Can someone explain this?

[wtl0715]

I'm deciding whether to change part of a design I'm building. I'm in the late stages, so I don't want to mess with anything unless I'm sure I understand what I'm doing.

I am building a twin-boom pusher. I would like to shorten the booms a bit to cut some weight. My understanding is that, in order to maintain constant pitch stability, I must increase the horizontal stab area to counteract the shortening of the booms. Is this correct?

The Horizontal Tail Volume Coeffiicient for my model is .35. That's on the low end of the desired Vh range, so I may even make the tail a bit larger than is necessary to counteract the shortening of the tail. Does anyone forsee any problems with this approach?

Here are some stats:

Wingspan: 66in
Length (currently): 67in
Boom Length: 40
Fuse Length: 37
*Booms overlap the fuse a few inches*
Current Weight (fuse, booms, wings, tail assembly - no gear, motor, or electronics installed yet): 4.5lb
AUW (educated guess - after cutting a bit of weight): 5.5lb
Motor: 600-850w outrunner (maybe Hacker or Power Series)

As mentioned above, some of these numbers are educated guesses. If I can take a few inches off of the booms, it will reduce the airframe weight. I also must cut some more access holes in the fuse, which will bring the weight down a bit.

-TL

[vintage1]

I'm deciding whether to change part of a design I'm building. I'm in the late stages, so I don't want to mess with anything unless I'm sure I understand what I'm doing.

I am building a twin-boom pusher. I would like to shorten the booms a bit to cut some weight. My understanding is that, in order to maintain constant pitch stability, I must increase the horizontal stab area to counteract the shortening of the booms. Is this correct?


Not exactly. no.
You can also counteract the effect of less tail volume my moving the CG forward. Howevver this does lead in the limit to situations where there is a large download on the tailplane, and you may run out of up elevator authority, or in really bad cases, the tailplane stalling.

Increasing tail area is the safe option, but its not the only one.

[BMatthews]

If you're doing this to reduce the weight in the tail then keep in mind that what you save with shorter booms will be made up for by the weight of the added tail area. Instead you'd be better off to figure out a way to make new proper length tail booms that are lighter than the present ones.

[wtl0715]

That's a good idea. Another thing I considered was using a single boom. What do I lose by switching from twin boom to single boom? I thought about using a thick carbon fiber strip instead of aluminum booms. That would allow me to attach the boom to the bottom of the fuse and run it out to the tail and cut at least 2/3 of my boom weight out. If it wouldn't cost me too much in stability, that might be a great option.

-TL

[BMatthews]

Careful. Carbon is great stuff but it needs to be used correctly. On it's own it is often too flexible to ensure that the tail surfaces will remain in the correct orientation with the wing. Carbon works well as a cap over some other material so the components form a locked stress bering structure with a wider baseline. Think I beam here. Two strips of steel with a third strip joining them. On their own the top and bottom of the I beam are strong but very flexible. Lock them together with the vertical web and the assembly becomes far more stiff and strong than the three components on their own. You need to use carbon like this. Use the carbon as caps to bind a structure between them and life will be good. Try to use small strips or tubes and think they will be as rigid as a much larger box made from something like balsa and you'll find yourself looking at a pile of sticks and carbon wondering WTH happened.

In terms of shifting from twin booms to a single boom there is nothing at all to lose. A single boom, due to its larger cross sectional size, will be more rigid than two 1/2 size twin booms.

[wtl0715]

Great! I'll consider making the switch. I'll also use something a bit less flexible than carbon. Thanks for the info!

-TL

[ghoti]

Well, two booms gives you the option of spinning the prop between them.
bill

[Alex.Schweig]

I've seen twin boom aircraft with VERY rigid booms, made out or hardwood dowels wrapped in lots of carbon tow.

This might be a good idea if you know your way around composite materials.

[wtl0715]

I'm not sure I feel confident working with much more than very simple, ready to use materials. I'm sure I could learn, but I'd rather not learn on an airplane. I'd be sure to make a mistake that could cost me (or possibly a nearby homeowner). If a boom ever broke and the tail assembly was lost, that thing would turn into a missile.

-TL

[BMatthews]

Great! I'll consider making the switch. I'll also use something a bit less flexible than carbon. Thanks for the info!

-TL

I think you missed my point. Carbon is a fantastic material if used correctly. I tried to explain how best to use it but I don't think it hit home. I tried the short method and failed I guess.

If you made a balsa box boom of reasonable size and then put carbon strips on the four corners and on top of that if you were to wind carbon thread or tow fiber around the boom and glue it down so it forms an XXXXXX style pattern or a /\/\/\/\/\/\/\/\/ pattern like those on a crane's boom you would be using the carbon for both uses in a good way. The strips along the corners would add a huge amount of rigidity and strength to a lightly built balsa boom while the diagonal or X pattern of carbon or kevlar fibers would give you a huge amount of torsional rigidity. All this comes from what ON ITS OWN is a very flexible material. But when you support it so it sees the load forces directly in line rather than from the side it is incredably strong and rigid. In this example the carbon is the top and bottom of that I beam and the balsa rectangular box shaped boom the web that joins the top and bottom cap of the I beam, The difference being you are making an assembly of 4 Ibeams that form a box. Or you could do a triangular boom that uses less material. In that case definetly study a building crane's boom that is a triangular shape with particular attention to the way the diagonal braces join. Copy this style with carbon at the corners of the basic triangular boom and add the diagonals. You'll get a strong and rigid boom.

[wtl0715]

Bruce,

I apologize for my ignorance here. I understand the idea of what you're explaining, but I'm not sure that I can visualize the way it is accomplished. The picture in my head is of a balsa box made from four .5"x2"x30" pieces forming a 2"x2"x30" boom (I understand these sizes might not be correct, but let's pretend those dimensions create an appropriately sized boom). Each corner has a strip of carbon fiber on both sides, meaning that there are eight pieces of carbon fiber total. Then some smaller pieces of carbon fiber attached to the surface of the boom diagonally. Each of the diagonal pieces, which alternate in direction, stretch from one carbon fiber end cap to another.

Have I come close to what you were describing, or am I way off? A flat view of one side of the boom that I'm picturing would look like the top two figures in this picture:

http://www.linkandpinhobbies.com/Graphics/SP_CRANE%20BOOM.gif

-TL