What is L/D Max?

[skydriverdc6]

Hey all,

Im working on my CFI and was wondering if anyone had a nice 3rd grade level explanation for what L/D max is, and what affects it?

Thanks

[ananoman]

Go buy 'The Illustrated Guide to Aerodynamics' by Skip Smith and 'Aerodynamics for Naval Aviators' then get back to us.

[skydriverdc6]

Thanks, but if I wanted to go buy books, I wouldnt be asking on the forum. I'm looking for a method of explaining this to a student who may not have the background to understand an aerodynamics book

[Ian J]

This is a pretty good explanation:

http://selair.selkirk.bc.ca/aerodyna...nce/Page3.html

[flyguy]

Originally Posted by skydriverdc6

Hey all,

Im working on my CFI and was wondering if anyone had a nice 3rd grade level explanation for what L/D max is, and what affects it?

Thanks

We know that parasite drag goes up when airpseed increases, and induced drag goes down as airspeed increases due to lower AoA. If at any point you add induced drag and parasite drag together, you get total drag. At high airspeeds, there is high parasite drag and low induced drag. Add them together and total drag is high, due to the high parasite drag. At low airspeeds there is high induced drag and low parasite drag. Add them together and total drag again is high due to the high induced drag. There is an airspeed on every airplane, where there is a medium ammount of parasite drag and induced drag. It is the place where the two are equal, where the parasite drag curve and the induced drag curve intersect. The total drag at this airspeed is the lowest. This is called L/D max, and yeilds the highest ammount of lift (L) for the least ammount of drag (D). This is your best glide speed. The easiest way to explain this to a student is to draw the drag curves, and the total drag curve on top of that.

[flyguy]

Originally Posted by skydriverdc6

Thanks, but if I wanted to go buy books, I wouldnt be asking on the forum. I'm looking for a method of explaining this to a student who may not have the background to understand an aerodynamics book

That's what you are for. to give them that background. You have to have a basic understanding of aerodynamics if you wnat to be a pilot. It dosn't take a degree from MIT to understand the aeordynamics that relate to L/D max. If the student has not been taught the four forces, you'll have to give them that lesson. If you cover everything that you should, then they will have the background to understand aerodynamics. However, "Naval Aviators" is a little over the top. Most CFIs can't understand it. I haven't looked at the other references suggested, but you should have something in your personal library that you can use to teach L/D max.

[skydriverdc6]

Thanks to all for the helpful feedback. I've been a long time frieght dawg but am finding out I have trouble teaching this stuff. I was just looking for some perspective on how to relate certain topics to a level a student could understand versus just reading a definition out of a book. For example, describing gyroscopic precession and relating that to the bicycle wheel, etc. So being able to relate L/D Max to best glide speed was a helpful perspective, thanks for the tips.

[Flysher]

is that a caravan on floats in your avatar?! now THAT would be fun!!!

[skydriverdc6]

yeah, i fly a caravan, but unfortunately its not on floats, just plain ol' landing gear hanging out there catching ice and snow, lol