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Apr 28, 2017, 04:23 AM
Design is everything.
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Aerodynamics of catapult gliders


I am trying to figure out how to increase the performance (range, flight time) of model catapult launched gliders.

This is my work from an earlier thread.

As far as I have it figured out, the flight profile is a follows:
  1. Launch
  2. Gradually slowing level flight (elevator trim is adjusted for a slight climb)
  3. Stall
  4. Fall to the earth

In order to increase the time of phase 2 of the flight, I have the following parameters to work with
  1. Weight
  2. Wing span
  3. Wing thickness
  4. Wing section (flat, or airfoil shape)
  5. Drag (streamlining)

Decreasing the weight has had a huge effect on performance for two reasons - half the weight means twice the acceleration so the model ends up travelling with greater velocity of the end of the launch. Also, the lower weight means less elevator trim has to be used to keep the nose up at lower speeds, and trim changes are less.

I am not sure how important the other parameters are. Need a little help to optimize the model. I have made good progress in improving durability and damage resistance.

Here is some of my earlier work for comparison. I guess I did not build my 52 prototypes fast enough, that explains the slow progress.

https://www.rcgroups.com/forums/show...models-of-jets

https://www.rcgroups.com/forums/show...full-fuselages

Lots of re-reading for me to do. This time, however I very close to having a finished model.
Last edited by Designer2010; Apr 28, 2017 at 04:36 AM. Reason: added words, emphasis
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Apr 28, 2017, 09:17 AM
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For this kind of a paper or sheet balsa aircraft, Ignoring profile drag losses, your objective function, t could be written as follows, by using energy conservation.

E = energy given by the the catapolt
g = gravitational accelaration
v = glide speed
L =lift
D=drag
m =mass

t= (1/2g (2E/m - v^2)*L) /v*D

So for fixed L/D, neglecting the profile losses, ( or easy to account by using a percentage) , possible way to increase "t" is to use higher energy per mass or less mass per given energy as you have already figured out.

Other way is to minimise glide speed.
As glide speed ~ sqrt(m/A) for given CL ; and m~A ; I would certainly plot Area vs time graph to find out ball park optimum and experiment within ~10% margin and few trim settings to have some fun.

I wouldn't bother with airfoil sections, thickness, drag. assuming that its typical balsa sheet plane.
Weight and span are somewhat taken into account at once when you take Area as shown above.

HTH

PS: For further info, please have a look at this attached document from Prof. Drela. Good stuff.
Apr 28, 2017, 06:34 PM
B for Bruce
BMatthews's Avatar
Quote:
As far as I have it figured out, the flight profile is a follows:
Launch
Gradually slowing level flight (elevator trim is adjusted for a slight climb)
Stall
Fall to the earth
Not quite....

1- Launch, model flies away in a spiral upward pattern that turns to the left for a left circling glide.
2- Transition, model reaches top of climb and slows down to below flying speed with the nose still pointed upwards but with some bank angle from the spiral of the climb. Model side slips and drops the nose. If this is done right and the trim is just right the transition slides down and the model glides away with little or no altitude loss. Any reasonable model will do this provided the trim is setup up for the proper climb pattern that finishes with the model having just the right attitude when it slows down and can't climb any longer.

3- glide, the model circles downward with a nice floating glide that is not fast and not stalling. If it flies faster than you think it should trim it for slower flying. If it stalls repeatedly on the way down trim it so it doesn't stall.

4- if things are done right it doesn't fall It just continues to glide in and touch down... .or it catches lift and flies away.

You said the model in stage 2 slows down and goes into a glide. It won't do it that way with any sort of reasonable free flight trim arrangement. Instead you need to work on a repeatable upward spiral that uses from 1/4 to 1 turn in the climb such that the arrives at the top of the climb in a good position to do a smooth transition to level gliding.

Or if we are now talking about RC gliders then you need to be ready to put in a stab of down elevator to achieve a non stalling transition at the proper time. I am assuming here that we are talking about free flight since most of your previous postings have been towards free flight designs.
May 02, 2017, 10:11 AM
Design is everything.
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Buwa - so it looks like I have to decrease launch mass or increase the strength of the catapult. Still working on reducing luanch mass and preserving some crash resistance - main point of concern is the nose and the part behind the nose - I am making progress in strenghtening these areas, its a difficult task. I may have to settle for a replaceable nose cone.

Your comment on glide speed is interesting - this means a larger model with bigger wing area, using the same materials I should get a lower mass per unit volume which is density by the way. Tests with a flying wing earlier on showed a slower glide speed could be achieved. So on to 1/32 scale models now, with 30 cm span wing.

Bmatthews - reading through your ealier posts so many years agoI realized you were absolutely correct about many things, still working on the crash resistance using the composite fuselage you also suggested, probably use a wooden spine.

The flight profiles for these models are not really gliding profiles - the model will be used for level flight a slight pull up at the end, level flight with a sharp pull up and vertical flight into a stall, and 180 turning flight achieved by launching be model with a varying degree of bank. I do not have the space to try long or high flights at the moment maybe total area of a basketball court. This is basically a little jet fighter that will do some aerobatic manouveres.

The main purpose of this exercise is to come up with a model to teach basic aerodynamics, lift, centre of gravity, model construction and finishing, and some crash resistance. The other models out there are either more costly, made out of foam, or stick models or gliding paper planes. Nothing in this niche as far as I am aware. At the same time it will have the appearance of a minature jet fighter.

I will be getting into real remote controlled aircraft soon, thats another post.
Last edited by Designer2010; May 02, 2017 at 10:14 AM. Reason: corrections
May 02, 2017, 10:16 AM
Design is everything.
Thread OP
Basically this toy but made out of cardboard and with a full fuselage.

https://www.amazon.co.uk/Rubber-Band.../dp/B00ESEHK6Q

This one is closest to what I want to achieve but it is made out of balsa. Time to make a card model then.

http://www.parmodels.com/Plans/images/MIG-15_top.jpg
Last edited by Designer2010; May 02, 2017 at 10:31 AM.
May 02, 2017, 12:38 PM
B for Bruce
BMatthews's Avatar
That sort of flight pattern still does not alter the fact that you'll only get it to fly that way with care in the trimming stages. You will still need the "marginally pitch stable" CG where the balance point is only a whiff ahead of the over all design's neutral point. But in this case a very slightly bigger whiff than for a competition glider. You'll still want to have the model set for some amount of turn and trim it so it starts out banked away from that turn and through a climbing S turn rolls from the launch bank across and through wings level and banks the other way and finally slows down enough to transition into the glide. You'll just do all this at a lower angle and gain less altitude. But if done well it'll get your little jet models up to a good height and get you a good 15 to 20 second glide that looks pretty good without a whole lot of trouble.

The little "boy's jet" shown below was intended to fly in much the sort of manner that you are considering. The fuselage is a little thicker from balsa for durability. But if made from laminated card stock perhaps with a popsicle stick in the middle between layers it might work out quite well too. I only flew it a little but it was well on the way to being trimmed out to give flights that were a touch better than just looping around and spiking the flyer or their dad in the back of the head.... But it did take some playing to get that sort of trim. It's not something you can "build in" easily. Although if you made and trimmed out a few "straight" versions you might well find that by deliberately building in wing warps or different incidence from side to side and some built in fin offset that you could get a model that is nearly "trimmed" right from the beginning. That is the reason why in my writeups on free flight catapult and hand launch gliders I mentioned that the wing skewing and stabilizer tilt was added during the construction.

Keep in mind that young folks are quite happy to have something that flies at all. A 10 or 15 second flight provides delight to young minds far out of proportion than it would to an adult with a lot of time in the hobby.
May 03, 2017, 12:48 AM
Design is everything.
Thread OP
Quote:
The little "boy's jet" shown below
not sure if you are referring to my picture - is this a model you built?
May 03, 2017, 02:21 AM
Design is everything.
Thread OP
This one is an example of what could have been built:

http://www.anticsonline.co.uk/720_1_1307982.html

Cardboard wings, if they had only molded the fuselage to represent a scale model of an F-106 or a Mirage 3 or even and F-16 it would be a very nice flying scale model.

Even this model is out of stock.

Then there is this one : not sure how it would glide, maybe like the real thing

http://www.guillow.com/spaceshuttle.aspx
Last edited by Designer2010; May 03, 2017 at 02:27 AM. Reason: added shuttle
May 03, 2017, 05:51 AM
Design is everything.
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Bruce

Quote:
... You'll still want to have the model set for some amount of turn and trim it so it starts out banked away from that turn and through a climbing S turn rolls from the launch bank across and through wings level and banks the other way and finally slows down enough to transition into the glide. You'll just do all this at a lower angle and gain less altitude. But if done well it'll get your little jet models up to a good height and get you a good 15 to 20 second glide that looks pretty good without a whole lot of trouble...
Something to try given enough space. Totally new to me, but won't the roll trim end up making the model spiral finally? Or just turn?

Quote:
The little "boy's jet" shown below was intended to fly in much the sort of manner that you are considering.
Any pictures?

Quote:
... my writeups on free flight catapult and hand launch gliders I mentioned that the wing skewing and stabilizer tilt was added during the construction.
Trimming is a complex business I suppose.

Quote:
Keep in mind that young folks are quite happy to have something that flies at all. A 10 or 15 second flight provides delight to young minds far out of proportion than it would to an adult with a lot of time in the hobby.
Well yes, the whole problem has been these foam gliders do not last long. If I build it myself of card, I can ensure a large supply of gliders that cost nothing and do not scatter foam to the winds. Funny thing is, building and flying these little gliders may just compare to the complicated expense of RC flight.
May 03, 2017, 11:54 AM
B for Bruce
BMatthews's Avatar
Quote:
Something to try given enough space. Totally new to me, but won't the roll trim end up making the model spiral finally? Or just turn?
A soccer pitch is more than enough room for small rubber band catapult gliders if it's not super windy. And you could get by with less if it's not a strong wind day.

The dihedral and rudder angle does make the model roll fairly strongly at faster speeds. But once it slows down it just makes it turn. If you start trying to get by with less dihedral then it becomes far more fussy to make the model turn much at all without wanting to drop into a spiral dive. But if you make up a modern style jet fighter the dual fins located strongly forward act a bit like high keels or fences and can substitute for dihedral to some extent. It's a tricky way to do it. But I had fairly good results from a small balsa F18 profile glider similar to your foam toy but done in balsa. It was far from a contest model sort of performance or flight pattern. But it did go up and do a sort of transition and fly for around 20 seconds from a light duty rubber band launch.

Forgot the picture in that post. It's attached to this one. And yes, trimming for a proper launch pattern so it puts the model in a good attitude at the end of the climb to do a smooth transition is a trick business. It takes time, fussing and consistency of the launch to get it right. Young folks don't do very well at this at all. But they seem to like it when the models do some aerobatics and don't crash.

The key though is to write about it less and build and fly more. These simple smaller gliders are cheap and fast to make. And when we are relying on model trim and launch attitude to allow a transition from climb to glide the little things become big things. You've flown enough of these things now to know that the change needed to turn a good flight into a crash or a crash into a good flight cannot even be seen by the eye after it is bent into the surface.

Anyway, here's the picture for you.
May 04, 2017, 12:23 AM
Design is everything.
Thread OP
Bruce

That's a nice little glider you have there. How much does it weigh? The aerodyamic design parameters have been discussed.

The construction of the model has been tested, and weight limits for performance are established at less than 18 g or so.

The final piece of the puzzle is the crash resistance part of the nose section. I bet your model just bounces off concrete, but what sort of nose cone could I use for the nose cone and front fuselage area (for a model of a Mirage III for example) this is the area I have been working on for weeks.

One promising design I had extended the wing area to the forward fuselage, with a little reinforcing this would really work I think.



Thanks for the input : my next post will be some pictures of the completed model.

Here's a site much in need of a re-vamp. https://sites.google.com/site/catapultjetgliders/
May 04, 2017, 09:11 PM
B for Bruce
BMatthews's Avatar
Thanks, It's roughly "square" with an 8 inch span and 8 inch length. No big attempt was made to keep it light and as a result the weight is up around 8 to 10 grams. I'm not in a position to weigh it just now for something more accurate.

Card stock can provide a fairly damage resistant model too if it is done where the nose is made up of multiple laminations all glued together to produce a more "wood like" density. On top of that a simple mold that uses common bathtub silicone caulking to make nose cones can aid in giving the laminated card stock nose area a rubberized spring like tip that takes the initial hit and bounces instead of crushing.

You should try to engineer these models so they come in at under 18 grams. In particular for the 1:48 size options. That is just far too heavy for that size to fly at all well. 18grams would be more tolerable in a 1:32 size though. Not great but tolerable. I know that this puts serious challenges in your way. But to fly well that's just the way it is I'm afraid. It also explains why foam is used instead of card stock on cheap and simple gliders of this size. Or balsa over foam for an even better balance of flyability and durability.
May 05, 2017, 11:47 PM
Design is everything.
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Modelling Engineering


Well this certainly is getting very interesting. Bruce you are right about the layered card. I actually have created a nose cone out of card, which seems to be holding quite well.

Mirage 2000 camouflaged model

I have included a picture of my prototype model here - the one I have tested the most. It has done over 50 flights, and had its nose cone replaced twice. The pink strip on the top of the fuselage indicates the centre of gravity. The wing is made out of corrugated cardboard covered with paper. The fuselage is all cardboard, which I later found is an unneccesary weight as the rear fuselage can be made out of paper on a delta such as this without buckling, though some strenghtening in the form of a roll of card must be used to secure the vertical tail.

No matter what I do I cannot get it to fly more than 10 metres, or climb more than a few centimetres in its flight.

Weight? That 26 on the side of the fuselage denotes the weight - 26g! I have the durability but not the light weight.
May 05, 2017, 11:52 PM
Design is everything.
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Back to basics


So its back to basics. I start with a simple card wing cut out of a cereal box or equivalent. It needs some reinforcement to avoid twisting, in the form of another card strip or wing fences.

Wing: 6g
Fuselge: 6g

Total 12g

Finished weight should be 15g
Last edited by Designer2010; May 06, 2017 at 04:15 AM. Reason: Added info
May 07, 2017, 04:03 AM
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cog flying wing


hey when using flying wing calculator do you also include the elavons as part of the wing calculation


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