wen tout to attempt some Sawtooth climbs as per the Nigel Speedy Kitplanes article.

I found that holding accurate speed during high speed climbs - e.g. 170mph and 160 was easy enough, As i got to the 150, 140 range it was harder

130 and 120 it was almost impossible to get stable on speed before the start

of the test band altitude.

I didn't combine a dive test after a climb test and perhaps that's the issue. I would just reduce power, get back to the starting altitude, try to burn off the speed before starting the climb.

Also, I had the directional AP on to eliminate as much work load as I could.
All I had to do was adjust pitch.

But I'm going to go out and try again.

I’ve been doing some prop testing recently which involves a lot of sawtooth climbs. The tricks are to do it first thing in the morning, when the air is very smooth, and to start well below your target altitude band, to give you time to get on speed.

I dive to 1,000’ feet below the planned band, throttle at idle for the dive, then slow to the target airspeed, then add throttle to climb power while concentrating on airspeed. There is a little bit of airspeed wobble going through the power transition, but by a couple hundred feet below the target band it is sorted out, and if the air is smooth, the speed can be held - you just have to divide your attention between the ASI and the pitch picture outside, and stay ahead of the oscillations.

Paul
 
I’ve been doing some prop testing recently which involves a lot of sawtooth climbs. The tricks are to do it first thing in the morning, when the air is very smooth, and to start well below your target altitude band, to give you time to get on speed.

I dive to 1,000’ feet below the planned band, throttle at idle for the dive, then slow to the target airspeed, then add throttle to climb power while concentrating on airspeed. There is a little bit of airspeed wobble going through the power transition, but by a couple hundred feet below the target band it is sorted out, and if the air is smooth, the speed can be held - you just have to divide your attention between the ASI and the pitch picture outside, and stay ahead of the oscillations.

Paul

Yeah I went 1000 ft below the start of the test band too in order to give myself more time to get on speed. 500 feet below was just not enough.

Helped a little, but I think your suggestion is key:

So you are saying that when you come out of the dive, the procedure is:

1) Slow to target speed while the throttle is back.

2) Pitch up AND add throttle at the same time trying to keep the speed on or near the target.

I wasn't doing that. I was going fast out of the dive and tried to burn the speed off in the pitch up. But that's a non-starter. I climbed way too fast and had no idea what the proper pitch attitude would be for the target speed I was after. By the time I got close I was well past the bottom of the band.

Next time out I'll do that....GET to target speed 1000 feet below the band and then add pitch and power to keep the speed in the vicinity of target. Steady up on target speed before, but near the band.

So instead of pitching up to GET to the target speed I'll pitch up (and add power) to STAY on the target speed.

That's somewhat similar to getting into slow flight - slow down then add pitch and throttle to keep the speed just above stall. I can do that very well.


Thanks!
 
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Actual engine out glide

I did my testing by pulling the mixture cut off and allowing the engine to windmill no-power.

RV-7 IO 390 3 blade MT prop

Found that prop control was still effective at all normal glide speeds. Full pitch of course provided the best glide.

It seemed that 12" MAP or higher improved the glide. Throttle at idle stop was the worst position. Again, no power just tinkered with windmilling MAP.

Did 5 MPH increments from 90-130 MPH IAS. Used time and TAS to determine best range. Sweet spot was 120 MPH.

Max endurance was quite a bit slower and while better at the slower airspeeds it wasn't really meaningfully different. But this was not something I specifically tested for.

As a side note: I use AFS avionics and once the parameters were updated I found their power out glide predictor to be extremely accurate as it includes wind. Did a few engine out glides from the edge of the envelope on the display and would have made the field without difficulty.

My best glide is faster than my best climb by about 12 MPH at full weight. An operating engine and its own efficiencies has a lot to do with that and it's not just a wing shape issue as some folks seem to argue.
 
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I did my testing by pulling the mixture cut off and allowing the engine to windmill no-power.

It seemed that 12" MAP or higher improved the glide. Throttle at idle stop was the worst position. Again, no power just tinkered with windmilling MAP.

Interesting. OK. I was wrong.
 
Effect of MAP while windmilling

I assume each engine/prop/airframe combination will be different.

With the prop at full coarse pitch I observed that the prop spun faster as I began to open the throttle up to about 12"

There is the conundrum that a spinning prop is extracting work from the air and it might be reasonable to assume that if it spins faster then it is taking more work. This was certainly true of decreasing pitch, but seemed not so much the case when simply tinkering with MAP.

I really got the impression that allowing a little more flow of air into the engine reduced the work being taken by the prop.
 
For a given pitch setting, the prop will spin fastest when no work is being extracted from it. Any braking torque from turning the dead engine will slow the prop down. So your observation of the rpm is consistent with your observation of the effect on glide.
 
The engine off glide testing we did this past summer indicated that prop control position had zero influence on glide performance. That at normal best glide speed’s, relative wind just doesn’t spin the engine fast enough for the governor to have control of the prop. At least that was our experience.
 
The engine off glide testing we did this past summer indicated that prop control position had zero influence on glide performance. That at normal best glide speed’s, relative wind just doesn’t spin the engine fast enough for the governor to have control of the prop. At least that was our experience.

This seems to be dependent on prop governor. On my IO-360A1B6 with MT (Avia) governor, I have full prop control down to very low engine speed. In my own "true power-off" glide testing (mixture pulled), at 100 kias with the prop lever forward, the engine windmills at ~ 1500 - 1600 RPM. with the prop lever pulled back it will windmill at 650-700 RPM. This makes ~ 20% difference in altitude loss through a 360 degree gliding turn at 45 degree bank. At lower air speeds, the engine will spin even slower with the prop lever back - down to 200 RPM. Also of note, if my engine is shut down with the blades at full coarse pitch, they will stay there for some period of time until I push the prop lever forward. I found that out when the engine was new and I cranked it with the plugs out to build oil pressure before first start. I inadvertently had the prop lever aft. A little while later, after I was done with the pre-oiling I noticed that the prop looked "different". I realized that the blades were in the full coarse pitch position, which I wasn't used to seeing. When I pushed the prop lever forward, they returned to fine pitch.

Skylor
RV-8
 
That's interesting Skylor. This suggests there is a check-valve feature in the governor that prevents or restricts bleed back of the oil pressure.

My old MT governor gave control to very low RPM, just as Skylor describes. My Hartzell governor loses control at 1600 RPM or so, so has no effect on engine-out.

Would be interesting to know how other mfg's governors behave.
 
That's interesting Skylor. This suggests there is a check-valve feature in the governor that prevents or restricts bleed back of the oil pressure.

My old MT governor gave control to very low RPM, just as Skylor describes. My Hartzell governor loses control at 1600 RPM or so, so has no effect on engine-out.

Would be interesting to know how other mfg's governors behave.

Steve,

I don’t know if there is a check valve in the governor or if the internal oil pump is a constant displacement pump that just didn’t have any significant internal leak-by when new and therefore just didn’t allow backflow when the governor control valve was in the increase-pitch position and the engine wasn’t turning.

Also, my engine will increase RPM if I experience brief oil starvation during aerobatics thus in flight the governor doesn’t seem to lock up pressure to the prop hub although I can speculate a number of reasons why this might be the case.

Skylor
 
I’ve been doing some prop testing recently which involves a lot of sawtooth climbs. The tricks are to do it first thing in the morning, when the air is very smooth, and to start well below your target altitude band, to give you time to get on speed.

I dive to 1,000’ feet below the planned band, throttle at idle for the dive, then slow to the target airspeed, then add throttle to climb power while concentrating on airspeed. There is a little bit of airspeed wobble going through the power transition, but by a couple hundred feet below the target band it is sorted out, and if the air is smooth, the speed can be held - you just have to divide your attention between the ASI and the pitch picture outside, and stay ahead of the oscillations.

Paul

Hi Paul,

I went out again after reading your post, here, and used your technique.

It made getting ON speed and staying on speed much easier. So thanks.

I also found that getting a simple to use digital stopwatch made things a WHOLE lot easier. I was using a timer that had buttons and it was hard to do everything I needed to do and get the fingers on the buttons.

During multiple practice runs, I detected another technique error I was making:

Once on speed I was correcting any slight deviation by using the same technique I used on the throttle when formation flying:

1. Add a little power (if sucked)

2. Just before position removed added power and then some - you have to stop the acceleration

3. Go back to the "nominal" power setting.

Well I was doing that with the pitch. At the slightest speed deviation I would adjust pitch by adding more (if I had to slow down)

then taking the addition out and then some

Then going back to the "nominal" stick position.

That, as you might expect, failed.

So I learned to just make the tiniest of stick adjustments in the proper direction and hold that. That worked much better.

I still have to practice - I'm looking at the airspeed indicator way too much - but that's for the next ride.

Thanks everyone.
 
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