Engine-out Sink Rate With a C/S Prop

I was pleasantly surprised to find out today that my Hartzell C/S prop will go to coarse pitch with the engine producing no power and as expected, this has a significant effect on sink rate. At 9,000 ft I set the plane up in a 70 kt glide and pulled the mixture full back to idle cut-out. With the prop control full in, I was showing 1,200 fpm on the VSI. When I pulled the prop to full coarse I could really feel and hear the prop go coarse and the VSI went to 800 fpm (at 70 kts). Have others had a similar experience? I was always under the impression that with the engine at low revs and producing no power there would be insufficient oil pressure from the governor to change the pitch.
9A, 0-320 with 9:1 C/R, MT governor.

Fin
9A

Finley Atherton said:

I was pleasantly surprised to find out today that my Hartzell C/S prop will go to coarse pitch with the engine producing no power and that as expected, this has a significant effect on sink rate. At 9,000 ft I set the plane up in a 70 kt glide and pulled the mixture full back to idle cut-out. With the prop control full in, I was showing 1,200 fpm on the VSI. When I pulled the prop to full coarse I could really feel and hear the prop go coarse and the VSI went to 800 fpm (at 70 kts). Have others had a similar experience? I was always under the impression that with the engine at low revs and producing no power there would be insufficient oil pressure from the governor to change the pitch.
9A, 0-320 with 9:1 C/R, MT governor.

Fin
9A

Click to expand...

Did you note what your RPM and oil pressures were at the different prop settings? Glad this theory works with the Lycoming/Hartzell setup, and I would recommend that emergency engine-out training include teaching such a configuration. As I had posted earlier, I hadn't considered this until training in the T-28, but it just makes so much sense! Maybe someone can research best glide speeds for the two different configurations. At the very least, this will demonstrate the need to need to not let the airspeed decay to the point that the prop/engine stops, at which point you will have no control over its pitch.
Very good report, Fin!

Safety?

Maybe Doug can move this to Safety thread?

Yep!

ABC's Wide World of Aviation did a video many years ago on "stretching the glide." this was one of the procedures used.
If you need to stretch the glide further, the next procedure is to stop the prop. This last one should only be done with sufficient altitude and assuming that you know there will be no chance of a restart.

Hi Mel, If you stop the prop, the blades will go back to flat pitch. Has it been proven that this is better (less drag) than windmilling at coarse pitch?

Definitely...

A stopped prop is always less drag than than the big disk area of a windmilling prop.
I forget the exact numbers, but the test on the video (a C-182) showed a dramatic increase in glide.
A side note; stopping the prop requires bringing the airplane almost to stall speed, so obviously it shouldn't be done close to the ground.

Estimating best speeds

captainron said:

.. Maybe someone can research best glide speeds for the two different configurations. At the very least, this will demonstrate the need to need to not let the airspeed decay to the point that the prop/engine stops, at which point you will have no control over its pitch.
Very good report, Fin!

Click to expand...

Yes, very interesting report.

We know that at best glide speed induced and parasite drag are equal. If we reduce parasite drag by changing prop configuration then best glide must be where induced drag is reduced an equal amount. If you were at best glide to begin with for either configuration.

We already know or can calculate the change in total drag because we know the two sink rates and could know the weight of the airplane and the IAS (assume it's equal to the CAS for this purpose).

Since induced drag declines with the square of the velocity, the new best speed for glide will necessarily be faster. Of course, we don't know that 70 kts is really best glide for this aircraft, so we need more data.

PM me for ways to quickly, safely and accurately determine best glide. I can't do this research because mine is FP.

Mel said:

A stopped prop is always less drag than than the big disk area of a windmilling prop.

Click to expand...

Not sure I completely understand this. The prop blades are the same size whether they're moving or not. Wouldn't the decreased drag of a stopped prop actually be due to not turning the dead engine?

Mel said:

A stopped prop is always less drag than than the big disk area of a windmilling prop.
I forget the exact numbers, but the test on the video (a C-182) showed a dramatic increase in glide.
A side note; stopping the prop requires bringing the airplane almost to stall speed, so obviously it shouldn't be done close to the ground.

Click to expand...

I'm sure there are going to be a lot of trade-offs here, but the origin of this thread was getting back to the airport, or at least selecting a safe landing area after an engine failure after takeoff. Slowing the plane to near-stall certainly increases drag and reduces the ground distances covered, and whatever altitude is lost during this maneuver and the corresponding pitch-over needed to regain safe airspeed (and best glide speed), together with the time involved that could be better spent planning the landing site, doesn't seem to suggest safe operation. All I'm advocating here is for pilots with constant-speed props to rethink their emergency procedures to include going immediately to Low RPM after engine failure while maintaining the safe airspeed needed necessary for both best glide and to avoid stalling. In aircraft (twins) with full-feathering props, you are taught that in case of a catastrophic engine failure (seizure) you must get the prop into feather before the engine RPM decays below a specified RPM, or else you're stuck with low pitch when the engine stops.

Anyway, basic emergency checklist memory items after an engine failure after takeoff should probable be:
1: Pitch to establish safe airspeed
2: Prop to Low RPM
3: Consider altitude available for landing maneuver (turn-back or not)

After memory items, and if there is time available, consider things like
4: Oil pressure?
5: Fuel source (switch tanks, boost pump on, fuel pressure?
6: Magnetos (try L or R separately)

What have we been taught for years regarding engine failure? I remember the familiar "Balls to the Wall", or, all engine controls full-forward.
Maybe it's time to rethink some basics.

Alan Carroll said:

Not sure I completely understand this. The prop blades are the same size whether they're moving or not. Wouldn't the decreased drag of a stopped prop actually be due to not turning the dead engine?

Click to expand...

I know I'd need convincing that Alan's not correct!

Possibly you are right.

Alan Carroll said:

Not sure I completely understand this. The prop blades are the same size whether they're moving or not. Wouldn't the decreased drag of a stopped prop actually be due to not turning the dead engine?

Click to expand...

But the end result is still the same. The glide will increase with a stopped prop.
Again, 99% of the time, this is not practical. Just an example.

Be very careful when practicing this technique during dead stick approaches. Bad things happen to jugs when you forget to push the blue knob back in before pushing the go knob.

John Penny described using this technique at Reno last year during his preplanned dead stick approach after winning the gold race and having a carb malfunction. He pulled the prop control back for the approach and then pushed it back forward when he knew he had the runway made.

This is one of the first things my instructor and I did during the insurance check-out in my RV-6. Big difference in glide...but it still comes down pretty fast!

Full Throttle

Peter Garrison talked about this in his latest column in "Flying." He tested with the prop in fine and course pitch and also with the throttle open and closed. He found that with a windmilling prop opening the throttle also significantly increases the glide. Maybe useful information for some situations.

Glide Ratio

I'm getting close to finished and wondering what kind of glide ratio does an RV have? For instance, a C172 is somewhere around 8.5 to 1 (give or take a little).

What should the air speed be trimmed to for the minimum sink rate? Or is there a simple trick like you can do in a C172 with a dead stick is trim full up and it will seek the best glide speed which is about 65 knots?

At 6000 AGL - I would like to know rather quickly "airport, road, field, or other" based on my distance options.

Not exactly...

Alan Carroll said:

Not sure I completely understand this. The prop blades are the same size whether they're moving or not. Wouldn't the decreased drag of a stopped prop actually be due to not turning the dead engine?

Click to expand...

It takes energy to turn the prop, whether or not it is connected to the engine. The dead engine would tend to make it harder to turn of course, which increases the energy required. But even if the crank snapped behind the main bearing (creating in essence a free spinning prop), the spinning prop creates more drag than a stopped one. It's easy to demonstrate with a rubber band powered model airplane.

Not much...

Webb said:

I'm getting close to finished and wondering what kind of glide ratio does an RV have? For instance, a C172 is somewhere around 8.5 to 1 (give or take a little).

What should the air speed be trimmed to for the minimum sink rate? Or is there a simple trick like you can do in a C172 with a dead stick is trim full up and it will seek the best glide speed which is about 65 knots?

At 6000 AGL - I would like to know rather quickly "airport, road, field, or other" based on my distance options.

Click to expand...

Look DOWN! RV's will never be mistaken for gliders My best glide is 85 KIAS. Below that the sink rate increases dramatically.
I haven't yet calculated a glide ratio. A wise man once told me "Don't ask the question if you can't stand the answer". It's not going to be 8.5 to 1.
I tell my students - look at the angle where the runway is on a normal downwind. That's where to look for an emergency field. You won't make anything farther than that. Altitude makes that distance farther, but that angle is a decent guide for what is available.

Webb said:

I'm getting close to finished and wondering what kind of glide ratio does an RV have? For instance, a C172 is somewhere around 8.5 to 1 (give or take a little).

What should the air speed be trimmed to for the minimum sink rate? Or is there a simple trick like you can do in a C172 with a dead stick is trim full up and it will seek the best glide speed which is about 65 knots?

At 6000 AGL - I would like to know rather quickly "airport, road, field, or other" based on my distance options.

Click to expand...

www.cafefoundation.org
has test results for a couple different RV's, both short-wing and long-wing. As one might expect, the -9 is significantly better in both glide ratio and sink rate than the short-wing versions.

DGlaeser said:

It's not going to be 8.5 to 1.

Click to expand...

You're right, it's not 8.5 to 1. Cafe says it's 9 to 1 on the 8a and a whopping 12 to 1 in the 9(9a I think). So while the VSI shows a bigger number in the RV's in glide, so does the ASI and thus glide ratio ain't that bad. Not sure why people assume they're bricks on the way down. On the rare occasion I try a no flapper in the -4 I'm amazed how much effort it takes to get slowed and down. Almost always takes a full rudder slip for a surprisingly long time.

Experienced the propped-stop glide once...

Back when I was CFI'n in college (OK, way back...sorta ), I experienced an engine failure in a C-150, while prepping a student for her check ride. Had the chance to see the glide rate both with the prop windmilling and with it stopped that day.

As Mel said, it was a marked increase in glide capability (reduction in sink rate), such that I quickly felt pretty high, and had to slip the aircraft pretty aggressively (though carefully) on base and final to hit my spot on the (appropriately enough) glider airpark that we happened to be above when it let go.

The gist of it is that while climbing out after GRMs to do high work, the engine started sounding a bit "loud", so I had the student climb and get high enough for crossing the SF Bay back to home field safely, just in case...and at about 2500' AGL a piston pin failed. The rod sounded like it was coming through the firewall, so I shut it down. At best glide, we did 2 360's to land at the glider airpark. The prop stopped on the second downwind (without any effort or slowing on my part...I was not even considering trying to stop it...just fly best glide and git 'er on the ground). It really became a glider at that point (it was a 150, not an RV of course). The change in glide/descent rate left a lasting impression on me, though at the time, I was looking out at the landing field, and not at the VSI, so unfortunately I can't give a quantitative VSI change. But it was a significant change in glide, from what I saw and felt.

My RV is CS, so I'd be very interested in seeing the descent rate delta in prop forward versus back...and it sounds like a very good step in the emergency engine-out procedures. I'd be interested in quantifying it, or seeing info from those that do...but if it happens for real, my guess is it won't be about what VSI numbers you get, it'll be about getting to best glide speed, doing what you can to improve the glide rate (ex., prop back) and flying the airplane to get to a safe landing spot.

I'm not advocating slowing to intentionally stop the prop...no internet instruction here...and I've personally not seen that put out as a single-engine airplane engine-out procedure anywhere. Just relating the experience.

As for the reduction in drag and increase in glide capability, I suspect perhaps Alan and Mel may both be correct. Though I've studied aero, I'm not an aerodynamicist, but I was taught that the windmilling engine will extract energy from the air, which translates to drag, as Alan stated. I've also seen it described as "flat-plate" drag of a windmilling prop somewhere, so it may be that the drag of a windmilling prop can be equated to an equivalent flat plate drag amount, and perhaps that's just an easy way to visualize it. Could be a combination of energy extraction and the drag of the disc, as Mel called it. Either way, when it stops, you do glide farther!

The note about the throttle is of interest...I just got that mag, so I looked it up...Peter Garrison wrote, "the airplane glides farther with the throttle open, because the engine offers less resistance to the propeller". My question would be, if the engine is really dead and windmilling, how does throttle position have an impact? I get the prop pitch thing, and I'll probably do a V-8 salute when someone explains the throttle part, but the brain is not connecting the dots on this one for now. PG also said that the difference between a windmilling CS prop in coarse pitch (low RPM) and fine pitch (high RPM) was about 10% in his plane, and he estimates that the delta between a CS prop in coarse pitch and a stopped CS prop might not be that large, and would be much less than the change with a fixed pitch prop (windmilling vs. stopped). All interesting stuff.

Cheers,
Bob Mills
"Rocket" RV-6
N600SS
4SD

Yeah, I'm sure you're right...

grantcarruthers said:

You're right, it's not 8.5 to 1. Cafe says it's 9 to 1 on the 8a and a whopping 12 to 1 in the 9(9a I think). So while the VSI shows a bigger number in the RV's in glide, so does the ASI and thus glide ratio ain't that bad. Not sure why people assume they're bricks on the way down. On the rare occasion I try a no flapper in the -4 I'm amazed how much effort it takes to get slowed and down. Almost always takes a full rudder slip for a surprisingly long time.

Click to expand...

After I sent that post I got to thinking about the speed and I think when I get around to doing the math I may be surprised. My best glide speed may also be a bit higher also, but in the air 85 feels good. Right now I have a ground adjustable prop, but a CS is coming one of these days. That's why I haven't spent much time with my performance numbers 'cause they'll be changing soon (but not as soon as I'd like).

rvmills said:

The note about the throttle is of interest...I just got that mag, so I looked it up...Peter Garrison wrote, "the airplane glides farther with the throttle open, because the engine offers less resistance to the propeller". My question would be, if the engine is really dead and windmilling, how does throttle position have an impact? I get the prop pitch thing, and I'll probably do a V-8 salute when someone explains the throttle part, but the brain is not connecting the dots on this one for now.

Click to expand...

On the intake stroke, an open throttle lets the cylinder fill with air a lot easier than with it closed.

It's an air pump!

rvmills said:

The note about the throttle is of interest...I just got that mag, so I looked it up...Peter Garrison wrote, "the airplane glides farther with the throttle open, because the engine offers less resistance to the propeller". My question would be, if the engine is really dead and windmilling, how does throttle position have an impact?

Click to expand...

Bob, The throttle situation is that the engine is an "air pump". Even if the engine is being driven by the air flow, it is still pumping air. The open throttle reduces the resistance of the intake air.

rvmills said:

My question would be, if the engine is really dead and windmilling, how does throttle position have an impact?

Click to expand...

The cylinders still need to fill with air on each intake stroke, and the pistons need to do work to pump that air past the throttle plate. Or to put it another way, with open throttle, the atmosphere is helping more to push the pistons down during the intake stroke.

Think of the windmilling prop as a 76"??? diameter solid disc. The windmilling prop disrupts airflow over a significant portion of the wing on relatively short wing airplanes . On a biplane this is almost doubled. Be careful about assumptions based on tests with an idling engine. Take away the oil pressure and the prop pitch will go wherever the opposing force(counterweights, spring, compressed nitrogen) take it. The Pitts S1S with a fixed pitch prop can be landed power off from 600' abeam the aim point. Put an aerobatic constant speed on the same airplane and the required altitude is almost doubled. The c/s also requires considerably more airspeed to reduce the much higher sink rate on final.

Remember that best glide speed is not the same as minimum sink rate. Minimum sink is normally slightly above stall, but best glide is normally well above stall.

TODR

2 "best" glides.

the_other_dougreeves said:

Remember that best glide speed is not the same as minimum sink rate. Minimum sink is normally slightly above stall, but best glide is normally well above stall.
TODR

Click to expand...

Best glide ratio gives max distance. Minimum sink gives max time.

grantcarruthers said:

You're right, it's not 8.5 to 1. Cafe says it's 9 to 1 on the 8a and a whopping 12 to 1 in the 9(9a I think). So while the VSI shows a bigger number in the RV's in glide, so does the ASI and thus glide ratio ain't that bad. Not sure why people assume they're bricks on the way down. On the rare occasion I try a no flapper in the -4 I'm amazed how much effort it takes to get slowed and down. Almost always takes a full rudder slip for a surprisingly long time.

Click to expand...

I'll take a wild stab at it and guess that your RV-4 has a fixed pitch prop on it.

Cafe measures their drag polars with the prop taken out of the equation (at least they did when testing the RV-6). They didn't even tell us what kind of prop was on the plane. It could only be inferred to be fixed pitch from their mentioning static RPM.

A zero thrust switch was installed on the engine crankcase so as to detect the aft displacement of the crankshaft during windmilling flight. By sensing the transition point from tractoring to windmilling, the zero thrust condition can be detected. At zero thrust, the propeller effectively becomes “invisible” and the aircraft becomes a “pure” glider.

Click to expand...

The way I'm reading this you can't trust these numbers because their testing effectively removes the prop from the airframe. Drag polars must be recalculated for the given prop installation to get an accurate power off L/D for your airframe propeller combination. The L/D will be considerably lower when you 'put the prop back on the plane'.

And, yes, the RV-6 can be quite a brick with a constant speed Hartzell windmilling against an O-360. I did some thumbnail polars on mine configured as described and came up with a glide ratio that was less than 6:1 with the prop at fine pitch. Pull the blue knob out and that number goes to about 7:1. This isn't necessarily a bad thing because when the engine is working as it should you can squeeze the plane into some pretty tight spots without having to slip it. I practice power off approaches frequently and it's a real sled ride. It's a tight, short approach at best L/D of 90mph with a turn to base not very far past the numbers, slowing to 80 over the fence, and a flare just 100-200' past the numbers in a no wind condition.

the_other_dougreeves said:

Remember that best glide speed is not the same as minimum sink rate. Minimum sink is normally slightly above stall, but best glide is normally well above stall.

TODR

Click to expand...

This true with a high aspect ratio wing, but minimum sink speed typically goes up as the aspect ratio goes down for a given wing area. The type of airfoil is also a factor. Vx usually corresponds very well with minimum sink and Vy to best L/D. Minimum sink in the RV-6 is considerably faster than Vs (somewhere around 80mph).

Big air pump?

I'm not sure I'm buying the open throttle theory. The engine is a big air pump, but if you can remove the air it should unload the "pump", just as blocking the intake on your shop vac causes the motor RPM to increase. The motor is operating under reduced load by not moving air.

captainron said:

I'm not sure I'm buying the open throttle theory. The engine is a big air pump, but if you can remove the air it should unload the "pump", just as blocking the intake on your shop vac causes the motor RPM to increase. The motor is operating under reduced load by not moving air.

Click to expand...

If I remember correctly, Peter Garrison had his own theory and some other people had their disagreeing theories. That is why Peter went up and shut off the engine and tested it. I did some engine off testing of my own with the prop windmilling and stopped, but I didn't think to test this one. I'm glad Peter did.

I'm not sure I'm buying the open throttle theory. The engine is a big air pump, but if you can remove the air it should unload the "pump", just as blocking the intake on your shop vac causes the motor RPM to increase. The motor is operating under reduced load by not moving air.

Click to expand...

The shop vac is essentially a centrifugal compressor that is unloaded by the blocked intake, while the aircraft engine is a positive displacement pump on which the intake is never totally blocked by the throttle plate. The only way to make it not "move air" is to keep the intake and exhaust valves closed. That's how it's done on auto engines that have an economy mode that involves running at partial displacement.

V-8 Salute complete...

Dennis, Mel and GG: Makes perfect sense...open, or declutter if you will, the path of the air that's going through the engine. Thanks!

To take this one further, seems like pulling open the alt air or ram air (IO engines) would further help in this situation, by allowing the air to bypass the air filter. Would you wise ones concur?

Might not be a massive difference, but with an 80" set of paddles, big motor, and clipped wings, I'd be looking for every bit of improvement. Why I fly tight patterns...a -9A bud calls me a dive bomber...'course I call him a B-52 (all in fun...we're both doing it safely).

Thanks again!!

Bob

William Slaughter said:

The shop vac is essentially a centrifugal compressor that is unloaded by the blocked intake, while the aircraft engine is a positive displacement pump on which the intake is never totally blocked by the throttle plate. The only way to make it not "move air" is to keep the intake and exhaust valves closed. That's how it's done on auto engines that have an economy mode that involves running at partial displacement.

Click to expand...

But if you have the throttle wide open where the cylinders completely fill with air, you then have to compress that air, which takes some effort. I agree with the partial displacement method, but we don't have that here. I don't know the answer here; a stopped prop in flat pitch is going to create a certain amount of drag, but if the prop starts turning again, does that necessarily create more drag? And, if the pitch is increased enough away from flat(therefore less drag) and that starts the engine windmilling, will drag then increase?

Free spinning prop

DGlaeser said:

But even if the crank snapped behind the main bearing (creating in essence a free spinning prop), the spinning prop creates more drag than a stopped one. It's easy to demonstrate with a rubber band powered model airplane.

Click to expand...

I'm willing to believe this, just not sure I understand it...

Think about one thing...

If you have an engine failure and push the throttle full forward to extend your glide, remember to pull the mixture, turn off the fuel and ignition.

Once you have committed to a landing, you don't want the engine to miraculously start firing again when you are about to touch down. There have been cases where this has happened and the engine dies again once the plane is 100 feet in the air with no place to land.

Just a thought.

RV-6A glide raio

MacNab said:

And, yes, the RV-6 can be quite a brick with a constant speed Hartzell windmilling against an O-360. I did some thumbnail polars on mine configured as described and came up with a glide ratio that was less than 6:1 with the prop at fine pitch. Pull the blue knob out and that number goes to about 7:1. This isn't necessarily a bad thing because when the engine is working as it should you can squeeze the plane into some pretty tight spots without having to slip it. I practice power off approaches frequently and it's a real sled ride. It's a tight, short approach at best L/D of 90mph with a turn to base not very far past the numbers and a flare just 100-200' past the numbers in a no wind condition.

Click to expand...

Ed Kolano had an article in the July 2002 SA, in which he presented flight test data he obtained for the Young Eagles RV-6A. The article doesn't say what engine/prop it has.

Kolano reported these numbers from flight testing at 1435# gross: best glide speed 92 mph, glide ratio at that speed 11:1. I presume that this was with the engine turning at idle. He also noted that anything between 80 and 100 mph gave close to best glide performance. He calculated that at 1650# (max gross) the best glide speed should be 99 mph.

captainron said:

Did you note what your RPM and oil pressures were at the different prop settings?

Click to expand...

I just downloaded the data from the engine monitor.
At 70 kts with the mixture at idle cut-off and prop control full in:
Oil pressure 55 psi
RPM 1350

At 70 kts with the mixture at idle cut-off and the prop control fully out (coarse):
Oil pressure 50 psi
RPM 900

Fin
9A

Finley Atherton said:

I just downloaded the data from the engine monitor.
At 70 kts with the mixture at idle cut-off and prop control full in:
Oil pressure 55 psi
RPM 1350

At 70 kts with the mixture at idle cut-off and the prop control fully out (coarse):
Oil pressure 50 psi
RPM 900

Fin
9A

Click to expand...

Wow, very cool. Thanks for sharing the numbers. They are way more interesting than all the talk .

captainron said:

I'm not sure I'm buying the open throttle theory. The engine is a big air pump, but if you can remove the air it should unload the "pump", just as blocking the intake on your shop vac causes the motor RPM to increase. The motor is operating under reduced load by not moving air.

Click to expand...

The drag from a windmilling prop will vary depending on the windmilling rpm. If the prop is windmilling at high rpm, it starts to approximate a flat disc, which has very high drag. If it is windmilling at very low rpm the drag approaches that of a stopped prop. If you push the throttle open, the extra work required to compress the additional air in the cylinders should cause the rpm to decrease, which should cause the glide ratio to improve.

Kevin Horton said:

The drag from a windmilling prop will vary depending on the windmilling rpm. If the prop is windmilling at high rpm, it starts to approximate a flat disc, which has very high drag. If it is windmilling at very low rpm the drag approaches that of a stopped prop. If you push the throttle open, the extra work required to compress the additional air in the cylinders should cause the rpm to decrease, which should cause the glide ratio to improve.

Click to expand...

As always, I have the greatest respect for what Kevin says and that just goes to show that the problem is not that simple. Extra work causing a better glide ratio reminds me of imaginary numbers in college. Not very intuitive, but useful nontheless .

If engine D-E-A-D

Ok folks, at idle, you still have some oil pressure to potentially change the pitch from flat to coarse. Fine and dandy for discussion.

If I'm 6000 AGL, set my glide speed to around 90 knots from what I'm reading, the engine is dead, what's the prop going to do if you have no oil pressure?

Also, what's my range (let's assume no head or tail wind)? IE: Do I have 5 miles to land at an airport, 7 miles, 9 miles, what???

Quite frankly, dead engine and I all about trying to decide where I'm going to put my bird down. I would like to have an ideal of a "go or no go" distance decision. Hit the nearest button on the GPS and then make a decision and stick to it.

If the range for 1 up is 5 out, it becomes pretty easy to decide. Good visual for roads and fields 5 out.

So what's the distance to draw the line?

I have some experience from a former life with the Diamond DA20-C1 with the FP Wood Sensenich and the DA40 with the Hartzell C/S doing engine off glides. In all cases(for a given IAS) the windmilling RPM was higher with the throttle open. Interestingly, the engine sounds different too! Glide ratio was better with the prop stopped and best with the prop rotated so the lower blade lined up with the nosewheel...quieter too.
Both aircraft had to be slowed to less than 50 kts(45kts for the C/S) to get the prop to stop and unless a slow deceleration was used (1kt/sec. or less) the props would continue to windmill thru the stall. Once stopped, 105-110kts would get them turning again.
These days, with my clipped-wing cub, the behavior is identical but with 10kt slower speeds.
Just my .02
Mike

This is what you flight test period is all about!

This and many more things will be determined during your flight testing. BTW, just because your engine is "dead" doesn't mean you won't have oil pressure. As long as the prop is windmilling it is turning the oil pump.

Glide data RV6A - gps gathered

Just this past Sunday I collected some data. I had my notebook computer on the right seat, with a little gps puck up on the glare shield. I used a program to capture the gps data every second during power off glide tests. I kept the prop at fine pitch, but pulled the mixture. There are a number of variables not accounted for, but I wanted to share the data. I started at 6000' msl, pulled throttle to idle, mixture full lean. I nailed 70, 80, 90 and 100 kias. Each speed was held long enough to stabilize things while the computer collected the data. The 70 knot run was around 5500', while the 100 knot run was at around 3500', the other two in the middle of that range. Indicated airspeed was held +/- 1 knot, since that is all I had to worry about. I used only the gps altitude for the data below, although I did use ground speed to bracket the periods when airspeed had been stabilized.

The fpm numbers below are averages of the individual delta altitudes collected, during the periods that the speed was held constant.

70 kias - 975 fpm
80 kias - 1002 fpm
90 kias - 1197 fpm
100 kias - 1287 fpm

Not correcting for true air speed, the glide ratios are:

7.3:1 for 70 kias
8.1:1 for 80 kias
7.6:1 for 90 kias
7.9:1 for 100 kias

I can't explain the glide ratios not being well behaved - need more test data, although it could be because of the continuously lowering altitude for these tests. As part of the additional testing, I need to have data for all airspeeds between the same altitudes.

I don't know the error from TAS vs IAS (vs CAS for that matter) for these runs, but I recall that the baro was close to standard, and the temp was not far off either.

More work is needed, but the automatic data collection is quite nice.

Just so there is no confusion, my engine was "dead" (posts #1 and 37). The mixture was pulled full back and the data downloaded from the monitor shows fuel flow flat-lined at zero.

Fin
9A

Glide ratio calculation

AlexPeterson said:

The fpm numbers below are averages of the individual delta altitudes collected, during the periods that the speed was held constant.

70 kias - 975 fpm
80 kias - 1002 fpm
90 kias - 1197 fpm
100 kias - 1287 fpm

Not correcting for true air speed, the glide ratios are:

8.2:1 for 70 kias
7.4:1 for 80 kias
7.9:1 for 90 kias
7.6:1 for 100 kias

Click to expand...

Alex,

You might want to recheck the math? Based on your data (and making an approximate TAS correction), I get a glide ratio of about 8:1 at 80 kias. The ratios are lower at the the other speeds. This seems reasonably consistent with Kolano's results for the RV-6A, assuming he was running idle power and had a FP prop.

Spinning Prop - Greater Drag

Mel said:

A stopped prop is always less drag than than the big disk area of a windmilling prop.
I forget the exact numbers, but the test on the video (a C-182) showed a dramatic increase in glide.
A side note; stopping the prop requires bringing the airplane almost to stall speed, so obviously it shouldn't be done close to the ground.

Click to expand...

Alan Carroll said:

Not sure I completely understand this. The prop blades are the same size whether they're moving or not. Wouldn't the decreased drag of a stopped prop actually be due to not turning the dead engine?

Click to expand...

DGlaeser said:

It takes energy to turn the prop, whether or not it is connected to the engine. The dead engine would tend to make it harder to turn of course, which increases the energy required. But even if the crank snapped behind the main bearing (creating in essence a free spinning prop), the spinning prop creates more drag than a stopped one. It's easy to demonstrate with a rubber band powered model airplane.

Click to expand...

Alan Carroll said:

I'm willing to believe this, just not sure I understand it...

Click to expand...

Sometimes this forum can be a curse... it's after 1:00 AM, and I'm lying awake trying to get my head around this one. And, it finally hit me. As Alan states, the prop is the same size whether it's moving or not. However, a spinning prop blade will displace a greater volume of air in a given distance the aircraft travels through the air, than a stationary prop blade.

Since I have a very simple mind, let's keep this very simple. Someday, hopefully in this decade, my finished RV9A will travel through the air a distance of 10ft. If the prop is stationary (zero RPM), the blade(s) will have traveled through 10ft of air, from point A to point B... displacing XXX volume of air mass in a straight line.

Now, if the blade is spinning XXX rpm and lets say it makes one full rotation in that given 10ft. of aircraft travel, then the blade is no longer traveling in a straight line between point A and point B. The blade has covered a greater distance and displaced a greater volume of air mass for the given 10ft the aircraft has traveled.

I'm sure there are a million other variables that effectively change the profile of the blade and reduce the volume displaced, ie. open throttle, blade pitch... and I know that all of you guys understood this immediately... but for this slow guy it was driving me crazy!!!

So, in order to get some shut eye, I just needed to vent my thoughts. Please, somebody smart, correct me if I'm wrong.

Alan Carroll said:

Alex,

You might want to recheck the math? Based on your data (and making an approximate TAS correction), I get a glide ratio of about 8:1 at 80 kias. The ratios are lower at the the other speeds. This seems reasonably consistent with Kolano's results for the RV-6A, assuming he was running idle power and had a FP prop.

Click to expand...

Alan, you are correct (see corrections in original post). I never saved the spreadsheet I made those numbers from, so I'll never know what I did wrong! Probably the usual problem - VAF tends to be late at night! Thanks for keeping me honest...

Those numbers compare very favorably with previously tested numbers for minimum sink and best L/D speeds - and they also match Vx and Vy.

What engine and prop combination are you running, Alex?

I think I need to do some more thorough testing on Friday.

AlexPeterson said:

J

70 kias - 975 fpm
80 kias - 1002 fpm
90 kias - 1197 fpm
100 kias - 1287 fpm

Not correcting for true air speed, the glide ratios are:

7.3:1 for 70 kias
8.1:1 for 80 kias
7.6:1 for 90 kias
7.9:1 for 100 kias

I don't know the error from TAS vs IAS (vs CAS for that matter) for these runs, but I recall that the baro was close to standard, and the temp was not far off either.

More work is needed, but the automatic data collection is quite nice.

Click to expand...

As far as drag with a spinning prop goes I don't know how much the disc area increases with rpm but what is 100% clear is it takes a fair amount of horse power to turn the engine. I'd guess on the order of 5-10hp (just a guess, maybe an engine builder can chime in here).

As far as the throttle open vs closed. The piston on intake is trying to suck air in. If the throttle is open it's very easy. If it's closed it has to pull against a vacuum which requires more horse power. Kevin is correct that the more air in the cylinder will require more effort to compress when the compression stroke comes around but where he is at least partially incorrect is ignoring that that same air gives power back on the power stroke. Once the valves close the air acts as a spring which yes requires energy to compress but it gives back that energy on the power stroke. This is why the engine spins easier thus less hp/drag penalty with the throttle open in the glide.

I for one will get the engine/prop stopped if at more than 1500 ft and trying to extend the glide. Slowing to near stall to stop the engine should not be an issue. If it is go get some time with a CFI.

What I'd REALLY love to see is good data on a cs plane for course, fine, and stopped. I'm a FP prop flyer so I don't have the 3rd option. I'd like to see if a CS prop benefits much from the feather position to the stopped prop.