quick PofF question

[spitfire747]

hello all, doing my FI course at goodwood and have been asked the following question

if the speed for the best lift to drag ratio in the cruise is say 85kts for the tommahawk, then why is the speed for the best lift to drag ratio in a glide 70kts and not 85kts...?

any ideas
Cheers
Spitty

[Snigs]

Engine slipstream provides some of the lift.

[djpil]

The standard derivations of L/D in the glide and speed for best range, best climb etc all assume:
- simple drag polar Cd = Cdo + K. Cl^2
- constant propeller efficiency and constant specific fuel consumption
and ....

The drag polar has a break in it - above a certain value of Cl, drag increases by more than K. Cl^2. There are slipstream effects on drag and .....

Main reasons:
- the characteristics of a fixed pitch propeller are very much different than assumed above.
- SFC is not constant

Hope that answers the question that you intended:
"why is speed for maximum range different from speed for best glide angle?"

[Tinstaafl]

More mundanely, perhaps the performance table in question is really a Long Range Cruise Table, and not Maximum Range at all.

LRC typically trades off a very small reduction in range for a significant gain in speed compared to maximum range speed(s).

[dreamingA380]

after a quick search on the net...

http://142.26.194.131/aerodynamics1/...nce/Page3.html

bitch of a question, throw this it your smart ass FI instructor.

All the best.

[whatunion]

this is actually a very simple question if you do not try to over complicate as some of the book instructors seem to do.

the best lift drag ratio will always occur at the bottom of the total drag curve. beware of explanations and diagrams that just show an aerofoil. bolt on an aircraft and an engine and things change.

to fly the a/c straight and level is going to require thrust and this in itself will cause a different amount of total drag than that in the glide and will therefore change the total drag curve.

thinking about a glider always helps, the speed is the same for the glide and the cruise!!!!!!

[BEagle]

Are we talking about gliding for range - or minimum sink?

I feel that the question posed by your FIC instructor wasn't quite the one you've posted here.

For max range in a prop driven aircraft, you would indeed fly at just over min drag speed (to avoid going below min drag speed with the consequent power increase needed to get back on the 'right' side of the drag curve). However, for minimum sink, the glide speed is roughly the same as minimum power speed - and that is about 3/4 min drag speed. As whatunion so rightly says - add a few effects for a real aeroplane and the 70/85 relationship seems fair.

[whatunion]

beagle glad u mentioned that, range or minimum sink i like it, quite a few think its the same.

i think i am right in saying range is not weight variable on the glide but min sink is.