View Full Version : Cg location on free-flight e-power conversions
01-06-2006, 12:36 AM
Hi to all! I would like to hear about Cg location of converted free flight models that use a lifting stabilizer. I've been told that the Cg will be much further back on the wing than normal...???????????? Thanks for any help. I have a friend who has a "Banshee" conversion and he doesn't have any tall grass around to test glide before he fires it up!:)
01-08-2006, 12:49 AM
To Gliderbill, regarding the CG, it's been my experience with 3 FF to R/C conversions that the rearward CG common to most FF's (50 percent of MAC or so) won't work with R/C. The upwind penetration needed to keep the model over the field isn't there with the CG position putting that much download on the stab. It works in FF because the model isn't supposed to penetrate; It needs to wheel around and stay in the thermal. That small amount of longitudinal instability actually helps a Free Flight model but hinders a controlled model. So the CG will need to be forward, 25-30% of MAC, but the normal FF model also has 3-5 degrees of decalage (difference of angles between wing and stab) built in, this needs to be lowered too. In effect you are "flattening" out all the angles as viewed from the side. Those angles (and motor downthrust, stab tilt, trim tabs, etc) are aerodynamic "tricks" to allow a model to fly without direct control. Free Flight. To fly with direct control these adjustments are counterproductive. So I'd suggest starting with a 25% to 30% CG, lowering the wing incidence to no more than 2 degrees and trim with the elevator control. The lifting stab doesn't really help R/C but in this case it won't hurt, either. At a very high speed there might be a tendency to nose-tuck, the lifting stab overcoming wing angle, but it's unlikely a FF model would ever reach that speed in one piece! If it glides nose down and fast, a small (1/32") shim under the wing Leading edge should flatten it out. You may be able to move the CG back to 35% but I wouldn't go much farther aft than that or you will end up chasing the model, it won't come out of a thermal!
Good Luck! Ron
01-08-2006, 04:45 AM
Hi Ron! Thanks for that detailed bit of info! My first FF was a Comet Sailplane with an Ohlsson (?) 60, built in the men's dorm at Purdue Univ. back in 1946. I had no experience with gas and had help from the guys in the Purdue Aeromodelors club. Did a lot of shimming of the wing before it flew well! I've fwd'd your reply to my friend. Thanks again!
01-08-2006, 06:48 AM
Ron: Gliderbill is my friend who wrote to you about my Banshee. I have since talked with the club member who sold me my kit (that is a story in itself). He told me that this plane needs the CG at about 50% to fly well, and that has been proven out by experience with quite a few of these kits being built by club members, and flown. An interesting issue, one I have had a ball of fun discussing with quite a few RC flyers (many of them old "free flighters."). I am going to take it very slow in breaking my Banshee in, let me tell you. It is such a cute and fun looking plane. Roger Q.
01-08-2006, 11:03 AM
Hi Roger, Good to hear from you. I am an old Free Flighter myself, also flew Control line "back in the day". If your club mates say that 50% CG will work on that particular design, go for it! It doesn't seem to work for me, and I've noticed a few other OTer designs having the same problem of upwind penetration (without power, I should add) in the typical "Limited Motor Run" events I've seen. That may be a difference too, with enough power applied, just about any design will move upwind, but a lightly loaded model with a high lift/high drag airfoil like most older designs, will have a harder time.
Another factor is you can't just move the CG forward; With a big lifting stab the model will dive. You might have to shim the wing or stab, or in the case of an R/C model, add "up" trim to get a good glide. My recommendation to lower the decalage was to compensate for the (normal) climbing attitude under power common to FF's. The power and glide phase require different trims, but with R/C the elevator does both.
It's all a balancing act, Hope it all works out!
Good Luck, Ron
01-10-2006, 08:40 PM
I just finished a conversion to electric/RC of a Goldberg 1/A Viking. I posted it here: http://www.wattflyer.com/forums/showthread.php?t=2629
I was wondering about the CG as well. I went with the plans suggestion which is about 70% or so from the LE. It actually flew really well. It doesn't want to fly very fast though. In fact, it gets a little nutty under full power, but putting it anywhere in the sky isn't a problem. The thermals here in Denver aren't as strong as I remember they were in Florida, so staying up is more of a task then getting down. The air here can be a bit turbulent closer to the surface and a bit of power is a very good thing to get to the cleaner air. As far as the wing/stab incidence, I believe they are pretty flat as is the thrust line, but it is hard to tell just where the horizontal reference line is on that thing. I want to use it to hoist up a camera to take some shots and that will nose-weight it a bit.
It does look cool in the air. definitely different!
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