Kestrel-RCU

Could 1/64" ply be used in place of 1/16" balsa sheeting? Would it be lighter and stronger?

Dragon Soaring

My samurai is 1/64th ply covered with Oracover. sure beats balsa for me. I dont know if it any lighter but is sure is stonger!!!

Kestrel-RCU

Thanks DH. I was pretty sure that it would be stronger. I am considering ways to build the tail planes lighter without sacrificing durability. I did find some weight comparisons but it didn't show any 1/64. And, showed 1/32 as being a lot heavier than 1/16 balsa, but it was a 5 ply.

I have also thought about the possibility of using carbon fiber. ????????

Ollie

The range of strength and weight of a balsa part has a range of 5 or 6 to one depending on its density. The lightest conterst balsa only weighs about 4 pounds per cubic foot and ther hardest balsa can weigh as much as 24 pounds per cubic foot. It doesn't mean much to compare balsa of a certain thickness with plywood of a given thickness without specifying the balsa's density.

Kestrel-RCU

Is strength sacrificed by using the lighter balsa?

Ollie

See:
http://www.uoguelph.ca/~antoon/hobby/sigbal.htm
If you scroll down the page you will see that doubling the density of balsa more than doubles its stiffness and strength. Matching the density of balsa parts to the application is at least as important as using the right size. Serious modelers select their balsa density and grain for each particular application.

Kestrel-RCU

Thanks Ollie. I did find a chart that showed that similar info. Would you happen to know of any listings for the 1/64" ply? I found a chart but it didn't list the 1/64"

I am planning the construction of a T tail. Definitely not an area to compromise strength for weight.

I have e-mailed ACP asking if they have a product that could be substituted for the 1/16" balsa skin. Rather expensive stuff though. Not sure if the $ is worth it.

seafury_fb11

Kestrel,
What kind of plane are you building - sailplane? sloper? If it's a sloper, you don't really need to worry so much about a little extra weight. I sheet the wings of all of my slopers with 1/32" ply then glass them with 6oz cloth. I built a high tail (not exactly a t-tail) plane a while back. For the vertical, I and used a foam core sheeted with 1/64" ply, then glassed over that. It is plenty rigid even though it is only about 1/4" wide where the horizontal goes through. I don't get any flutter at all - and this plane hauls.

http://russ.rcplanet.com/slopeships/me262.html

Russ.

Ollie

Often the most highly stressed part of a T-tail is the fuselage just in front of the fin. In a hard landing the inertia of the stab and vertical tail are the culprets. Saving weight (mass) in the tail surfaces means less stress on the fuselage just in front of the fin and lighter turns out to be more survivable.
Also, in a ground loop, the inertia of the tail surfaces torques the tail boom. Unfortunately these loads are hard to quantify and the structure is difficult to design rationally. In cases like these, one falls back on an empirical approach.

Build one and see where it breaks. Then lighten the next one where it doesn't break and beef it up where it does. After a few iterations you will have a design that is refined for the conditions of use.

Kestrel-RCU

Seafury, the model is a Mystery Ship. I have been told by some that it is a decent model and by some that it is a stone! One owner who had an early version claimed its biggest downfall was a heavy tail. It seems that the later models had the wing moved to require less nose weight. Anyway, I like the looks of it, the price was in the budget and it looks like a nice large building project for next winter. So for now I am looking for some things to perhaps improve the model before i start to build. Lightening the tail planes looks like a good place to start.

Ollie, you are right on in regard to the weak area in front of the fin. Some have said that it is a weak area on the Mystery Ship. I looked one over carefully this past Sat. It doesn't look to be a flaw in the model, but rather the weakest link in the chain inherent to the T-tail design. As you suggest. The one that I looked at had never had a problem in that area. The gentleman is a good pilot so it perhaps never had a hard landing. I was considering a .007 by 1/2" wide strip of carbon fiber on the sides of the fuse in the fin/fuse junction to beef it up without adding a lot of weight.

Hope the weather shapes up pretty soon so I can get out flying. Won't have quite as much time to think up all of this crazy stuff.

Ollie

It's not just the Mystery Ship. Every T-tail model faces the problem.

The area on top of the fuselage comes under severe compression in a hard landing. The compression is a particular problem with fiberous materials like wood and carbon fiber which are much weaker in compression than in tension. The compression is caused by the inertia of the stabilizer and to a lesser degree the inertia of the top of the fin. Anything you do to reduce the mass in this area will pay off bigtime. Minimizing the metal in this area is one measure. Use plastic rather than metal clevices. Keep the threaded rod as short as possible. Replace any hard or medium hard wood with contest balsa. Use the lightest covering film. Every gram saved reduces the inertial loads. Don't do dorky landings.

If you still want to reinforce something, reinforce the top of the fuselage, not the sides. Unidirectional cloth is much stronger in compression than woven cloth.

Kestrel-RCU

Thanks Ollie. Very helpful info. I don't have the kit yet but do have the instruciton manual. I believe it says to plank the top of the fuse with 1/8" Balsa running cross grain. Which is common practise. Wouldn't it be stronger under compression if the grain were running lengthwise? At least for a few inches forward of the fin?

Ollie

Yes, Balsa is about 17 times stronger in compression with the grain than it is across the grain!

Even so, the leading edge of the fin puts a compression load on the side grain of the top sheeting, no matter which way the grain of the top sheeting runs. You might want to replace the leading edge of the fin and several inches of top sheeting with a single laminated piece of balsa that is bent to conform to the angle between the top sheeting and the leading edge of the fin. This introduces a radius to the outline of the side view. The result is that the bearing loads are transformed into bending loads, for which the balsa is much stronger. So you end up making that area of the structure much stronger at little or no weight penalty but at the cost of building complication.

Kestrel-RCU

I had considered something like the darkened area in this sketch but your method would spread the stress over a larger area forward without adding the extra weight.

MTT

My Feedback: (8)

I can confirm that the fuse of the Mystery Ship just in front of the vertical stab is too weak, from own experience. I had a not too hard landing, and the fuse split open just in that area. I have now repaired it with the grain running lenghtwise. Still haven't been able to fly it though.
Another thing with the Mystery Ship : I liked the kit very much, fit of the parts is excellent, instructions are very good, it is a very enjoyable build.
However, I think that the vertical fin is way too weak for that relatively heavy flying tail, so now I am considering either to take the weight penalty and reinforce the vertical fin somehow, or change the whole setup altogether to a conventional tail.
Another thing : The kit comes with a already hollowed out nosecone for balancing the plane, but there is not nearly enough space in the nose cone for the amount of lead required, so you have to find room for more in the fwd part of the fuse, and there is not much space to begin with ! Try to save as much weight on the tail as possible, since the nose is relatively short, and any excess weight on the tail requires a lot of weight up front to balance it.

Michael

Kestrel-RCU

Thanks MTT for the input. I am looking at parts to swap to help reduce weight especially in the stab area as Ollie has suggested. Just a thought, but I don't think it would be to hard to build it as a mid-tail. Which would alleviate most of the inertia related problems. Perhaps a V-tail would be better yet as it would also cut the tail weight and gross weight. I think I could figure out the redesign for midtail but the V-tail would be a whole new challenge. Not quite sure how to construct it in addition to angles and dimensions reqired.

Still thinking about it. Perhaps I will build it just to see if I can get it built and make it hold up. Then, if it breaks I can reconstruct it as a "V". All part of the "FUN" of the hobby.

BTW MTT, how old was your kit? I had read someplace that the wing was going to be moved back a little to help reduce the amount of nose weight required.

MTT

My Feedback: (8)

Kestrel, my kit was purchased november last year, I don't know if it has the moved back already. I'll measure it and get back to you, so you can compare.
BTW, any idea why Bob Martin quit producing them ?

Michael

Kestrel-RCU

Your model should be the latest. The info I had in regard to that was from something around 1996. In other words, the real early ones.

As I understand it, Bob sold Major Hobby and is now working for a company doing CAD design.

BTW another suggestion I had gotten to beef up the fin was to laminate 2 pieces of 1/32 sheeting with the grain at 90 deg. Use it in place of the 1/16. Although, I hadn't heard anything about the actual fin giving a problem. One had a problem in the stab pivot area but most seem to be the fuse just forward of the fin.

Actually, the mid-tail idea seems to be the easiest fix for the fuse problem. Does anybody have any opinions on this?

Chachi639

Regarding the question about strength:

I'm assuming that the application is sheeting over a framework of balsa sticks. While strength is important, if you've got unsupported spans of balsa that you're changing to plywood you'd do well to make sure that the stiffness of the material you're changing to is no less than what it replaced.

Otherwise, although your replacement sheeting might be plenty strong, and won't break while flying or landing, if it's too flexible weird things might happen in flight.

Home brew way of finding out if you're okay: take a piece of the balsa you're replacing and a piece of the ply you're considering, cut a piece of each into, say, a 1" x 8" plank. Put a couple of same thickness books on a table about 6" apart, lay both planks across the gap between the books, like bridges. Line them up alike. Stack pennies on the middle of each plank and look to see which one bends less with a given number of pennies stacked on it.

Voila! you're an engineer, and you've found out which material is stiffer! Let us know what you find out.

Kestrel-RCU

I have gotten info and advise back from a number of different sources in regard to sheeting built up structures. Many of you perhaps know these things but for those that don't, here goes.

Carbon fiber sheeting is not meant for and wouldn't work well over open frame structures. Excellent over foam.

The 1/64" ply usage is basically the same as the C/F sheeting. It was suggested by Balsa USA that the ply could be used over built up framing if the ribs were very close together. Which is not the case with the Mystery Ship as the ribs are around 2.75 apart. The ply is tough but so thin that it could sag between the ribs.

The main idea for looking for a replacement for the 1/16" balsa sheeting was to make the tail assembly lighter while maintaining the durability of the structure. The ply perhaps could maintain the strength in the fin but may end up heavier depending on the density of the balsa being used. I didn't get an actual weight on the ply but it would be close to about an 8to 10#/cu. ft. balsa.

So it would seem that the best approach is to build the tail area as light as possible by being conservative on adhesive, substitute lighter components where possible, lighter balsa on the rudder and stabs, (or lightening holes)maintain the integrity of the fin and reinforce the fuse in front of the fin and finish with a light covering.

After describing all of this to a friend he said, "That's they the way a model should be built." HMMMMM

Kestrel-RCU

I did some messing around with a cheap CAD program that I have. (Keep in mind, I am not a draftsman!) Drew up a mid tail design for the Mystery Ship. I don't have the kit and plans yet so the actual dimensions may not be right but the basic concept should be pretty close.

Please look it over and let me know if there are any changes that should be made. Lighter or heavier in any area etc.

I hope it comes out OK. I had to draw it, print it , scan it, convert to jpg then resize it!

Thanks for your time. Am I boring anybody yet? Duey

Ollie

The fin above the elevator mount is a very low stress area and needs no sheeting. Lighter ribs and cap stripping will do. The area around the elevator pivot tube is a high stress area and needs washers or cheek rib fairings of 1/16 plywood on the surface to distribute the load. Soft rather than hard balsa is OK for the core.

The pivot line of the stab should run through 25% of the mean aerodynamic chord of the horizontal tail to avoid flutter or excessive servo loads. This will give it aerodynamic balance.

Kestrel-RCU

Thank you Ollie. Got it! That is exactly the info I was looking for. I have an instruction manual but it is not here to lookat. (At work in my desk. I peek once in a while.) I believe there are two sets of 1/16 ply roots. One for the end of each stab and the other pair were for the center fairing/ mount for the t-tail. They would work great as the washers you are refering to . Cutting them short right at the control wire slot.

The pivot tubes are located in between the main spars of the stabs. I think that they are probably back 25 to 30% from the leading edge. I should be able to leave the stabs positioned as I have them in the drawing and just move the main support in the fin forward to line up.

I can't tell right now but I am guessing that I can cut an oz. or more from the tail. Which could mean as much as 5 oz. form the total weight. Shouldn't have to beef up the fuse either.

Thanks agan for your time. Duey

Ollie

I forgot to mention: Because the horizontal stabilizer no longer acts as an end plate on the vertical tail the vertical tail area is less effective. Some vertical tail area needs to be added. Increasing the vertical tail height by an inch or inch and a half should do.

Kestrel-RCU

Thanks Ollie. Now all I have to do is decide which way to build it. It is interesting though. I have this all entered into PCSOAR. With everything the same. The t tail design out performs the mid tail very slightly. If I remember correctly the edge is in the L/D. ?????Is this because the lower stab is running in more turbulent air?