|Oct 04, 2012, 02:35 PM|
Thoughts on rocket powered gliders...
Wanted to post a bit about my rocket powered glider designs and things that I think about when building them.
On the surface, profile depron planes look simple, and a lot of people disparage them because they don't look real(in their eyes....) They are simple conceptually, but that takes some work to make them simple, strong and have good flight performance. In general cruciform designs will give the highest surface area and strongest structure for the weight. They have low frontal area and low drag. By using a cruciform type structure, you can get things to fly with extremely small wing area, but due to fuse lift, have very light loadings, very gentle flight and landing characteristics. I have done several full fuse designs, and they look great, however flight performance and weight is not as good as the profile designs, it all depends on what you are trying to achieve.
I like rockets that look like the original, ie motor in the tail, I'm not a pod and boom person, although that is the easiest way to combat CG shifts and get good flight performance.
I like to keep things simple, which means not having to move balast, centerline thrust, and minimal control surfaces. I normally run flat plate wings. The advantage of this is that you can boost perfectly straight up and not have any pitching moments, all you do is keep it straight if it strays due to wind, and then pitch over into glide. You don't need a special launch pad.
I like full flying tail designs. By using long stiff pushrods, you can get servo weight up forward, and you are only running two servos with direct torque linkages, with no slop and hinging worries. At the slow speeds that these fly in glide, the full flying surfaces provide plenty of authority and plenty of trim capability for nose heavy condition post-boost.
There are a few magic parameters that work well. For the aerotech E-6 motor, if you can keep the weight in the 8-11 ounce ready to fly realm, and have a fuse in the 39-42" regime, your CG shift between boost and glide will be about an inch which is easily trimmable with large surfaces. An aerotech E-6 weighs about 1.7 ounces, about the same as a 2 cell 800mah lipo pack, so in a perfect design you won't need extra nose weight. Unfortunately due to wing location etc, you often do need to add a bit anyway. Of the small RC BG's I've done the ones that have the best flight performance are the X-15, Interceptor, and A4B winged V-2. The E-6 has a long burn time and low thrust, so that you don't have to be quick on the sticks. As soon as you jump into E-12 and E-15/E-30 range, you have to use more reinforcement and worry about flutter. Higher velocity means higher drag with these and generally you won't get a higher boost.
I use velcro to hold the battery in place, it is light enough under mild boost to not come off. I use velcro for the bec and rx as well so I can remove them later if needed. I use foam safe CA or epoxy to hold the servos in place because I've had servo tape come loose under high heat in the summer.
For launch guidance I like rails. They have no wip and are small and low drag. In general I try to mount them on the sides of the model so that the flat surface of the fuse helps keep them from torquing on the pad if it is windy. Sandwich the sides of the depron with .03 styrene and then mount a T nut. The styrene is firm enough to suport the launch buttons.
For the smaller models I do glide tests, holding the nose in a 10 degree up attitude I push the model horizontally forward. I do this before I mount any electronics so that the weight is lightest. If the model pitches over and into a nose down glide, it is fine. If it pitches up at all or goes into a flat mush, it is tail heavy. You want to figure out the rearward CG. For boost, you can run slightly tail heavy as it will become nose heavy on glide, but you don't want to push that for the first flights, just use plenty of control throw to allow trimming. I normally set my glide trim about 3/16" up and adjust as needed.
For profile designs with a horizontal forward fuse, you need to model that as part of the wing when doing cg calculations, the CG is usually much further forward than a pure wing CG calculator will calculate. I use http://wingcgcalc.bruder.com.br/en_US/?
It allows you to add multiple wing panels.
For glues I use foam safe CA for the fuse joints and the motor tube, UHU POR for small laminations, or 3m-77 for large applications. I use trim self adhesive monokote for trim that I cut myself, vinyl letters from Great planes, stars and bars from sig, and sharpie pens in silver or black for panel lines. If I need paint, I use testors model master or regular small can testors as it is foam safe. Krylon short cuts will also work(small cans) but they don't have a huge color range. I don't run any sealer before painting, so I avoid having to mask over other colors to prevent pulling off paint. Again, easy and simple...
In the end, these models are very different looking, impressive in the air and very majestic in glide. Hopefully I can encourage a few more out there to try it, it's a great way to mix two neat hobbies.
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