# Designing a far flying catapult launched model glider

1. Jul 20, 2014

### aeroseek

Many of you are familiar with the catapult launched profile fuselage gliders that are sold in hobby stores. Since these tend to rather fragile, I am designing gliders with better durability out of card, and also with a full fuselage. These will have a 20 cm wingspan typically.

Given that low drag and a thin wing are required, how do I maximize the range and / or flight duration of these gliders?

Specifically, what is the science behind the way these operate : they fly using the kinetic energy imparted in the launch, this can be made high as possible, however once launched they begin to slow down due to air resistance.

A heavier glider will have greater inertia and slow down less. A heavier glider will also cause higher induced drag towards the later potions of flight where the nose will be pitching up slowly to maintain height.

What are the equations I should look at to maximize distance flown or duration ?

2. Jul 20, 2014

### Simon Bridge

Use a really strong catapult ;)

Same as the science behind any flight.
The forward motion produced lift depending on the shape of the wing and the angle of attack.
Gravity makes weight, and the passage of air makes drag ... both these things will eventually overpower unpowered flight.

Assuming the nose pitches up more ... bottom line: a heavier glider falls faster.

Complicated.

Considering your materials you should experiment.
You have thrown gliders before so you know the basics already.

3. Jul 20, 2014

### rcgldr

Maximum distance from a bungee / catapult type launch required being able to convert speed into altitude, so more weight would help here. Maximum duration is more complicated, since once at maximum height, lighter weight means lower sink rate, which translates into longer duration.

Instead of a catapult launch, there's also the option of a hi-start (launch like a kite):

4. Jul 20, 2014

### aeroseek

Thanks for your help, however I let me clarify: right now I am thinking of low level flight - high speed distance flights of about 50m or so, loops, turns, glides, etc.

What about the kinetic energy possessed by the glider at launch?

5. Jul 20, 2014

### aeroseek

Hi start is one thing I will be looking at for my RC glider project. The glider moves upward under the net combination of forces due to tow cable and lift.

6. Jul 20, 2014

### rcgldr

Is 50m the distance or max altitude of the expected flight? The problem with this idea is that a safe distance would be limited by your vision. You wouldn't expect to fly the model an 1/8th of a mile or more away and be able to land it. 50 meter distance wouldn't be difficult to achieve (a hand held sling shot with a marble can accomplish this).

Most of the model gliders I've seen catapult (or the more common term bungee) launched are foamie (EPP or Z foam) slope type models at a flat field side (no need to do this at a slope with an updraft). The flight times aren't that long compared to thermal duration type models which are usually launched with a hi-start or winch.

On a side node, the record for pumpkin chunking via a high pressure pneumatic cannon is over a mile. In this case the pumpkin crashes into an area void of people so no one is at risk.

http://en.wikipedia.org/wiki/Pumpkin_chunking

For high speed flight, the model needs to be radio controlled so it lands instead of crashing (especially if could crash into someone or something). The limitation on weight would be depend on how much energy the catapult can generate. There's still the risk that something could go wrong with any model that can fly at high speeds.

In the video I posted above, the model weighs 4 1/4 lbs, and the max tension in the 7/16" outer diameter, 2/16" inner diameter tubing I use is about 27 lbs (it drops off unless there's sufficient head wind to keep the tubing stretched). I'm using 60 feet of tubing and 210 feet of monofilament line, pulling the line back 210 feet (pulling the tubing from it's initial length of 60 feet to 270 feet, so I need 480 feet for a max pull with that hi-start).

There used to be even biggger "hose monster" "bungee" tubing, but I haven't found any links. As an option, you could double, triple, or ... , up the tubing I used to get even more tension as long as you have the strenth to pull it back, but you'd need two people, one to pull, the other to control the model with the transmitter (to be safe).

For a high speed model, something between a normal model and a dynamic soaring model such as the Kinetic, which has a 100 inch wing span and all up weight over 12 pounds would work. The last I read, the dynamic soaring record was 498 mph, so speed from a catapult wouldn't be an issue. Again, high speed presents a potential danger, since if something goes wrong, it goes wrong quickly and with a lot of energy if there's a crash.

Last edited: Jul 21, 2014
7. Jul 21, 2014

### CWatters

Look at things from an energy perspective. The launch provides the initial energy. Anything you can do to preserve that energy is going to extend your flight. Drag represents a loss of energy so design your plane to have low drag.

One problem with catapult gliders is the wide speed range they need to operate over - it can be difficult trimming them so they glide well at all airspeeds.

8. Jul 21, 2014

### rcgldr

Assuming it's a radio control model, it's common to use a switch to change the center trims on the control surfaces, sometimes 3 modes, one for hi-start and/or winch launch, one for normal flight, one for fast flight. For catapult launch, the fast flight mode would be used (neutral trim so the model tends to flight in a straight line regardless of speed). Slope models are also sometimes trimmed to be neutral, since efficient gliding isn't required with an updraft, the speeds are typically higher, and aerobatic maneuvers are easier if the model isn't speed sensitive (no pitch stability which would tend to nose the model up at higher speeds).

F3J and discus launch contest gliders are launched with a lot of speed, so it's not uncommon to have a wide speed range.

9. Jul 21, 2014

### enorbet

Gosh gliders are beautiful things :) This thread has me wondering about design tradeoffs. It seems to me that the common form of catapults used, bungies, are quite violent in acceleration requiring considerable strength (and therefore weight) to resist coming aprt upon launch.

I'm wondering if it might be more workable ie: produce greater final launch velocity while keeping weight at a minimum, to design a catapult similar to those use aboard aircraft carrier vessels. While those employ steam, any hot gas can be used (even commercially available model rocket motors) and can be throttled for a smooth acceleration. Is anyone doing anything like this?

10. Jul 22, 2014

### aeroseek

Many interesting replies: I will respond to them in due course, however I have come across a brick wall in my analysis by simplifying the flight of a glider to that of a projectile:

Which will fly further, a heavier arrow or a lighter one?

The lighter one will be accelerated to a higher speed by the bow, but will face increased air resistance.

As C. Watters says

"Look at things from an energy perspective. The launch provides the initial energy. Anything you can do to preserve that energy is going to extend your flight. Drag represents a loss of energy so design your plane to have low drag. "

While I agree with him, I cannot figure out if a heavier arrow will fly further.

Since K.E. increases linearly with weight and exponentially with speed, there has to be an optimum weight for an arrow launched by a bow of a fixed maximum extension...

Looks like there is some discussion here:

http://archeryreport.com/2011/01/heavy-vs-light-arrows-speed-power/

Last edited: Jul 22, 2014
11. Jul 22, 2014

### enorbet

Is this accurate and actual? If I have 2 gliders with rigid shells, and one is filled with some dense foam and the other is empty but for air, since both have identical geometry, how does the heavier, filled one have less air resistance?

I can see how the lighter one might be buffeted more easily, subjected to air currents more readily because of the difference in momentum and inertia, assuming both were launched at the same velocity but that implies also a more powerful launcher to accelerate greater weight to an equal speed.

It's easier to think of each profile in a wind tunnel and with the same cross-section, size, shape, etc I can see no difference in air resistance. Am I missing something?

Last edited: Jul 22, 2014
12. Jul 23, 2014

### CWatters

I used to fly F5B models and this involved a rapid climb then gliding around a course to complete as many laps as possible in a fixed time. We found it was best to try and fly at a more or less constant optimum speed. If you flew some laps faster and some slower you got worse overall performance. The thinking was that drag is proportional to velocity squared so you lost more energy while going slightly "faster then average" than you saved when going slightly "slower than average". Sorry if that's not explained at all well.

13. Jul 23, 2014

### aeroseek

"enobert" You are right, I did not mean increased air resistance, but an increased deceleration due to air resistance.

I think the solution is to build the required strength for flight loads and impact loads, keeping the structure as light as possible, and then test the effect of added weight.

Distributing the weight throughout the fuselage will help pitch stability - like an arrow.

I understand your post, but for a moment I thought you were referring to the Northrup F-5B!

14. Jul 23, 2014

### Simon Bridge

http://hyperphysics.phy-astr.gsu.edu/hbase/airfri.html
... both are approximate.

Note: a light glider may slow faster, but it requires less lift (and thus: speed) to stay aloft.
It'll also hit the linear-drag speeds sooner. I'm afraid you have to experiment.
Start from designs others have used in the past.

I've seen gossamer designs (mostly powered gliders) which stay aloft for 10s of minutes in still air - but they are so delicate that they cannot be flown out-doors.

Mythbusters ep66 (2006) built gliders out of concrete.
Noteable was the aero-engineer they had on the show could basically work out the launch speed and pitch to get pretty much any geometry to glide - using a computer program.

I've seen a paper-glider competition won by someone who exploited some sort of ground-effect: his glider went right to the floor but stayed there, just above the ground. That also involved a tricky launch angle though. There's a bunch of stuff online about it.

I guess a serious designer should use a CAD:

But if you just want approximate solutions - to get an idea of what to try - there are aerodynamics primers online.
http://wright.nasa.gov/airplane/lifteq.html

Aside:
... all unpowered flying things start out with some store of energy providing the initial KE in one go. Once the energy supply is removed, i.e. the tow cable is detached, the craft is in unpowered flight. During flight this energy gets used up.
http://en.wikipedia.org/wiki/Unpowered_flight

Last edited: Jul 23, 2014
15. Aug 26, 2014

### aeroseek

Actual testing has shown that a light glider, delta shaped and weighing 20 g, with a 15 cm span will fly quite well.
Drag effects of a cylindrical fuselage with a tapered front end do not seem to be very significant - I am in the process of improving the durability of the model.

Last edited: Aug 26, 2014
16. Aug 27, 2014

### CWatters

Myself and the kids have some flying wing type gliders that fold up for launch called Zing Wings. They are launched vertically using a catapult. At the top of the climb they slow down and drag reduces to the point where an elastic band can unfold the wings and they glide down. Can be tricky to trim them but they go pretty well. There are several variations on this theme out there.