Converting rubber FF kits to RC (weight reducing tricks)

[David Lewis]

I've noticed that receivers, servos or other heavy items on the CG or behind it often contributes to an overweight condition. It's better to locate the motor, RC gear, and especially the battery, as far forward as practical because you'll need less nose weight. Extending the nose a little longer than scale in order to make short-coupled subjects more flyable is also an option to consider.

Another common pitfall is to use battery, propulsion or control systems that are too big and powerful compared to the size of the model. For best performance on a small airplane, equipment should be just big enough to do the job, and no more. Likewise with covering material. The lightest mylar covering available is much more than strong enough, and the weight savings is considerable. Caution is advised when it comes to "Super Light", "Ultra Lite", "Solite" and so forth because, while a few live up to their name, some of these so-called extra super duper light coverings are too heavy for this type of model.

A non-scale FF sport airplane often features a long nose chord because it allows a long rubber motor and moves the CG forward. The same rule still applies, however. It usually works best to locate the heavy items as far forward as you can, even if it would make the airplane nose heavy because you can always shorten the nose, and shortening the nose has the side benefit of making the airplane more stable.

[davidchoate]

I have attempted many conversions on yhe 500's, and some 900's. my problem is that the weight seems to always go over th 50g mark, and thats about all those little 8mm brushed motors will fly, and my hinged tails are very prone to warpage. Not to mention the unreliability of the micro Rx / servo board. I am not doing anymore micros unless I can fit a true Rx, and even those 3g mini servos in it.

[David Lewis]

I'm glad you mentioned that because right now I'm selecting a radio for my 1:15 Aeronca Champ (kit #301), and I want something rock solid reliable.

To prevent warping, you can switch to contest balsa and increase the thickness of the parts. For example, if 3/32" is too flimsy, go up to 1/8" and reduce balsa density by 25% (e.g. from 8 lbs./cu. ft. to 6 lbs./cu. ft.). It will weigh the same but be 33% stiffer because bending stiffness is proportional to the square of the thickness. (I'm assuming elastic modulus of balsa is proportional to density.)

[Bill Gaylord]

David the Champ should be a good subject. I did the Cessna 150, and it's one of the easiest flyers I have, on 3 channel. I could probably hand the transmitter to a complete beginner, and it's anything but light with a sheeted fuse. The Coverite Microlite, or Solite, is really not too bad on weight if not painted, since tissue has it's own added weight of dope/coloring and/or paint. As for efficiency in gear placement, there's no substitute for building a boatload of electric rc conversions. You can locate the gear and battery pretty much off top of head based on previous experiences, given the building methods used, (sheeting/covering/etc) and hit the cg dead on. Did it perfectly with my last project, the Stearman, with no ballast. Really nice model to fly.
https://www.youtube.com/watch?v=U7gySPVBtyo

[davidchoate]

I'm currently building a plane from plans. It's a good Trainer,and easy build. I spent $14 for the plan, and maybe $20 on materials (not including electrics). But I have alot of servos and stuff. Just need a motor, and a Rx. I have 2 Rx spektrum, and I use them in different planes because they are expensive. I'm glad David Lewis is talking about this stuff because he is very knowledgable on how these things fly, and how to change the aerodynamic settings for RC, or FF. Do you have any formal training David?, or just learned from doing, and reading?. I recently discovered a magazine called Micro Rc World, and signed up for a free issue. At first I would gut a Champ after using it to practice, and transfer the parts to a similar size Guillows, but a Champ weighs 38g's with batt. I cant even get my rubber powered that light. I digress, but bet a Champ would do damn good on rubber powered flight. I have one perfect except the Rx burnt out,and they dont sell just the Rx anymore, you have to buy the whole fuselage with electronics for $50.00/ I think a Chomp without the transmitter is the same price. But anyway I'm gonna say i'm done with the brick boards til theyre more reliable. and brushed motors are not good.Cheap,but no guts. I put one (a 370 ) in My 404 Zero just cause I needed the weight up front. Mounted the servos,and batt all forward, And still just balanced. Dont have the guts to maiden her yet. I'm worried the Elevator is a bit small.Oh, and I found out about Hinge tape recently. Awesome for micros,and Firelands Corp? Ares makes a nice little brushless for their Taylorcraft 130. The plane comes as a trainer, but has capability in the Rx/servo brick to add aelerons, and a ESC. My next attempt at a convert of micro size will be usisg that setup. Spektrum does make receivers that accept the 3g micro servos without need of adapters, but they are expensive. I'm just glad someone posted on conversions,and i'm glad to see that Guillows all but gives you how to do it on the newer plans. Like the Beaver & Edge with larger control surfaces etc. For flying. I use solid balsa now for tail surfaces that warp. Like they do on the 400 series plans. I'm learning, and thanks Dave and Bill for sharing your knowledge.

[Bill Gaylord]

Since you brought up the Zero, here's how I'd do it, if I built another. It's a great flyer, and would even be better if I didn't have the added nose ballast. I built the little Guillows 190 with the tail servos/batt box in the cowl, using the batt/box for the motor mount. Even with a 1/16" sheeted fuse, I actually overshot the cg! The battery is mounted on the wing! As for the lengthening idea, the Guillows 190 is an A3 model. Since the A5-A8 models are around 6" longer in the full scale, this allows the model to be built as one of the later models with a slight nose lengthening, and still be scale. I did that with both the small and large FW190, if I remember correctly.

[davidchoate]

I like the 190. I also like to add cap strips and some 1/32" sheeting on the wings on RC. I have'nt had much luck so far in getting aelerons to work good on the 16" planes. I've read of guys using elevator and aelerons, but no moving rudder. I just cant fit it all in there. I have the FW-190 400 kit, and may use your idea on the cowl..

[David Lewis]

I learned everything just by doing, copying from experts, trying all sorts of ideas and discovering a lot of things that didn't work. I enlarged the Champ plans to 28" wingspan and omitted a lot of parts to get the wing loading down.

Here is a comparison table of covering weights in grams per square metre:

2.2 g/m^2 = 2um clear mylar
2.6 = 2um aluminized mylar
6.8 = 4um aluminized mylar
7.0 = 5um clear mylar
10 to 15 = Risteen MicroLite
12.5 = Esaki Jap tissue
19 = Esaki Jap tissue doped
18 to 20 = Coverite Microlite

I believe what happens with ailerons at very low Reynolds number is that the wing upper surface boundary layer doesn't have enough energy to stay attached when the aileron is deflected down. So instead of increasing the lift coefficient very much, you get bluff body drag and adverse yaw. If this theory is correct, then lots of differential throw should increase effectiveness. (Differential throw means the up-going aileron goes up a lot, while the down-going aileron goes down very little. That is, the upward deflected aileron does most of the work.)

The second problem is that roll stability (dihedral) has to be kept relatively low in order for ailerons to have adequate authority. Not a problem with a larger model because the pilot can keep the wings level, but a very small airplane can react too fast for the pilot to stay ahead of it.

[davidchoate]

I converted a ME109, and used scale aeleron size, not so good. If I ad aelerons, I make them much larger than scale, to correct that problem.

[David Lewis]

Bill Gaylord wrote: "I did the Cessna 150, and it's one of the easiest flyers I have, on 3 channel. I could probably hand the transmitter to a complete beginner, and it's anything but light with a sheeted fuse."

Good point. There isn't a clear dividing line between too heavy and acceptable weight because it depends on design goals and what kind of flying you like to do. Most of the little R/C conversons I've seen fly too fast for my taste. Personal preference. I will go to great lengths to save weight. Most model builders are less extreme, but there's nothing like a model coming in at amazingly slow snail crawl speed for landing!

[davidchoate]

I have learned that the formers rearward especially of the wing can be greatly reduced in mass by opening up the center and scalloping. As there is no need for the strength because there is no motor and peg apllying a force on them. . and as well it gives more room for control rods, and I think scalloping gives an all in all better appearance to the finish. ( no B formers visible through tissue).

[David Lewis]

Virtually all the formers can have their width reduced, the side keels can be eliminated and you'll still have more than adequate strength. I modify the main keel, though, so that it forms a continuous loop with no gaps.

[Konrad]

I for one like to make laminated hoops for most of my curved outlines. I'm also of the opinion that the rear fuselage on most rubber band scale model are too strong and heavy for R/C (there is no need to hold the torque of a wound rubber band). I like to take as much as 2/3 the balsa out of anything aft of the center of gravity. I often add this back in the way of motor mounts, hatches and stronger landing gear.

With what I show here I can make most Guillow scale models land at just over a walking pace. OK, truthfully at a man's sprinting pace. Most of my 24" models come in at 100 to 150 grams. They are not Gossamer nor are that lead sleds. They are sturdy enough to handle real world R/C mishaps.
http://www.hippocketaeronautics.com/hpa ... ic=12443.0

Aerodynamically I like to give the airfoil of my Guillow models a full radius (no sharp leading edge on the bottom of the airfoil as is often shown on the plans). I also lower the stabilizer incidence (declarage) as they often don't need or I want the added pitch stability with an R/C model that a negative (up) stabilizer affords.

All the best,
Konrad

[davidchoate]

Thanks Konrad, I joined youre forum last year, and forgot all about it. There IS alot of very useful info on there. Thanks.

[Konrad]

Dave
Yes, there is a lot of good information on that site free of shilling. Sorry, I have to correct you in that it is not my site or even my thread. There are many contributor that have added value to the site and thread. I hope to see your input soon.

All the best,
Konrad