Boost Glider - how big is too big??

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James D

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I've just built a Quest Flat Cat Boost Glider and while I was completing it I was already thinking - "How can I make a bigger and better one?!" :) (I have a basic knowledge of how they work - that the CG must be at about 50% of the wing chord when gliding and the CG must be at least one body diametre infront of the wing when boosting etc - so I aint that much of a rookie!). So, looking at what balsa I had available, I knocked up another 'bigger and better' one kinda based on the Flat Cat, the old principle - "If it looks right it is right", the odd "Guestermation" and a few ideas of my own.

It is longer: 600mm long, with a 700mm wingspan. I used 1/8th balsa for the wings with 15 degrees of dihedral on the wingtips - strentherned with 1/16th ply. 2/32nd was used for the stabiliser and rudder assemblage 1/2 inch by 1/4inch hard balsa strip used for the fuselarge (which was shaped, i.e I tapered it so it thinned out towards the tail). The stabiliser and fin were made bigger too - the stabiliser (300mm wide) was also mounted about 1/4 the way up the fin as so not to be in the 'dirty' or turbulent air from the wing. All of this was epoxed together bar the wings which are attached by nuts and bolts. I copied the same method as the Flat Cat to attach the booster pod and space in the nose to mount and additional nose weight. It probably weighs in at 50 - 60 grams without the booster pod. I also built it with the booster pod slung 'underneath' the glider - so it has a conventional layout - stabiliser and fin on 'top' rather that 'below' the fuselarge (as on the Flat Cat).

My plan was too use the booster pod of the Flat Cat and stick a C5-3 motor in it. But....when I looked back over some info I had on a boost gliders in a book on Space Modelling (Published by Traplet), it recommeded a wingspan of around 400mm (!) for boost gliders and a area of 300 to 400 cm2. Now im no genius but at 700mm that gives far more wingspan and gives far more area than suggested! This led me to rethink my plans a little! Is this beast of a glider gonna boost up to a decent height or crash about 2ft from the launch pad coz its too heavy or creates too much drag? I have thought about building a bigger booster pod, with a D12-3 in. But will the higher impulse just turn the glider to matchwood upon launch? Oh, my finally though was am I ever gonna see it again if it does launch - will it just glide off into the horizon never to be seen again? Will I need to stick some kind of de-themalisers on it?

Any advice and or suggestions would be warmly welcome,

Thanks,

James
 
Oh, what a great can of worms you have opened! I share many of your same theories and thus admit that I cannot add much that would help you. I just had to comment though because I too have been wanting to hear some input on those that have gone before me. From what I have heard, everyone seems to agree that anything with any kind of power greater than Estes would shred the glider. Bah, humbug. I disagree. People once told me that I would never successfully cluster 2 AT E30s and one AT G80. Well, it worked by golly...AT Copperheads and all.

So, whatever you do decide on, I support it. If it works, I'll do it too! Just remember to take pictures!
 
Okay, I've never done this, and am kinda extrapolating on stuff I've read, so take it for what it's worth. When you read the reviews on EMRR re: the Edmunds Aerospace Deltee Thunder, which has a pretty huge glider on it, by the way, several of the opinions mentioned on the review stated that they built the glider and it worked beautifully WITHOUT sanding airfoils into the wings. Seems to me that without sanding an airfoil into the wing, you could upscale that design almost as much as you want, with the only limitation being the strength of your materials (Kevlar/carbon weave?). Again, I'm just extrapolating, and my materials knowledge isn't that great either, but someone's GOTTA be able to do something of the sort.

Of course, the more I think about this, the dumber my thought process seems to be.

Never mind...move along...nothing to see here!
 
Well the beast is finally completed, had to add, what felt like half a ton of lead to the nose of the glider to restore the CG to 50% of the wing chord, so it would glide ok!I think i've probably doubled its all up weight! (Opps) So, decided to launch on a C5-3 to begin with and pray that it works!If it doesnt well, back to the drawing board. Im also going to build a new motor pod for it, to make it more stable in the boost phase. At the moment its a bit touch and go if the Flat Cat Boost pod willl be ok.

Hopefully it will be strong enough. Epoxy sticks most stuff pretty well aslong as you prep the area proper first. But if it isnt, then carbon wing spars or kevlar cloth and epoxy will have to used to strenthern weak areas I feel.

I'll try and take picture of either a sucessful launch or the resulting mess if it all goes horribly wrong!Now just need to wait for a ice bit of high pressure and calm weather!(a slight prob over here on old blighty!)

James
 
What struck me is that boost gliders are nightmares when it comes to rocketry. On the one hand, a better glider will have a sgood amount of lift (or is this wrong, too?), which in turn makes a lousy rocket as that lift will try to pull the rocket horizontal. Probably why they say the shuttle flies like a brick.

SO, even in the case of the Deltie Thunder when you don't sand an airfoil into the wing, the lift has to be coming from somewhere. Sooner or later, the whole setup has to break down as either a) the lifting body carries the rocket horizontal, or b) you toss a powerful enough motor in there that it rips the lifting body to shreds. Could the answer be canted fins to put a spin on the rocket, making the horizontal pull variable? It'd probably make a fun corkscrew smoke trail at least! :D

Speaking of lifting bodies, has anyone ever tried to make either a boost glider or a rocket glider out of an actual lifting body instead of something with wings? That'd be a neat project!
 
Thats why, during the boost phase the CG must be infront of the wings by at least one body diametre, so they act like a set of fins, as opposed to lifting divices. (If the CG was further back, I think it would kind of back loop into the ground upon launch!). This achived by sticking the booster pod infront of the wings, bringing the CG forward. When the pod departs the glider at the top of the launch the CG moves back and the glider, glides back down to earth. Well thats the theory, i've yet to prove it!

I have read in various articles people puting small amouts alieron into the wing, so it does a corksrew on the way up. I have yet to fathom why.

Yeah ive heard that space shuttle "rockets" fly like bricks too, and there a pain in the bum to build because the trst line is all wrong.

James
 
An airfoil that produces lift is going to produce lift, regardless of CG. I think the question is what effect that lift is going to have on the rocket during boost. In the case of a standard boost glider, the force of the lift is going to be away from the rocket, and putting the CG in front of the wings serves to make it so that the lift pulls the back of the rocket up, allowing the glider to be on top at separation instead of pulling the front of the rocket over causing separation to occur with the glider upside down and the airfoil to be pulling down. Again, the issue becomes one of the efficiency of the airfoil. An airfoil with lots of lift glides great, one with very little launches well, but doesn't glide well (think fighter jet--really has to be moving pretty fast to fly at all). I was wondering: would it be possible to create a boost glider that weathercocked to horizontal at an altitude that would allow safe recovery for the booster, but would allow an inefficient airfoil to fly? Either way, the drag and lift of the airfoil is probably going to cause some pretty huge stresses on the airfoil itself, especially in HPR, where speeds of 400-500 MPH aren't considered "fast".
 
Very true, I didnt really think that angle trough completely. Fair point. Anyhow, that was one of my original points - though i didnt think it from the lifting aerofoil angle. I just assumed that to much power would shred the glider due to the high drag of the wings (I guess thats why they use a B4-2 to give them a slow Woosh, rather than 0 - 100mph in half a second!as would occur if a B8 was used for example). So, are we saying that is I used a D size engine in my beast of boost glider I knocked up, its is well, going to disintgrate on launch? (I guess thare is only one way to find out though.....! :) ).

I think it is also why you have to fly the wing at 0 degrees incidences, rather than say at 1.5 or 2 degerees (as on RC rocket gliders) - too much lift would be created and either tear the wings of the glider or cause it to back loop.

Going back to the lifting body glider idea. Have you ever seen the prototype lifting body craft which lockheed built to test ther ideas before building the shuttle? Its smaller and not as coplex a shape as a shuttle. That would make a great project for RC.

James
 
Not really being able to visualize your rocket, I don't know for sure, but I think you'll be okay with D's. I'd just be really nervous designing one for H's (0 to 400 mph in a second would be freaky bad). Again, to use the example, the Deltee Thunder will fly on D's if I recall correctly.

I saw a few of the lifting bodies they worked on before the shuttle at the National Air & Space Museum. One of them is the thingy that Steve Austin smacked up in the opening credits of the Six Million Dollar Man. I think they might do okay as an R/C rockt glider, but "lifting body" is a wee bit of a misnomer, as I think you'd have a doozy of a time actually making them go up on a prop engine. It was pretty much an experiment in dropping a rock without breaking the guy inside of it. ;)
 
I was just looking through the LDRS 21 special projects webpage and saw that somebody was building a 10' long X-30. It will be a HPR K or L powered RC boost glider :) I would sure like to see that thing fly, but cant make the LDRS trip :mad:

-Brian Barney
 
OOF!! I forgot about that thing. Saw it a few weeks ago on there. I think it'll be interesting to see if that thing actually lands okay. I don't recall whether it was R/C or not, but I'd sure hope so unless he plans on launching it a mile away from the parking lot. That thing could put a heckuva dent in a vehicle (or a person, for that matter), as I'll bet it won't exactly glide gently.
 
OK, since Ken hasn't chimed in here yet...I'll do it.

I think the key to a successful glider flight is a good motor match. You need a motor with enough push to get it going, but not so much you rip the wings off. I have flown several gliders now and can tell you that the 5 to 1 thrust ratio doesnt apply here. A 17oz model will fly just fine on an E12, which is more like a 2.5 to 1 thrust ratio. Remember, these are gliders with flying surfaces, not a rocket with minimal fin size, so they will actually start gliding much earlier than a rocket would. Ken actually flies his combat jet on an F40W, but I have seen the glider and it definitely has some weight to it.

Now, as far as too much push and ripping wings off, it all depends on the model. I have "tried" to fly a thin foam profile jet on a larger motor; since it was a jet with swept wings and low profile airfoil, it handled the motor, but it was still a bad choice as you have to little time to react with the fast boost off the pad. The wings on the jet did remain in tact, but I won't be trying that again anytime soon.

Ken's Combat jet handles F40's well because it is heavier and has a very thick and sturdy airfoil. If you were to try this on a regular R/C glider, it would be a bad thing as I know the wings would collapse under the stress of an F40.

Remember, you can try a low end motor, like a D12 or such for your project. If it seems the motor is not enough as you didnt get the elevation and push you wanted, got to the next size up. Start small because they are gliders and do take little speed to actually fly (exception: R/C JET style gliders do require speed to maintain lift because of their profile); experiment like this until you find the perfect match for your plane. Even if you get little boost, it is a glider and you can fly it back to the ground, then try again. Don't expect it to take off from the pad like a rocket either!!! Remember, these are aircraft, NOT rockets...so consider it a rocket assisted airplane and take it from there.

btw...blackjacks and white lightnings would be the best choice...al longer burn would be better too, as you have time to get it up and fly it a bit before burnout.

Carl
 
Brian,

I haven't had a chance to see the BIG glider yet...could you post a link?

FYI, Ken Parker and I both have large HPR sized R/C foam Jets we each purchased many years ago and still have them NIB. These were made for .90 sized motors, so they are pretty big. We both plan on building them up as HPR R/C Rocket Gliders and, with a little luck, may have them ready for Whitakers next spring.

They are Military Foam models of the MIG SU-27 Flogger Jet with a length just over 6' and wingspan of 5 1/2'. Weight range will probably be in the 5-8lb area...so we are talking some serious weight compared to our current stock of R/C rocket gliders.

This will be our first attempt at HPR R/C rocket gliders, so stay tuned to the Rocketry Forum for further details...they should make for a good "winter" build project this year and I would love to see Ken and I do a "drag race" at Whitakers next spring!!!

Carl
 
Ooooo...that drag race would be cool! Just one extra channel on the R/C to remote fire an Estes Sidewinder and we have a dogfight!! bwahahahaha

Here's your link Carl...scroll about 2/3 of the way down...turns out the thing IS R/C.

https://www.ldrs21.org/page8.html
 
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