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  #211  
Old 07-28-2008, 09:19 AM
Steve Brown Steve Brown is offline
 
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Default Thanks Pierre!

Pierre,

Now we can discuss how it can be done safely for those of us that don't have your glider pilot skills.

Your airspeeds are higher than what I used in my one test at altitude and on my spread sheet.
-We're in different airplanes. The 6 has higher best glide and stall speeds
-Your flying at a lighter weight with more horsepower, putting you higher, closer to the runway
-Your better at doing this than I am

That last one means I'm never going to make this from 250 feet.

I re-crunched some numbers at higher airspeeds, estimating sink rate, and what I found was that I increase sink with speed, but being able to confidently turn back with 60 degree bank can completely make up for this in terms of altitude lost in the turn.

I need to go back up and do some glide/sink rate testing at higher airspeeds, then try some turn backs at those higher speeds.

I must say, going from 75 kts to 85 kts, or even 90kts, my enthusiasm for practicing this maneuver increases considerably. That allows bank angel to be, as yours, between 45 and 60, without fear of accelerated stall.

Sounds like you were using flaps. Did you use them while turning?
Another angle on this is to use full flaps and tighten the turn by keeping the airspeed low, say 70 kts for me, and using 60 degree bank. Still allows a healthy margin over stall.

As a glider pilot you probably already know whether or not that is viable.

For someone of my very modest stick & rudder skills, I need to reduce this to a procedure that is not a death defying feat and be willing to accept the reduction from best available performance that goes with that.
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  #212  
Old 07-28-2008, 10:56 AM
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Question Pierre's accent... and Slipping a Cessna with Full Flaps?

This has been a very interesting thread. Thanks, Pierre, for providing some REAL data...with the appropriate caveats, of course.

I'm a long way from flying my RV-7, but I do have a few hours in a friend's (a CFII) RV-9A and a little unofficial time in another friend's RV-6A.

One thing I learned from this is that your accent wasn't what I expected. I guess I thought that with a name like "Pierre" and with your South African heritage, you wouldn't have the typical Southern accent. I was pleasantly surprised. Being from Alabama, I like the Georgia accent. There is a slight difference.

Another comment...someone in a previous thread referred to slipping a Cessna with full flaps. My CFI demonstrated that once for my BFR and about a week later told me he made a mistake by doing it. He said Cessna doesn't recommend that maneuver for a 172. Their concern is that full flaps can block airflow over the horizontal stabilizer and elevator. Ever heard of that?
Also wondering if that should be a concern for our RV's.

Don
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  #213  
Old 07-28-2008, 11:03 AM
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hevansrv7a hevansrv7a is offline
 
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Default Best Sink vs. Best Glide

Quote:
Originally Posted by pierre smith View Post
....according to the CAFE Foundation is around 105 MPH with the engine windmilling. If you use 75 or 80, then yes, the sink rate approaches that of a set of car keys! They also point out around 11:1 glide ratio at that speed.
...


Pierre, thanks very much for doing this - I agree with you 99%. That said..

At best glide, the sink rate in terms of f/m is actually higher. Best sink is at a lower speed but with a reduced glide ratio. Best sink is found at 76% of best glide. In the CAFE 6A that's about 80-81 mph vs. a best glide of 106 mph. The sink rate at best sink/endurance speed for the CAFE 6A is between 666 and 715 fpm depending on which of their observations are taken as primary (they have data which don't match up completely and admit to a 5% fuzziness due to air conditions during the test). Sink rate at best glide speed goes to 815 fpm, an increase of 100 fpm over best sink speed.

So those are some pretty aerodynamic car keys.

However, it is worth noting that for this test, CAFE was using the zero thrust method which could be expected to give a better glide or sink than a stopped prop. That's because a stopped prop must produce some drag - negative thrust - and CAFE's test had zero thrust.

BTW, In my opinion, even the short wing RV can out-glide most fixed gear Cessnas. Here's some logic to consider. If an airplane had no drag it would have an infinite glide ratio. If you add drag it sinks. If you add more drag it sinks faster. Now, which airplane has less drag at its best glide speed? That will be the one with the better glide ratio. It is not a secret that the RV's are much cleaner. How else could we go 100 knots on 3.5 gph? The only flaw in this would be that Cessnas have a lower best glide speed.
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  #214  
Old 07-28-2008, 12:27 PM
cnpeters cnpeters is offline
 
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Default

Quote:
Originally Posted by rv7boy View Post
Another comment...someone in a previous thread referred to slipping a Cessna with full flaps. My CFI demonstrated that once for my BFR and about a week later told me he made a mistake by doing it. He said Cessna doesn't recommend that maneuver for a 172. Their concern is that full flaps can block airflow over the horizontal stabilizer and elevator. Ever heard of that?
Also wondering if that should be a concern for our RV's.

Don
I trained in the 172/T-41 (high performance, stripped out USAF version), and yes, it was in the manual to not slip at full flaps. The large flap area and high wing position don't apply to the RV's, so there shouldn't be the same factors as for the 172 of turbulent air masking for the empennage. In fact, I'm not aware of any low wing aircraft like the Pipers that have any limitation (anyone know of exceptions??).
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  #215  
Old 07-28-2008, 12:37 PM
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Default DUH!

Thanks for the reply, Carl.
As I was coming back from lunch just now, I realized I did my usual thing of posting without much thinking. With RV's being low wing, there shouldn't be the same concern with full flap slips as with high wing Cessnas.
Don
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  #216  
Old 07-28-2008, 01:00 PM
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pierre smith pierre smith is offline
 
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Default Actually.......

Quote:
Originally Posted by Steve Brown View Post
Pierre,

Now we can discuss how it can be done safely for those of us that don't have your glider pilot skills.




Sounds like you were using flaps. Did you use them while turning?

.
My glider skills are probably very rusty since it's been 20 years or more since I've flown one.

Flaps...Negative...I did start putting them down right after I'd turned around to see the runway and that I was high.

I'd credit my 38 years of Ag experience more than my glider skills though....making a living at 250' or so during turnarounds over trees near a stall and sometimes a wing "burble".

You should do whatever your comfort level needs. You could go to 2000' over an open area and use that number as your "floor" for the runway. Climb from there to 800' higher, for example, then pull the power, do a 180 or 240 but over a road or big powerline so you can simulate a runway and get back to it on a simulated final. Keep lowering your "climb to" altitude and practice your turnarounds. You'll be surprised at what you learn in a very short time. Then as your skill and ability improve, go lower, to maybe 1000 or 1500 feet and repeat the maneuvers until you're comfortable. Work progressively lower but don't get ahead of yourself.

Regards,
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  #217  
Old 07-28-2008, 01:02 PM
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rv6ejguy rv6ejguy is offline
 
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Default

Quote:
Originally Posted by hevansrv7a View Post


Pierre, thanks very much for doing this - I agree with you 99%. That said..

At best glide, the sink rate in terms of f/m is actually higher. Best sink is at a lower speed but with a reduced glide ratio. Best sink is found at 76% of best glide. In the CAFE 6A that's about 80-81 mph vs. a best glide of 106 mph. The sink rate at best sink/endurance speed for the CAFE 6A is between 666 and 715 fpm depending on which of their observations are taken as primary (they have data which don't match up completely and admit to a 5% fuzziness due to air conditions during the test). Sink rate at best glide speed goes to 815 fpm, an increase of 100 fpm over best sink speed.

So those are some pretty aerodynamic car keys.

However, it is worth noting that for this test, CAFE was using the zero thrust method which could be expected to give a better glide or sink than a stopped prop. That's because a stopped prop must produce some drag - negative thrust - and CAFE's test had zero thrust.

BTW, In my opinion, even the short wing RV can out-glide most fixed gear Cessnas. Here's some logic to consider. If an airplane had no drag it would have an infinite glide ratio. If you add drag it sinks. If you add more drag it sinks faster. Now, which airplane has less drag at its best glide speed? That will be the one with the better glide ratio. It is not a secret that the RV's are much cleaner. How else could we go 100 knots on 3.5 gph? The only flaw in this would be that Cessnas have a lower best glide speed.
No way I'm anywhere close to the CAFE figures. These tests are at zero thrust which is a lot different drag than a windmilling prop and a dead engine. The sink rate is dramatically higher on my 6A below 70-75 knots power off. CAFE should shut the engine down to give us some useful real world data in this area.
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  #218  
Old 07-28-2008, 01:34 PM
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Mel Mel is offline
 
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Default

Quote:
Originally Posted by rv7boy View Post
He said Cessna doesn't recommend that maneuver for a 172. Their concern is that full flaps can block airflow over the horizontal stabilizer and elevator. Ever heard of that?
Don
Don,
I think this "rule" is a carryover from the C-170B. I tried it at altitude in my 170B and found that a very extreme slip with full flaps did indeed blank the horizontal and the nose pitched down violently. In all honesty, I have not been able to repeat this phenomena in a 172. I believe it is because the 172 doesn't have the rudder authority of the 170.
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  #219  
Old 07-28-2008, 03:58 PM
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rv7boy rv7boy is offline
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Default

Thanks, Mel
He demonstrated it in my 172 but did not ask me to do it for the BFR. I've never tried it since.
Don
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  #220  
Old 07-28-2008, 04:42 PM
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hevansrv7a hevansrv7a is offline
 
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Default You are right but I'm still right, too

Quote:
Originally Posted by rv6ejguy View Post
No way I'm anywhere close to the CAFE figures. These tests are at zero thrust which is a lot different drag than a windmilling prop and a dead engine. The sink rate is dramatically higher on my 6A below 70-75 knots power off. CAFE should shut the engine down to give us some useful real world data in this area.
The speed for best sink is just a hair above where the drag and sink increase suddenly and in a 6 or 7 that's around 75 kts. You can test this easily, working your way down 1 kt at a time. You can make it an easier test to perform by using partial power. The speed will be the same for both best glide and best sink; only the sink rates will vary with power.

I agree about CAFE's zero thrust being a better-than-best case. It's academically interesting but not something we can achieve with a dead engine. It's a better, more consistent way to compare airframes independent of the many variations in propulsion setups but does not predict engine out behavior.
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We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
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