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gereed75

Well Known Member
Went out Sunday to get some glide data relative to prop settings. Did'nt believe it the first time so did it again. Didn't believe it that time so did it again today. Here's the data, all at 80 KIAS, throttle closed:

Prop full RPM (fine pitch) - oil Press 62- MAP 8"-1090RPM - 1050 FPM Descent

Prop Full Coarse - oil Press 56 - MAP 8"- 700-1100RPM - 1050 FPM Descent

No Change!!! I found that you could vary prop RPM with speed and took it down to 60 KIAS and 400 RPM. Pretty uncomfortable shaking (think continuous shut down shudder). At 400 RPm and 80 KIAS, still no change in descent rate.

Tried WOT. Yep,less pumping work resulted in faster RPM to 1300, and increased descent to 1300FPM.

Not calling BS on anyone else's experience. I was also a believer in coarse prop, more glide. I suspect it is highly engine/prop related so for me with a 200 RV and IO-360, it just didn't happen. Wanted it to happen, tried three times, just didn't happen. Can't argue the data - No increased glide for my set-up. YMMV.
 
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interesting

Seems like the prop blades are stall in all cases. I think the big difference is if the blades are not stall and freewheeling.
 
Glide ratio and descent rate are two different things. What was the airspeed for each test? As an example, if you were descending at 1000fpm at 60 knots, your glide ratio is roughly 6:1, if you were descending at 1000fpm at 120 knots, your glide ratio is rought 12:1. These aren't exact numbers, but you see what I'm getting at. A Garmin will give a more accurate glide ratio, as has been discussed in other threads.
 
Buff, All at 80 KIAS.

Ken, I was looking for comparative data - stabilized descent measured with altimeter vs clock for 1000', 5000' through 4000'.
 
Too slow - repeat at 105 kts

You flew the test at such a slow speed relative to best L/D that your sink rate is dominated by induced drag. The change in drag from the prop was insignificant compared to the high induced drag.

If you repeat the test again at 105 kt, and again do it with both coarse and fine pitch, and open and closed throttle, I will bet you will see more of a difference.
 
Thanks Steve

This has got me thinking about all of my "best glide" emergency data. I had generated most of this during Phase 1 and will review some of that and go fly a few more tests.

Thanks for making me think about this again.
 
I agree and will strongly suspect if you increase your speed, you will see a better descend rate. Also, a fair amount of difference if you are idling as oppose to engine out. My test with engine tuned off, I was getting 700 fpm @ 95K speed, this is with the prop pulled.
 
remember: min sink not best L/D

Remember that the minimum sink speed is slower than the speed for best L/D.
Don't pick a glide speed based on minimizing sink rate. That will give you best endurance, but not best gliding range.

Of course if you are already where you want to go, and just want to delay your arrival, then min sink speed IS best:D
 
There is a bit more to it than just taking a stab at the glide ratio. I think one should have a solid understanding of polar curves, and I find most power pilots lack it's basic understanding. Everyone should take a number of things into considering when developing a Polar Curve for their aircraft. Try to minimize a number of variables such as convection, weight and CofG, wind etc when plotting a curve.
 
A few years back, ABC Wide World of Flying did a glide ratio test in a Cessna 182 with prop in fine pitch, prop in coarse pitch and with the prop stopped. Dramatic differences.

Rate of descent and glide ratio are not the same thing.

As others have said, try your test at "best glide" speed and you will see a difference in glide ratio.

Another clue; don't trust your ROC instrument for this. It has too much lag. You must actually "time" the event.
 
Use GPS for VSI

I agree that the baro VSI is not good for this. I also agree that timing the sink from one altitude to the other is accurate if you can manage it and if you keep the intervals small (but that impairs accuracy). The same Garmin that can give you glide ratio can give you sink rate and it's probably very accurate. If you have the ability to record your airdata, then do that, too, analyze it later.

In an ideal drag curve, the sink rate will be about 14 to 15% higher at best L/D than at minimum sink speed. The speeds will be 1 vs 1.32 or 0.76 vs 1.0. It's the same relationship. However, with prop drag in the mix, these numbers may not hold up well. For example, my minimum sink is at about 76 KIAS and my best L/D is at about 96 KIAS.

If you want to know what the actual or calibrated air speeds are, you will have to calibrate the ASI using GPS and density altitude conversion for the speeds at which you are doing this.
 
Another possibility

Gary,

Are you sure you were affecting the prop pitch at all? Consider that your prop lever doesn't control prop pitch directly, but rather target RPM via the governor. Pulling the prop lever all the way out is not "coarse pitch", it's "low RPM". So, what is your prop governor's low RPM stop set to? If it's set higher than 1100 RPM, then you weren't actually changing the pitch of the prop at all. You were off the governor at that point, and with your prop lever full forward or full aft, it wouldn't matter -- either way your prop would be at fine pitch. Just a hypothesis.

-Roee
 
gereed75 said:
Went out Sunday to get some glide data relative to prop settings. Did'nt believe it the first time so did it again. Didn't believe it that time so did it again today. Here's the data, all at 80 KIAS, throttle closed:

Prop full RPM (fine pitch) - oil Press 62- MAP 8"-1090RPM - 1050 FPM Descent

Prop Full Coarse - oil Press 56 - MAP 8"- 700-1100RPM - 1050 FPM Descent

No Change!!! I found that you could vary prop RPM with speed and took it down to 60 KIAS and 400 RPM. Pretty uncomfortable shaking (think continuous shut down shudder). At 400 RPm and 80 KIAS, still no change in descent rate.

Tried WOT. Yep,less pumping work resulted in faster RPM to 1300, and increased descent to 1300FPM.

Not calling BS on anyone else's experience. I was also a believer in coarse prop, more glide. I suspect it is highly engine/prop related so for me with a 200 RV and IO-360, it just didn't happen. Wanted it to happen, tried three times, just didn't happen. Can't argue the data - No increased glide for my set-up. YMMV.
Click to expand...

You pose an interesting question and I know what you report is true.

It is also true that with the MT-7 prop and the Subaru H6 engine, descent rate at fine pitch was like a brick, and at course pitch about 800 fpm.

My oldest son was flying a C-182 with the Peterson STOL mod a few years back and its descent rate was the same, just like what you reported no matter where the prop was set.

It is also true, any fixed pitch propped RV will glide better than any CS propped RV. That course blade angle may be the key. The MT-7 goes very course, probably more than most Lycoming CS installs. That may be the answer.
 
In 1996 our 6A was built in 3 locations, wings one shop fuselage and tail in another and the engine at the airport. When the plane was assembled there was a 5/8" gap between the lower wing skin and the fuselage. Questioned the factory and it was everyone puts these parts together before assembly and we havent' changed the plans yet. The last four words held the clue, the plane would decend at 1000+ fpm no matter what. When we fit the skin to the fuselage, the plane became a glider. So the fit does make a HUGE difference.
 
6A Glide Ratio

Bob Axsom said:
As in falls like.

Bob Axsom
Click to expand...

Bob's plane may not glide well, but the actual L/D for the 6A that CAFE tested was 12.25. Of course, that is without prop drag. It would not be as good with prop drag. You can see the numbers in the spreadsheet on my website.
 
Last evening was perfect testing conditions. No wind, still air. I did a lot of dead engine (ICO) glide testing 80, 90, 100, 110 and 120 KIAS. Each airspeed at three conditions - prop fine, prop coarse and coarse WOT. All timed descents - start at 5500, stabilize and timing 5000' to 4000' (Dynon D-10 timer mode), and back up again.

I built a spread sheet to log the data and computed descent rates and glide ratios, and graphed it all. I thought I would see some smooth curves with clearly predictable relations ships, but it was a bit more confusing than that.

With Prop fine, glide ratios started at 7.46 at 80 KIAS and went down from there with increased speed (and proportionaly increased RPM). About as expected.

With prop coarse - Glide ratio started at 7.46 @ 80 KIAS, bottomed at 7 @ 90 KIAS and went up in a predictable curve to peak @ 8.2 at 110 KIAS, sliding off from there. Interesting to note that RPM stayed a constant 1350 through out.

The prop coarse WOT gave similar results but with a higher peak 8.7 glide ratio but at a lower 100 KIAS, sliding off more gradually than the throttle closed data. RPM also stayed constant @ 1350.

In short, Steve (and others) were right, prop coarse can yield better glide ratios at higher airspeeds (and greater rates of descent).

So what is the practical benefit (IMHO)?? For my money, in the event of an actual engine failure at cruise, I am going to transition to 80 KIAS and work with that first - clean up the cockpit, troubleshoot while deciding on a field and attempting a restart if prudent. Once all that is done, if and only if I need the extra glide and can positively control the prop, would I go to prop coarse, WOT and 100 KIAS.

My reasoning?? - after seeing 100's of students dealing with simulated engine failures (and doing hundreds myself) I know that committing to a known course of action (committing to a field, running the procedures and flying the airplane at known and practiced performance parameters) gives you the best chance of success in the short time you have. The last thing I want to do is screw up a good thing while I was trying to decide if I could get better glide if I could just optimize the airplanes configuration.

Whatever your choice - go practice and practice with a consistency and a method!!!

I am glad I went and revisited this and got better data. I learned a lot.
 
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