Expert: Paul Soderman - 2/27/2005

Question
Hi,
Sorry to bother you again. But i just cant seem to get the iteration formular right. Is it possible for you to further elaborate the method of iteration you mentioned earlier?

Thanks alot and thanks again..

>>Question -
Hi,
I have asked about the jettison of the engine the last time round, and now i faced another question...
I am suppose to design a non-powered glider to carry a 10kg load with a wing span of maximum 3 meters and make it glide the furthest distance. This glider is launch from the top of a ramp(10meters from level ground) and rolls down bypassing a curvature inclining at +12 degrees(Upwards as positive from horizon). The point where the glider leaves the ramp is 4meters from level ground. Using the Energy Equation, I calculated the velocity when it leaves the ramp.

But, I need to find the optimum Angle of Attack(AoA), Optimum Aerofoil Shape and Opimum Chord. As for what i did, i took the Coeff of Lift over Coeff of Drag Max, will give me both the optimum Aerofoil and AoA.
1)The real Problem is how do i get the optimum Area?
2)And when considering the CL over CD, should i include the induced drag in or just use the CDo?

By the way, i am a student studying in Singapore Polytechnic for a Aeronautical Diploma.

Really Thank you very much! Hope this wont take you too much time.


Paul Soderman Answers -
Ngoh

You must include the induced drag in your calculation since it can be quite large. Once you do, you will find that the aspect ratio must be as large as possible to keep the induced drag low. Since your wing span is fixed at 3 m, the chord must be as small as possible to get a high aspect ratio. But you need enough wing area to get lift to carry the payload and glider weight. So, you need to iterate on wing chord, area, weight, lift and drag until you have the optimum wing.

Paul<<

Answer
OK - you have airfoil shape, span, airspeed, and payload.  Pick a chord.  Compute area and weight.  Weight (aircraft + payload) equals lift in level flight.  Find the required angle of attack and lift coefficient for your airfoil to get the lift.  Compute drag using parasite drag and induced drag, which involves lift coefficient and aspect ratio.  You now have L/D.  Pick other chord lengths and repeat until you have optimized L/D.  You will find a chord that is not too small (requires high angle of attack and drag) nor too large (too much weight and too small an aspect ratio).

Take a look at a real glider and see the shape.

Good luck.

Paul