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  #1  
Old 04-26-2016, 10:12 PM
SMO's Avatar
SMO SMO is offline
 
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Location: Salmon Arm, BC
Posts: 933
Default Glide Ratio Weirdness

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.



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.
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  #2  
Old 04-26-2016, 10:54 PM
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logansc logansc is offline
 
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Default

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...
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  #3  
Old 04-26-2016, 10:58 PM
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Default

Oh yeah, should have mentioned - all glide tests done with prop all the way back (full coarse) for maximum glide.
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1987 RV-4 Sold
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F1 EVO Rocket, first flight May 31/14
First in line for the Sonex JSX-2T kit
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  #4  
Old 04-26-2016, 11:18 PM
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Bob Kuykendall Bob Kuykendall is offline
 
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Location: Vallecito, CA
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Default

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.
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  #5  
Old 04-27-2016, 12:48 AM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Southwest
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Default 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.
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  #6  
Old 04-27-2016, 06:57 AM
BillL BillL is offline
 
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Location: Central IL
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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-Progr...07.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.
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  #7  
Old 04-27-2016, 08:28 AM
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F1Boss F1Boss is offline
 
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Location: Taylor Texas
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Default 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
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  #8  
Old 04-27-2016, 09:33 AM
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erich weaver erich weaver is offline
 
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Default 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
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  #9  
Old 04-27-2016, 09:58 AM
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vfrazier vfrazier is offline
 
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Blake has the "rockblocker" tape on the Flyboy Accessories site.

http://www.flyboyaccessories.com/Pai...ilm-p/2301.htm
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  #10  
Old 04-27-2016, 01:42 PM
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I'll try taping off the leading edge screws, retest, and provide an update.
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First in line for the Sonex JSX-2T kit
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