downthrust and PVA on tissue

[stx44]

So Ive just returned to the hobby after a 30 year break...

The first attempts have lead to 2 questions-

How much downthrust is usually required? Ive built the Skyraider, added a small stringer offcut for downthrust, then a large stringer and stil getting a power stall with more than 50 turns.....

Second , I used thinned PVA on the tissue per a suggestion from Guillows, but it didnt turn out as hard as I remember the old dope being. So is this normal, or is it a case of mixture to thin and/or too few coats?

Thanks for any advice!

Simon

[Wildpig]

Welcome back.

How much downthrust is difficult to say offhand. Usually, putting a 1/16" square piece of stringer stock under the thrust bearing is plenty.

Are the basics of trim correct?
The C.G. is in the right location.
Gliding is straight and shallow.
Also, you might try a different rubber motor. The kit rubber, which is probably 3/16" wide gray rubber, gives a huge burst of initial power. That may be too much for your model. You might try 1 loop of 1/8" rubber, about 1.5 times the hook to peg length.
Lube the rubber and stretch wind it.

Other than that, all I can say is put in a shim as needed.

[Phugoid]

Wildpig is 100% right, check the C of G and the glide first.

However, I have always found that low wing monoplanes need a hefty dose of down thrust, this is only in my experience mind you, I don't know exactly why. The only thing that I can think of is that they want to fly fast, to fly fast equals more power, that equals having to have a bigger power burst at the start.

I built a Keil Kraft Spitfire that needed (at a guess) 10 degrees down thrust to fly properly.

If it does glide ok and the C of G is right, but is still "stally" under power even with a lot of down thrust then tweak the "elevators" down a bit (or add some paper tabs bent down). The problem with test glides is that they can be a fair bit slower than the flight speed, so when under power and moving faster the down elevator is needed to get things back in check.

Andrew

[stx44]

Im figuring it must be downthrust- no power glide is dead straight and at a shallow angle- slightly fast maybe, but other than that ok.

I will crank in a little more downthrust and see what happens...

[Steve Blanchard]

If you are really only giving it 50 winds and it's power stalling then I have say you really need to look at the power. What is the weight, Wing span, and size of prop? These are all important factors into what size rubber to use. Just so you have a gauge, a good performing rubber model should take around 800 to 1000 winds or more.

Steve

[WIDDOG]

What size Peck Polymer propeller do you think is the best for a 900 Series kit?

[Steve Blanchard]

I would start with a 6" silver Peck prop. The rubber should be two loops of 3/32" Tan Super Sport at least 1.5 times the hook to peg length. It's also important that the trust button or noseblock is seated snugly or or the thrust changes won't remain and be effective.

[WIDDOG]

Thanks much Steve I will note that for my kit 904 Skyraider build. I am getting better at trimming with each model I make. However, I seem to still have a lot of trouble finding a good starting set up.

[kittyfritters]

Next time you tissue a model try fixing the tissue with a mist of Krylon Crystal Clear #1305 acrylic clear coating. (Use 1309 for matte finish) It is almost as hard as dope, quite light, if not over applied, much less toxic than dope, and UV resistant. The only disadvantage is that it is water resistant, not waterproof...no seaplanes. I have 8 year old models with the original tissue, done with Krylon Crystal Clear, that are still flying.

When you first apply it the tissue will sag slightly but will return to full tightness in 10 minutes. On mornings with heavy dew the tissue will sag outdoors so don't fly the model until the dew lifts and the tissue tightens back up.

[Steve Blanchard]

I second Howard's advice about the Krylon and I'll add that if you want and even finer spray just fill a cup with hot water and put the can of Krylon in it for about a minute. Then when you spray it comes out as a fine mist. You can do 2 to 3 coats this way and get a nice light even finish.

Steve

[davidchoate]

i recently returned to this hobby too. and the dope aint what it used to be,i noticed the same thing, and they dont even make colores anymore ! so Ive been using spraypaint, and they do sell a clearcoat spray at hobbytown. i just bought a can and am gonna try it on the ME109 im building now.

[davidchoate]

i recently returned to this hobby too. and the dope aint what it used to be,i noticed the same thing, and they dont even make colores anymore ! so Ive been using spraypaint, and they do sell a clearcoat spray at hobbytown. i just bought a can and am gonna try it on the ME109 im building now.