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Glide Ratio Weirdness

SMO

Well Known Member
I have never satisfactorily determined the "Best Glide" speed for my F1 EVO Rocket. I have now done a few flights trying to pinpoint this, and while I think I need to change my process a bit in order to get more accurate, I have discovered a weird anomaly and I wonder if others have seen something similar. The following graph shows the data points I have determined in 3 different flights on different days. The thing I think is weird is that every flight shows that at 105 knots the glide ratio is worse than the preceding or subsequent speed.

Glide%20Ratios.jpg


Anybody have an explanation for such a result?

The method I have been using: Determine the number of seconds it takes to glide from 3500' to 3000' at a constant IAS (+-1 knot). I get this from the data log of my G3X. I then convert the speed to "feet/minute forward" (knots * 101.269) and descent to "feet/minute vertical" (60/(seconds * 2) * 1000). Dividing feet/minute forward by feet/minute vertical provides the glide ratio. The speed with the best glide ratio is the best glide speed.

The problem I have with this method is that I need a more granular measure of time, or I need to increase the glide distance to 1500 or 2000 feet. However I don't believe making these changes will impact the seemingly repeatable event that shows 105 knots having a poor glide ratio.

I also understand that flying on different days with different weights, temperatures and air densities will also give differing results, so I am not unduly concerned that each days flight has some differences.
 
I don't have an answer Mark, but I can confirm that my sport wing Rocket seems to "give up" pretty quickly shortly after 100 knots as well. My testing was not as repeatable nor as precise as yours and also I didn't press the issue after the descent rate dropped off so obviously shortly after 100 knots. As a result, I did not pick up that things might improve again 5-10 knots faster. I was a bit surprised that (apparent) best glide was as high as it was, however.

Maybe one of the other technical guru's will chime in here, but I wouldn't be surprised to learn that this anomaly is related to prop drag somehow. You didn't specifically mention what you'd done with prop pitch during your testing (I'm assuming these were not actual engine out tests?)


Lee...
 
Oh yeah, should have mentioned - all glide tests done with prop all the way back (full coarse) for maximum glide.
 
I'd ping Steve Smith (scsmith on this forum), this is his area of specialty.

Looking at the data, I'd be tempted to suspect a leak in the static system used as an altitude and descent rate reference.

Thanks, Bob K.
 
Laminar flow trip

Last time I saw a curve like that was when Dick Johnson was doing sailplane testing. If I remember right, His cause was the laminar flow not tripping properly. He resolved it but photographing the laminar flow trip using dirty oil on the wings. And using a zig zag strip on the wings to trip the laminar flow. Maybe it is worth a flight with dirty oil and a GoPro to make sure the laminar flow is tripping properly. Dick Johnson wrote a whole series of articles on this subject and should be searchable.
 
Intelligent thread. Here is a link about the deturbulators that solved the problem. Note that it was dubbed the Johnson Effect.

http://sinhatech.com/SinhaFCSD-Progress-12012007.asp#article

If someone could help with the physical description of these deturbulator tapes please do so. It looks like they do not protrude above the surface except a tiny bit. Are they like s tiny flapping film? I will be doing more searching, meanwhile.
 
Try this:

With a good coat of wax on the wing, tape over the screw line that attaches the leading edge, and repeat the experiment. Could be that the screw line is tripping the flow locally, and it does not re-attach at that AOA/IAS.

If you find this to be the case, Rockblocker tape could be used to cover that screw line in a more permanent manner. I'd recommend the .020 thick stuff for that application.

Interesting experiment indeed!

Carry on!
Mark
 
Real time glide ratio display

FYI, the garmin x96 series gps units can display glide ratio in real time on their data page. I would guess the newer units can do the same but I am not familiar with them.

Erich
 
Can I ask about the underlying data for that curve?

It appears you have either fit a curve to the data or used Excel's smoothing function to draw a curve between the points. Do you have an actual data point at 105 kt? Does it correspond to the curve?

If you fit a polynomial to the data, of what order is the polynomial?
 
...If someone could help with the physical description of these deturbulator tapes please do so...

"Snake Oil" is the closest thing I can think of. So far as I know, it has not proven to be widely applicable.
 
Last edited:
Pretty simple process:
Established cruise attitude
Pull power
Hold cruise attitude
Aircraft will achieve best glide speed itself.
Not complicated!
 
Can I ask about the underlying data for that curve

It's just an Excel scatter graph. Here is the test results:

March 18 - 90, 7.7; 100, 8.8; 105, 8.3; 110, 8.9; 120, 8.9
April 25 - 90, 8.2; 100, 9.2; 105, 7.8; 110, 8.7; 130, 9.1
April 26 - 95, 8.0, 100, 9.1; 105, 8.4; 110, 9.6; 115, 9.7; 130, 9.5

Where I have holes in the data I just interpolated to smooth the graph (not included above), but in all 3 cases I have actual data for 100, 105, and 110 knots plus additional data above and below those numbers.
 
I wonder if possibly the engine power is varying in a strange way as the glide speed and AOA vary. Perhaps something is happening at 105 kt that affects the mixture.

Is there anything in the engine data that suggests that it is running differently (or not) as the glide speed varies? How do the idle speed and EGT vary with glide speed?
 
Building on what Kevin said, I wonder if the governor is just slightly tweaking the pitch somehow, or if a transition through 100kt is taking the slop out of some system that has a little "give". Trim tab shifting? Flaps extending or reflexing a tiny bit?
 
I'd be tempted to try again with the prop forward to see if the 'break' stays the same, goes away, or moves to a different speed range such that the area of interest in the curve is usable.

==dave=
N102FM
 
I'd be tempted to try again with the prop forward to see if the 'break' stays the same, goes away, or moves to a different speed range such that the area of interest in the curve is usable.
That would be an interesting test, and might shed some light on what is going on, or at least rule prop governor issues. But, I'm not sure it would make the curve any more useable, unless the question is "what would my glide ratio and best glide speed be if the throttle is ever jammed at idle?"

If the question is "what would my glide ratio and best glide speed be if the engine failed, but was not mechanically damaged, and thus allowed the prop to windmill?", then the only way to get an answer is to test with the mixture pulled to ICO. My testing with idle vs engine off showed the best glide speed and glide ratio on my aircraft differed substantially between engine at idle and engine off.
 
It appears the curve fit is hiding the real variability of the numbers. Fitting a polynomial with its greatest exponent (order) equal to the number of data points will always make a mathematically perfect fit that is almost alway useless.

A different approach to looking at these numbers: Collapse the data from the different runs and plot the mean glide ratio for each airspeed. Then place error bars around the mean for each airspeed based on the the variability (standard deviation) of each of the glide ratios observed at that airspeed.

Then entertain the idea that the 105 results may be just noise.

-Dave
 
Intelligent thread. Here is a link about the deturbulators that solved the problem. Note that it was dubbed the Johnson Effect.

http://sinhatech.com/SinhaFCSD-Progress-12012007.asp#article

If someone could help with the physical description of these deturbulator tapes please do so. It looks like they do not protrude above the surface except a tiny bit. Are they like s tiny flapping film? I will be doing more searching, meanwhile.

What turbulators usually do is the same thing that dimples on a golf ball do. They transition the flow from laminar, which is very unstable, to turbulent, which is much more stable. Turbulent flow is often confused with separated flow, which it is not. At the aft portion of an airfoil, where the accelerated air is now slowing back down, the boundary layer gets thicker. Usually it does separate towards the TE and then the drag goes up substantially. A turbulent boundary layer has higher skin friction drag than a laminar boundary layer, but it is much more robust and stays attached much further after on the wing than a laminar layer will. Despite the increase in friction drag, if you can get the flow to stay attached for a greater portion of the wing chord then you reduce drag - just like on the golf ball.

Laminar flow is very difficult to maintain for any significant portion of a wing's chord or length of a fuselage because of skin waviness, fasteners, panel edges etc. Only the most high perforance sailplanes that have really high quality surfaces free of waviness can maintain significant laminar flow. It is the holy grail of drag reduction but it is so difficult in practical terms that it is not widely used. Certainly not on our RVs.
 
I'd be tempted to try again with the prop forward to see if the 'break' stays the same, goes away, or moves to a different speed range such that the area of interest in the curve is usable.

==dave=
N102FM

+1
Just to see if there's something wierd with the prop or governor at that speed. But more than academic; it will give you a worst case (no engine oil) scenario so if it happens you'll have some idea of how far you can go.
 
....Only the most high perforance sailplanes that have really high quality surfaces free of waviness can maintain significant laminar flow....

Actually, that's not entirely true. Many years ago I was taking some performance data on a T-33, which at that time had to have been at least a dozen years old, probably more. This was before digital recording equipment; we wrote the data on test cards in real time.

I cross-plotted the data to see if we could catch any errors while still airborne and found that the curve was becoming quite non-parabolic. I'd expected that we'd see a pretty parabolic drag polar. What developed, though, was a classic laminar drag bucket.

I can't say what design feature of the airplane caused that; it might have been an engine characteristic, the wing airfoil (which was a laminar section) or the inlets or what. Kind of doesn't matter: there it was, still working fine on an airplane stored outside.

Dave
 
That would be an interesting test, and might shed some light on what is going on, or at least rule prop governor issues. But, I'm not sure it would make the curve any more useable, unless the question is "what would my glide ratio and best glide speed be if the throttle is ever jammed at idle?"

If the question is "what would my glide ratio and best glide speed be if the engine failed, but was not mechanically damaged, and thus allowed the prop to windmill?", then the only way to get an answer is to test with the mixture pulled to ICO. My testing with idle vs engine off showed the best glide speed and glide ratio on my aircraft differed substantially between engine at idle and engine off.


That's very strange Kevin. In my testing, I found going to ICO hardly made a difference in my glide ratio, maybe 10%. I know we have the same engine IO-360 angle valve but we do have different props. I still have the standard compact hub Hartzell.

During testing, I found my best glide speed to be about 80 KIAS which yielded about an 1100 fpm descent with the prop windmilling at idle. Going to ICO increases the the descent by about 100 fpm. I verified the engine was off by moving the throttle forward with no response. In fact, unless I stared at the VSI, I don't think I would have been able to tell the difference. The sound didn't even change. I haven't tried it with the prop stopped because it's almost impossible to get slow enough. Most of the testing was done with the prop lever forward. I did try it with the lever all the way aft (coarse pitch), but I found the prop didn't start to govern until I hit about 100 KIAS. I found that at increasing airspeeds (10 kt increments up to 140 KIAS) the decreasing drag of the prop was offset by the increasing drag of the airframe almost perfectly resulting in a constant glide angle.

I still do the testing ocassionally to verify my numbers. It drops like a rock unlike a fixed pitch RV or a sleek jet. At least it doesn't fall out of the sky like a C-17 with the speedbrakes deployed and all four engines in idle reverse!
 
....tape over the screw line that attaches the leading edge, and repeat the experiment.....

Did this today with electrical tape (.007" thick) and got a very similar result to previous testing: 95 knots - 8.6; 100 knots - 9.2; 105 knots - 8.9; 110 knots - 9.2; 115 knots - 9.2; 120 knots - 8.8; 130 knots - 8.6

I did make a change in the test procedure - ran the test from 4,000' ASL through to 3,000' ASL (instead of 3500' - 3000') in order to get better granularity regarding the time measurement. I am satisfied the results are reasonably accurate.

Pretty simple process:
Established cruise attitude
Pull power
Hold cruise attitude
Aircraft will achieve best glide speed itself.
Not complicated!

I find this quite interesting as an accepted process. I did this today and found the speed at which the airplane glides at this attitude is ~80 knots. I let it glide at that for 500' of descent and looked at the data - it results in a glide ratio of 8.3:1, whereas at 110 - 115 knots my glide ratio is 9.2:1, quite a bit better.

It is interesting that the documentation with my AFS AOA Sport suggests that, after calibration, the glide speed attained with the 3rd dot lit (first amber one) is the L/D max, which also is supposed to correspond to best glide. On my AOA, the glide speed that lights the 3rd dot (with prop all the way back) is ~80 knots. So the AFS documentation would appear to support the process you have suggested.

Problem is, my empirical testing refutes it for my particular airplane.

Is there anything in the engine data that suggests that it is running differently (or not) as the glide speed varies? How do the idle speed and EGT vary with glide speed?

Checked the data. Idle speed varies between 1440 and 1460 rpm at every glide speed. EGTs for 100 knots and 110 knots vary from the 105 knot run by 50* or less, some cylinders are a little hotter, some a little cooler. FYI, coolest cylinders are ~800*, warmest are ~1050*


I will do some glide tests with the prop full fine and see what I get from that.
 
That's very strange Kevin. In my testing, I found going to ICO hardly made a difference in my glide ratio, maybe 10%. I know we have the same engine IO-360 angle valve but we do have different props. I still have the standard compact hub Hartzell.

During testing, I found my best glide speed to be about 80 KIAS which yielded about an 1100 fpm descent with the prop windmilling at idle. Going to ICO increases the the descent by about 100 fpm. I verified the engine was off by moving the throttle forward with no response. In fact, unless I stared at the VSI, I don't think I would have been able to tell the difference. The sound didn't even change. I haven't tried it with the prop stopped because it's almost impossible to get slow enough. Most of the testing was done with the prop lever forward. I did try it with the lever all the way aft (coarse pitch), but I found the prop didn't start to govern until I hit about 100 KIAS. I found that at increasing airspeeds (10 kt increments up to 140 KIAS) the decreasing drag of the prop was offset by the increasing drag of the airframe almost perfectly resulting in a constant glide angle.

I still do the testing ocassionally to verify my numbers. It drops like a rock unlike a fixed pitch RV or a sleek jet. At least it doesn't fall out of the sky like a C-17 with the speedbrakes deployed and all four engines in idle reverse!
My three-bladed aerobatic MT prop does have a lot more discing drag than two-bladed props. A non-aerobatic prop would probably have less discing drag, assuming that the builder set the fine pitch stop correctly. With an aerobatic prop, the coarse pitch stop is the one the builder can adjust, and the fine pitch stop requires prop disassembly to adjust. I suspect mine goes fine enough to disrupt the airflow over the inboard wing at idle or with the engine off and prop windmilling. Pulling the prop control full aft didn't make any difference with the engine off, as that just changes the target rpm, and the rpm could never get that high, so the governor was setting fine pitch.

The different props almost certainly explains the differences between your results and mine.
 
I have now done a few glide tests with the prop set for 2500 rpm and the power pulled all the way back. The results (first number is speed is in knots, second is glide ratio) are:

75 - 8.1
80 - 8.1
85 - 8.2
90 - 7.3
95 - 7.3
100 - 6.8

This results in a curve more like I would expect, albeit quite a bit flatter than I would have thought.

Bottom line learning out of all this for me is - with the prop in fine pitch best glide target will be ~80 knots for a glide ratio of 8:1. If I can get the prop to go coarse, then best glide target will be 100 knots for a glide ratio of 9:1.
 
Kevin H,

Interesting data you've provided on the time, distance, and added drag (full flaps to get the airspeed low enough) involved in stopping the prop while calculating best glide numbers. http://kilohotel.com/rv8/index.php?option=com_content&view=article&id=776

Since you did those tests, have you explored an alternate method of stopping the prop? I'm thinking about pulling the nose up smartly, relaxing the back pressure, and flying something approaching a .5 g parabola. The drag penalty and altitude lost with this profile seems like it should be minimal.

Thanks for the information you've shared.

Cheers, David
RV-6A
 
2 different responses in one

FYI, the garmin x96 series gps units can display glide ratio in real time on their data page. I would guess the newer units can do the same but I am not familiar with them.

Erich

1. The whole x96 series through at least 496 do that. However, they are reporting the result of a calculation from sink rate and ground speed. Therefore the best use of that instrument for this purpose would be either in zero wind or 90 degrees to a very light wind.

That said, it would show a change such as the OP found, even if it distorted it due to wind.

2. "Best glide" is different depending on what you are trying to achieve. If you want to find the best glide in an emergency then simulate that emergency as best you can and ignore other approaches. If you have a GRT it will show you the best glide on the flight display and it would be the way to manage an emergency descent. Other EFIS systems may also do that. However, if you want to know the best L/D speed of your aircraft I suggest an indirect approach:

1. fly the aircraft in level flight as slow as you can without adding power. This is not easy to do accurately. You want IAS (or CAS if you care, later). When you are reasonably certain of the speed that does this, multiply by 1.32 to get the best L/D speed in A] level flight and B] theoretical "no prop" situation. But this will almost certainly not be the speed for an engine-out event. If the wing is significantly laminar (like a Vari-EZ) then this will not produce accurate results but for a normal GA wing like an RV, it will. You will want to know your airspeed correction factors before this (not a simple issue).
 
Kevin H,

Interesting data you've provided on the time, distance, and added drag (full flaps to get the airspeed low enough) involved in stopping the prop while calculating best glide numbers. http://kilohotel.com/rv8/index.php?option=com_content&view=article&id=776

Since you did those tests, have you explored an alternate method of stopping the prop? I'm thinking about pulling the nose up smartly, relaxing the back pressure, and flying something approaching a .5 g parabola. The drag penalty and altitude lost with this profile seems like it should be minimal.
David - that is an interesting thought, but it took so long for the engine/prop to wind down once I got slow enough that I really doubt it would work. I would need to check the recorded data to confirm, but my recollection is that the rpm decayed to a fairly low value once I slowed to just above the stall, then took over two minutes for it to decay to a stop. The technique you propose would allow a momentary period of slower speed, but I would be extremely surprised if the duration at slow speed was long enough to get the engine to stop. But, actual results don't always match the theory, so I'll add it to the list of things to try sometime.
 
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