Building a glider that can get from point A to B fastest.

Mr VP

Hello everyone,

I need to build a glider that can get from point A to B fastest. Point A is about 35ft off the ground, and Point B is about x = 190ft away from point A.

I know the basic equation to consider here is L/D. I want this ratio to equal the (distance traveled/ascent lost).

Thus my Lift-to-Drag ratio needs to be about 190/35 = 5.429 (assuming zero wind conditions).

Is there a certain wing structure or certain fuselage structure to consider here?

Any tips?

Simon Bridge

Welcome to PF;
Is this a project you have been given as part of a course?
The object that gets from A to B fastest is probably a projectile.
But the project specifies "glider" so...

The time it takes to travel a particular horizontal distance depends on how hard you throw it and the drag. Lift-to-drag affects the rate of descent. So you also need to account for the initial height.

Not really - the whole thing is wide open. There are literally infinite possible configurations. That is why this makes such a good project for engineers.

There are computer models that will tell you needed air-speed and angle of attack for a particular design but the best approach is to experiment with different glider designs to gain a feel for how different parameters affect what's important to you.

AlephZero

An L/D of 5.5 shouldn't be a huge challenge. Full size sailplanes used for competitive flying have L/D more than 10 times better than that.

L/D is determined by the geometry of the plane, not the materials it is built from. For the fastest gliding flight (in a straight line at constant speed), think about whether the weight should be large or small.

StrayCatalyst

Even the old Rogallo wing hang gliders had a better glide ratio than that, and the modern ones are about twice that efficient. At point A, are you starting with airspeed of zero?
(As your glider drops out of the sky like a rock)

Five to one glide ratio is fairly easy to attain. Even the shuttles had about 3:1, and they weren't gliders at all.

Simon Bridge

@Mr VP : how are you getting on?

AerospaceEng

Hey I see you haven't posted in a while but hopefully if you do check back you'll find this information useful.

If you think about it a glider doesn't have an engine so its getting all of it's speed through a component of gravity thus the steeper angle you fly at the faster you'll fly! In a perfect run you'd use all the gravity possible to accelerate your glider and land just after flying 190ft.

The following equations apply to unpowered aircraft and can be easily derived:

Your glide angle can be calculated using Cd and Cl:
tan(γ) = Cd/Cl where γ is your glide angle.

Your Velocity can be found:
V = SQRT(2Wcos(γ)/ρSCl) where W is the weight and S is the surface area of the glider

Using this equation you can see what factors come into play when designing your glider.
A more detailed explanation and derivation of these formulas can be found in chapter 8 of "Airplane Aerodynamics and Performance" by Roskam and Lan which i think can be downloaded online.

Also if you don't mind, I think it's a neat project so if you wouldn't mind updating us with pictures and such that'd be sweet thanks.

Best of luck!

Matt

Velikovsky

It's a big ask for any conventional winged glider. Have you considered the better speed characteristics of a wingless lifting body?

http://history.nasa.gov/SP-4220/sp4220.htm

AerospaceEng

Interesting idea for sure but,
A glider without wings would just be a brick. The only reason a wingless lifting body works is that it's capable of traveling at the speed that allows it to generate enough lift to stay aloft. Without a propulsion system I highly doubt the glider could ever reach the speeds required.

Velikovsky

I don't think describing wingless lifting bodies as bricks is at all correct. Bricks, unlike "lifting bodies" don't generate lift, the clue is in the name! Not many unpowered supersonic vehicles can glide from space and land on a dime aka the Space Shuttle which follows a glide return trajectory and not a ballistic one.

AerospaceEng

My point was that if they wanted to try doing a lifting body glider it probably wouldn't be able to fly fast enough to generate the required lift and unlike the space shuttle which does have wings and travels relatively fast.

Matt

Simon Bridge

I recall some others in my Masters year experimenting with a kind of ground-effect - they had gliders that dropped at a steep angle but then skimmed the rest of the way just (1-2 mm's if that) above the ground. They were very fast - but needed a very smooth floor. Cannot tell if it could lead to something useful but it sounds like it suits the kinds of constraints implied by the description.

iirc: a brick can be made to generate lift - but it has to go very fast, and hold the correct angle of attack (things bricks tend not to do for long) ... supporting AerospaceEng's position really. (I suppose someone could crunch the numbers and prove me wrong... but the gripping hand is:) The project described in post #1 will be severely constrained in terms of initial speed and height... otherwise, why bother with gliding: just throw it hard?