Best Glide

[FlyingMonkey]

Just for reference, I was looking through the CTLS manual and found some interesting differences. One jumped out at me:

 

In the CTSW manual, best glide is defined as 63 knots at 15° flaps.

 

In the CTLS manual, best glide is defined as 78 knot at 0° flaps.

 

This is a big difference! Are these airplanes really different enough to warrant such a big gap in glide speeds? Or was the 78kt number gained by experience of the FD team, and is that really a better speed and flap setting for the CTSW as well. In absence of definitive information I will definitely follow the book, but it sure is one of those "things that make you go hmmm"...

[Ed Cesnalis]

Think best L/D, there are 2 components.

 

Find best glide for the CTSW at 0° flaps, then compare

 

or

 

Find best glide for the CTLS at 15° flaps, then compare

 

ask yourself this "is best L/D for the SW found at 15° while best L/D for the LS is found at 0° and if not why the different configurations?"

[GravityKnight]

Best Glide (speed and flaps) is given under Emergency Procedures in the revised POH according to weight.

 

 

 

What about the CTSW??

[Ed Cesnalis]

What about the CTSW??

 

 

http://documents.fli....com/SW-POH.pdf

[mocfly]

If I remember there was quite a long thread about glide speed distance some time ago, with a few actually experimenting.

[coppercity]

Here is the thread

 

http://ctflier.com/index.php?/topic/589-glide-ratio/page__hl__glide

 

 

[FastEddieB]

This is the first time I recall a plane where flaps increase glide distance.

[FlyingMonkey]

I wonder if the situation here is that the 63/15 glide ends up being about the same distance as the 78/0 configuration, so it really becomes a matter of choice.

 

I'm with you Eddie...not sure how increasing drag can help glide...

[Ed Cesnalis]

I've heard 15 degrees being described as a big increase in lift while only being a modest increase in drag where 30 degrees becomes a big increase in drag and just a modest increase in lift.

 

Best glide is a ratio, there are 2 components lift and drag.

[C ICEY]

my CT2K allows me to stop at any flap setting. I trim to 65 and then play with the flaps, using the old garmin 196 to guide me to best glide ratio, while monitoring glideslope to the nearest known outfield landing fields. \ for me, the best glide is around that 65kts and somewhere closer to 15 flaps than 0 flaps. In anything other than still air, the results are variable!!! I have seen but don't count on 15:1. A landing field target at 12:1 would appear to be a challenging straight glide, especially for the last few hundred feet. I plan target fields for arriving at +1000 feet! ICEY thermals like a plank, but a bit of a slip into the core of the circle can sometimes help it tighten it up.

[Jim Meade]

Last year I took the CTSW to 10,000 feet and shut of the engine. As I recall, I set flaps to 0° and trimmed for 69 kias. The results of a glide to 4,500 feet were that the glide ration was about 9:1. Determined by time of altitude change.

[FastEddieB]

From Aerodynamics for Naval Aviators (free as pdf here http://www.faa.gov/r...a/00-80T-80.pdf):

 

 

page 372: The effect of configuration has been noted previously in that the addition of parasite drag by flaps, landing gear, speed brakes, external stores, etc. will reduce the maximum lift-drag ratio and cause a reduction of glide ratio. In the case where glide distance is of great importance, the airplane must be maintained in the clean configuration and flown at LD(max).

[Ed Cesnalis]

From Aerodynamics for Naval Aviators (free as pdf here http://www.faa.gov/r...a/00-80T-80.pdf):

 

 

page 372: The effect of configuration has been noted previously in that the addition of parasite drag by flaps, landing gear, speed brakes, external stores, etc. will reduce the maximum lift-drag ratio and cause a reduction of glide ratio. In the case where glide distance is of great importance, the airplane must be maintained in the clean configuration and flown at LD(max).

 

Your quote is correct but it doesn't tell the whole story. Compare gear to flaps, extending gear will add dramatically to the air frame's parasitic drag but will do virtually nothing to add to lift. The result will be as stated above.

 

Now look to flaps, the quote above would suggest that best L/D would be with reflex flaps because it is the lowest drag configuration but neither the LS or the SW POH recommends reflex flaps for best glide, the recommendations are either zero or fifteen.

 

The rest of the story is this '... the addition of parasite drag [lift] by flaps, landing gear, speed brakes, external stores, etc. will reduce [increase] the maximum lift-drag ratio and cause a reduction [increase] of glide ratio.' When you increase flaps you increase drag and degrade the glide but at the same time you increase lift and enhance the glide. You have to dived the resulting total lift by the resulting total drag to get the new L/D and see what direction it moves.

 

Each setting results in a different LD ratio.

• Reflex to Zero - result is to slow down
  
• Zero to Fifteen - result is to balloon 1st, then slow down
  
• Fifteen to Thirty - result is a big pitch change and steep descent.
  

When you consider the increase in lift as well as drag it is easy to see that approach flaps can and do enhance the glide where landing flaps degrade it. Many designs will balloon abeam the numbers when approach flaps are deployed.

[Jim Meade]

Does anyone know what FD means by "best glide"?

Is it L/D max, the farthest distance one can glide?

Or is it minimum sink, the longest time one can stay aloft?

Minimum sink is slower than L/D max.

One would presume it to be L/D max as that would give the greatest area in which to select a landing spot. But, other than guessing, does anyone know for sure?

[Ed Cesnalis]

Best Glide Speed

Definition: The speed of the aircraft when it has the best glide ratio. This results in the least amount of fall per distance traveled.

 

http://open-site.org/Recreation/Aviation/Terminology/

[Jim Meade]

OK, it's best L/D.

[Tom Baker]

This is the first time I recall a plane where flaps increase glide distance.

 

Eddie, This isn't your everyday normal airplane, it is something better. That is why some of us do things different than in every other plane we have flown.

[FastEddieB]

Eddie, This isn't your everyday normal airplane, it is something better. That is why some of us do things different than in every other plane we have flown.

 

It must be the fairy dust from the Black Forest that FD uses!

[Ed Cesnalis]

DA20-A1 Katana has a best glide with flaps; it calls for take off flaps for an engine failure in flight. Interestingly it calls for no flaps for engine failure on departure.

 

[WmInce]

. . . "This isn't your everyday normal airplane, it is something better." . . .

 

We know . . . we know . . . it's "fuel injected and carbon-fiber."

Got it!

[Ed Cesnalis]

it is generally assumed that as flaps are extended past 15 degrees, they begin generating more drag than lift. As the wing's lift increases, so does the drag, and the nose must go down to maintain speed--which increases the glidepath angle. The increase in lift from 15 degrees to 40 degrees of flap extension is small, but the drag skyrockets.

 

http://flighttraining.aopa.org/magazine/2008/February/200802_Features_Controlling_Your_Approach_Path.html

[josjonkers]

my CT2K allows me to stop at any flap setting. I trim to 65 and then play with the flaps, using the old garmin 196 to guide me to best glide ratio, while monitoring glideslope to the nearest known outfield landing fields. \ for me, the best glide is around that 65kts and somewhere closer to 15 flaps than 0 flaps. In anything other than still air, the results are variable!!! I have seen but don't count on 15:1. A landing field target at 12:1 would appear to be a challenging straight glide, especially for the last few hundred feet. I plan target fields for arriving at +1000 feet! ICEY thermals like a plank, but a bit of a slip into the core of the circle can sometimes help it tighten it up.

I have done this experiment a number of times and every time I did it , it was quite conclusive. Anyone can try this experiment.

 

Here is what I did:

Fly a long final and be quite high and be prepared to eventually overshoot on a little wind day.

First part of the final at 0 flaps and you see your aim point on the rwy. Next dial in +15 flaps and after the initial bounce upwards due to increased lift and then wait for the new approach to establish itself you will find yourself very quickly below your original glide path at 0 degrees.

Once you try this experiment yourself you will have no doubt about best glidespeed and configuration. It is 0 degrees at around 70kts.

[Ed Cesnalis]

Josh and Eddie,

 

When you select 15° your glide will steepen, your aiming point will be lower unless you pitch for a much slower speed. If you do slow down 17kts from 80 to 63, and if the CTSW POH is correct you should see a better L/D.

[Ed Cesnalis]

What about wind?

 

However, like Vbr, wind direction and speed have to be taken into account before you can choose the Vbg speed when in a forced glide; for more information on the power-off glide speeds read the 'Know the best glide and minimum descent airspeeds' and 'Know the practical glide ratio and terrain footprint' sections in the 'Coping with emergencies guide'. In lower wind conditions, Vbg is increased in a headwind by around one quarter of the windspeed, but is decreased in a tailwind by a similar amount. In higher wind conditions, say above 25 knots, the speed changes required would be around one half of the windspeed.

 

http://flysafe.raa.a...le2.html#vspeed

[josjonkers]

Josh and Eddie,

 

When you select 15° your glide will steepen, your aiming point will be lower unless you pitch for a much slower speed. If you do slow down 17kts from 80 to 63, and if the CTSW POH is correct you should see a better L/D.

 

I understand what you are saying, however you can never regain your original aiming point you could achieve at 0 flaps at any speed you fly when you select +15. You will quickly drop below your glidepath and your aiming point will now be well before the original one. I have tested this a number of times with similar results.

It is an easy worthwhile exercise by going high on final at 0 flaps with best glide speed of around 65 to 70 kts and mark your aiming point. Next select +15 flaps and try any speed to better that, and I will bet you that you can not..