stunthanger.com
Building Tips and technical articles. => Building techniques => Topic started by: Bill Johnson on November 22, 2013, 01:34:25 PM
-
Looking at Tom's thread on building the new RSM electric P40 profile kit, one of the first comments is about lightening holes. This brings a question to mind I've wondered about for some time.
If a model comes out tail heavy, is it practical to cut a series of lightening holes in the stabs and cover them?
Not being experienced enough to tell light from heavy wood when opening a kit, I had an RC model some years ago that came out quite tail heavy. Adding weight to the nose to get the aircraft within CG, the end result was a very heavy model. This bit me square in the butt a few months later when I stalled it, destroying the aircraft. I've thought about this ever since.
-
I'm wondering things like that myself. I'm building a plane now that needs a light tail. I guess you could first calculate the amount of weight that you can take out of the tail by cutting the holes, then calculate the amount of nose ballast that can be removed. That might tell you if it's worth it.
-
In my not-always-humble opinion, you have to cover the stab, etc., anyway, so why not cover as much air (sp-holes) as possible. It's bound to be lighter, but by how much is the question. If you do just the stab, I don't think it will be significant. However, if you do the stab, fuse, vertical, etc., they add up.
-
Guys,
Unless you are cutting very large lightening holes they don't gain you much. It is better to start with light wood or use thin sheets and build up the surface with a hollow center. I recently started building a new profile for our local events and got a rib set that had lightening holes plus holes for jig rods. Just for grins I put all the cutouts in a container and weight it to see what was gained - 1/8oz out of a 4oz total with glue wing (42" span). If the holes were not in the rib set I would have stacked the light ones on the inside and the heavy on the outside and called it a day.
One area you can save significant weight is in the control hardware. If you use 1/8" horns with long arms and a heavy pushrod it will have and impact. There is a small carbon fiber arrow shaft that is used for competition archery that is a little over 1/8" and half the weight of the old 1/4" ones.
On most of my ships lately I have done a crude weight and balance estimate to see how much trim weight I might need. On my El Diablo I found that I would have be nose heavy so I really needed to make the tail surfaces out of "Bomber balsa" which I also glassed (1/8" sheet surfaces on this OTS model) and it just made the balance point.
Best, DennisT
-
I do a mix of solid and open bays but more driven by warp resistance down the road. I'll live with a little more weight if I think the surface will stay straight. Sort of a draw.
Dave
-
When I was breaking my RC glider repeatedly (about 20 years ago as a poor HS student) I went with semi-built-up slab stabs. I used fairly stout (dimensionally) stock of the right thickness, added bracing, and after it was dry I cut the stabs out like it was solid, then covered. It allowed use of "scrap" pieces to make much of it, as opposed to cutting away large holes that make scrap that cannot be reused easily if at all. (I was in high school with a minimum wage, part time job! Frugal material use was more important than weight.)
Phil
-
If a model comes out tail heavy, is it practical to cut a series of lightening holes in the stabs and cover them?
Bill,
Ya gotta dope or put some chemical on the wood to cover the grain. It's common to apply silk, glass cloth or silkspan over wood to make filling the grain easier and for strength.
Seems to me, the less material the less weight.
"Holes" that's exactly what I have here in this stab!
Charles
-
I have 1/2" profile fuselage in a Sterling kit that's bowed with the concave on the outboard side. I was looking at the RSM build too and got to thinking of sanding the fuse flat, boring holes in the balsa and laminating a 1/16" ply sheet on each side to hold it straight. The ply and epoxy might replace all of the lost weight from sanding and boring, but I'm thinking it will keep it straight and rigid. What do you guys think of that solution?
Rusty
-
I have 1/2" profile fuselage in a Sterling kit that's bowed with the concave on the outboard side. I was looking at the RSM build too and got to thinking of sanding the fuse flat, boring holes in the balsa and laminating a 1/16" ply sheet on each side to hold it straight. The ply and epoxy might replace all of the lost weight from sanding and boring, but I'm thinking it will keep it straight and rigid. What do you guys think of that solution?
Rusty
Rusty,
I would purchase a piece of 1/2" stock that is straight and light. Use the piece in the kit as a template for the new one.
Charles
-
Rusty,
I would purchase a piece of 1/2" stock that is straight and light. Use the piece in the kit as a template for the new one.
Charles
Yeah, Charles I know. It was just too easy. ::) But probably what I'll do, thanks for giving a kick to my sensibilities.
Rusty
-
Howard is right. You can EASILY compute how much weight any cut-outs save you. Then multiply that by, say, four, and that will give you a ballpark estimate of the total weight you'll save when you remove enough material up front to re-balance the model. Whatever you just cut out will weaken the model. If you really need to save weight, remove enough material and replace with triangulated trusswork with the grain going in useful directions and make sure you retain spanwise stiffness. Depending on the plane, this may be more complicated than just replacing the whole tail surface with better wood, less finish, or a lighter engineerd structure. If it's a profile, you can lighten the aft fuselage with some clever/judicious surgery.
Most important though is just to compute the change and see whether it's worth the trouble to weaken your model.
Idle thoughts...
SK
-
Replacing balsa with 1/16 plywood will add weight. I would make a new fuselage with 1/2" sheet or build it up with 1/2 x 1/2.
Joe
-
Howard is right. You can EASILY compute how much weight any cut-outs save you. Then multiply that by, say, four, and that will give you a ballpark estimate of the total weight you'll save when you remove enough material up front to re-balance the model. Whatever you just cut out will weaken the model. If you really need to save weight, remove enough material and replace with triangulated trusswork with the grain going in useful directions and make sure you retain spanwise stiffness. Depending on the plane, this may be more complicated than just replacing the whole tail surface with better wood, less finish, or a lighter engineerd structure. If it's a profile, you can lighten the aft fuselage with some clever/judicious surgery.
Most important though is just to compute the change and see whether it's worth the trouble to weaken your model.
Idle thoughts...
SK
I guess it's a "balancing act". (Haha). but rather than just computing weight, you actually have to compute moment, which is weight times arm. As the arm to the tail is relatively long, a small weight reduction there equates to a relatively large weight reduction in what would be needed in the nose to bring it within CG.
-
One area you can save significant weight is in the control hardware. If you use 1/8" horns with long arms and a heavy pushrod it will have and impact. There is a small carbon fiber arrow shaft that is used for competition archery that is a little over 1/8" and half the weight of the old 1/4" ones.
That's what I'm thinking. The brazed steel control horn I've been using is 10.3 grams. Comparing it to holes, that's how much 13 square inches of 8-lb., 3/8" balsa would weigh. I hesitate to try new technology in my new plane, though. It's gotta work.
-
That's what I'm thinking. The brazed steel control horn I've been using is 10.3 grams. Comparing it to holes, that's how much 13 square inches of 8-lb., 3/8" balsa would weigh. I hesitate to try new technology in my new plane, though. It's gotta work.
Look into a similar construction aluminum horn, I am sure someone makes one. 1/3 the weight, the same strength (ok, it is softer and 3 times more flexible, but the same ultimate strength)
Phil
-
Keeping the wire part of the brazed steel horns short is essential, but I've always thought that there must be a way to make CF control horns. I just don't know how to go about it. I'm leery of fabricating a CF horn from plate, tube and rod, all epoxied together, but then Howard epoxies the axles into his CF LG, and that seems to work ok. But not as critical, and easier to repair on the field. If I remember correctly, PW uses 3/32" wire horns, not 1/8". D>K Steve
PS: If you have trouble with advanced math, I offer this help: ;)
-
Thanks, Ty! Today's moving day. We're moving out of a 1BR apartment into a rental house. It's on Oakline Dr in Madison. Since we had to sign a 1 year lease, the wife's letting me put up 1 work bench. When I built it, we were at FT. Riley and the only place with enough room was our bedroom! That won't fly anymore, though. mw~
-
While not related to cutting out holes in the stab I build the wing, tail and fuse then hold them all on with rubber bands to finalize the the right layout for proper CofG. Only then do I cut out the hole for the wing.
Balance just slightly nose heavy (there is more wing aft of the CofG than ahead and paint weighs a lot). If you do it properly a few grams in the tail will move the CofG to the optimal location.
Kim.
-
While not related to cutting out holes in the stab I build the wing, tail and fuse then hold them all on with rubber bands to finalize the the right layout for proper CofG. Only then do I cut out the hole for the wing.
Balance just slightly nose heavy (there is more wing aft of the CofG than ahead and paint weighs a lot). If you do it properly a few grams in the tail will move the CofG to the optimal location.
Kim.
Kim,
That is brilliant! I've built hundreds of models over the last 50 years and never thought of that. I've used the same idea by moving the engine fore and aft to get the CG placement, but your idea is so much easier. Thanks for the information!
Paul
-
While not related to cutting out holes in the stab I build the wing, tail and fuse then hold them all on with rubber bands to finalize the the right layout for proper CofG. Only then do I cut out the hole for the wing.
Balance just slightly nose heavy (there is more wing aft of the CofG than ahead and paint weighs a lot). If you do it properly a few grams in the tail will move the CofG to the optimal location.
Kim.
Kim,
That is brilliant! I've built hundreds of models over the last 50 years and never thought of that. I've used the same idea by moving the engine fore and aft to get the CG placement, but your idea is so much easier. Thanks for the information!
Paul
Both ideas work together too.
A bit of masking tape in the right spots and one can even paint and cover the surfaces before setting the positions. (This doesn't work well on all models though. I have used it for RC though.)
Phil
-
The great thing about Kim's method is that you are affecting both moment arms (nose and tail) at the same time. Just moving either the tail or the engine has much less effect than moving the wing. Very smart!
Paul
-
Bill,
Ya gotta dope or put some chemical on the wood to cover the grain. It's common to apply silk, glass cloth or silkspan over wood to make filling the grain easier and for strength.
Seems to me, the less material the less weight.
"Holes" that's exactly what I have here in this stab!
Charles
Yes, you have holes in your stabilizer. I notice that it's a very narrow stabilizer, and the elevator is very large in comparison. The stabilizer wood must be very strong and heavy? I would have made them about opposite in areas. Works better; check out the Flite Streak.
As for the holes, they obviously do take out some weight, but I would much prefer to fabricate from strips and ribs, so that the grain of the wood is going in the correct direction. I would strip the spars and LE's from light sheet stock. Why? Because, as everybody knows, the balsa companies cut strips from heavier wood, so it will perhaps survive better in the hobby shop or craft store until it is sold. Waste not, want not. H^^ Steve