Dave Kovensky

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I am scratch bulding a three channel R/C version of Lou Garami's "Stratostreak" pylon free flight model. This was an Atom and Arden .099 gas engine powered airplane that I built many years ago that I have many fond memories about and want to replicate. I was able to get plans for this model and am in process of building it, but need to correctly establish the C/G. The plans do not show any C/G location. None of my research for this design or any other old timer pylon models for C/G location has been successful so far. My memory tells me that these type free flight gas models would balance at about 50 percent of the chord and have a lifting stabilizer design to compensate for this. I don't trust me memory however and hope someone can help me out with some valid information.

BMatthews

Your memory hasn't failed you.

The final balance location was a somewhat flexible thing. Free flighters move the final balance and adjust the stabilizer as part of the power pattern trimming to get a good power pattern, good transition and good glide. To do this the stability margin has to be small compared to what most folks set up RC models for but it has to be "just" the right amount too. Hence the playing with the balance and stab angles and downthrust to get just the right combination.

For your case where it'll have RC to aid with the transition 50% isn't a bad place to start. Even better would be to ask on an old timer free flight forum such as Hip Pocket Aeronautics where some folks with Strato Streaks will be able to tell you exactly where they put the balance.

Dave Kovensky

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Hi BMathews,

Thanks for your rapid response to my query. I tried searching in R/C Universe but came up with no results. I Googled this address and was successful in getting on the site. I then signed up and became a member. Finally, I wrote up a description of my needs and am waiting to see if I get a response.

Dave

Lifer

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Go to a plans service and pull up a competition free-flight model. Generally, the balance point will be indicated. Likely you'll find the 50 percent point is the standard. From my free-flight days, the first step in the process is a test glide over tall grass!

Dave Kovensky

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Thanks for your reply to my question. I already have done a search of many plan service competition free-flight pylon models and was unable to find any that did show the C/G balance location. And from my free-flight experience (of many years go) always did a hand glide over tall grass before attempting a powered flight. However, I now live in New Mexico which is high desert and tall grass is only a fond memory, I was hoping to start out with a proven C/G value to minimize instability issues due to the combination of a lifting stabilizer and a 50 percent or higher C/G rearward chord location..

Lifer

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http://www.airplanesandrockets.com/a...s-2000x927.jpg

Dave,

Check out the c/g on this satellite. Large stab equals a more rearward balance point. Looks like 80 percent.

Hossfly

Now you are building a RC model and not a Free Flight. The balance point is not as it was on FF. While I never built that exact model, I did build a lot of FF and won a number of competitions back in the '70s. Remember that your stab is -- if like all the FFers I competed with -- is in reality a Clark Y or close to it. That means a lifting airfoil.
On your RC, that stab is in what aerodynamically -- current RC model -- is in a mode of a lot of down elevator. For your RC you have some choices.
1. Place the chord line of the stab to be much more in line with the wing chord to start.
2. Most FF models have considerable down-thrust. You could take a lot of that out.
3. Re build your stab to a flat stab. Sticks rather than ribs. Use some diagonal sticks to enhance the twisting flat plate.
4. As others state, start your glide tests at not more than 50% of the CG. Most of my competition models (from 1/2 A to class C) usually trimmed out at about 55-65%
Using RC with reasonable stab curvature, 40-50% should be good. With same as a FF, 60-70% may well be best.
5. If wing is polyhedral, I strongly suggest washing out the wing tips starting at the break, to about 5-8%.

Makes me ready to build another FF. Have several kits and many plans.

Dave Kovensky

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Hi Horace,

I just finished my R/C version of the Strato Streak and it incorporates most of your recommendations. Thought you might want to see what it looks like.

Dave

Strat2003

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When I flew SAM Limited Engine Run R/C, my pylon models (Playboys and Bombers) balanced around 55%. I had down thrust in the engine and used full down trim in the climb, transitioning to some degree of up trim for the glide.

Hossfly

Thank You Much, Mr. Dave. Very pretty. Hope you have a ball with it. Since I know nothing about electrics, I can't really say concerning wing wobble, yet I will pass on my thought. I would add some dowel or wire hooks to allow for a tie-down over the wing platform. That center-only rubber band could give you a problem. I well remember that in the older RC Trainers, guys came to the field with only a couple rubber bands. They could not understand why the model kind of went crazy when in loops, hard-turns etc. That extra G-Force in a tight maneuver would cause the wing to lift some, changing the complete aerodynamic forces. Once they would finally quit calling the radio "BAD" and added rubber bands, the radio would get better!!! I learned that when I was flying FF.

BMatthews

I agree with Hossfly on the wing retention thing. Free flight models fly at a fixed trim that involves very slight pitch recover due to being set close to neutrally stable. So when they pull out of a stall or a dive it's with a SLOOOOOOOOOWWWWW pitch rate. So the wing rubber bands just don't see a lot of stress.... unless the model hits "the big green thing" But your RC model will have an elevator and you'll be using it for various things that are really going to strain those rubber bands. So use lots and stretch them pretty tight.


Hossfly, my own feelings on the trim are a little different from yours. I don't mind running the trim at closer to a FF setting and just live with the touchy elevator issues. I get around that by using a lot less area and travel. But then I used to fly competition sailplanes that were set close to but not quite at neutrally stable. I later shifted the balance ahead from "3 click critical" on the elevator trim to get something closer to a 7 click range before seeing a dive or a slight stalling. That seemed to work out nicely for me. I've always tried to set my RC old timer models up the same way. The result seems to be that I end up running somewhere between 5% to maybe as much as 10% ahead of the optimum FF CG position.

Strat2003

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I think you'll be fine with the center-dowel wing attachment. I used it with no problems on my Supertigre and Jett powered SAM contest Playboys and Bombers. Just use plenty of rubber bands!

Hossfly

[QUOTE=BMatthews;11950668

Hossfly, my own feelings on the trim are a little different from yours.[/QUOTE]

No big problem there, Mr. Mathews. Lots of folks feel as you do.

Way back in the 60's I was an instructor (T-33 & T-38) at Laredo AFB, I met a true modeler, Mr John Valls, that could make Balsa kiss his behinder. He tried to teach me Hand-Launch Glider but I had problems. Another Pilot, "Bear" Snyder taught me some about HLG that worked pretty well. Each differed considerably. I did win some trophies, first place, in the San Antonio area, Chicago-Land, and some USAF annual model airplane events. To each his own I guess.
What John could never teach me was rubber-power. John would wind up and release a rubber competition model, it would run down just fine until prop hit the "stop".
He would wind it up, hand it to me and it would roll over and die on the climb-out. Guess it did not like my smell.
FF was a lot of fun, but it could be a problem now and then while not knowing what went wrong.