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Balancing and Control Throws

Old 04-20-2002, 02:04 PM
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Jeff396
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Default Balancing and Control Throws

Hey all,

First time poster. Recently I have been building a Midwest Pitt's special. I bought the kit in 1985 and have finally gotten around to putting it together. Being an old kit it is somewhat less helpful than the newer ones. Specifically, they do not mention suggested control throws nor do they give details on balancing. This plane has a 4 ft wingspan and is powered by an OS .91 FX. My questions are,

1. how do I determine what control throws to go with for the first flight? Should I use exponential or dual rates?

2. The balance point is shown on the center on the fuslage and is in front of the leading edge of the lower wing. I have decided to balance by turning it upside down and balancing on the top of the upper wing. I then put a level on the horizontal stab to get it level and observe which way it tips. Is this the correct way to do it or is there another preferred method?

Any help will be GREATLY appreciated!

Jeff
Old 04-20-2002, 07:25 PM
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BingoFlyer
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Default Balancing and Control Throws

The balance point first (most important). I would project the point on the plans up to the top wing and us that point to balance from, just lift it with finger tips at each side at the point above shown on plans. I usually use a jig that I build using two dowels with those pointed ereasers on the end (better than your fingr tips) level by eye.

First flite I would set controls so Elevator was about 1" Rudder 1.5" and ailerons 3/4". Use dual rates at aprox. 50% of this.
Old 05-09-2002, 12:44 PM
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Jeff396
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Default Balancing and Control Throws

Quick question.....the throws you suggest are total or one way? ie, if you suggest 1 inch for elevator is that 1/2 inch up and 1/2 inch down or 1 inch up and 1 inch down? I'm thinking you mean total throw of 1 inch for elevator.

Jeff
Old 05-10-2002, 05:07 PM
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MinnFlyer
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Default Balancing and Control Throws

IMHO It doesn't matter too much as long as you don't really go crazy with the throws. Your first flight will tell you if it needs more or less. Nobody takes a plane out for a maiden voyage expecting to fly the pattern with it. That's why they have the term "Shake Down Cruise". Just take it easy the first time out, then trim accordingly.
Old 05-10-2002, 05:55 PM
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Default Balancing and Control Throws

I'm afraid that at my field there is a very low opinion of the Pitt's and I guess it's getting me a bit spooked. We just had one of our members put his in the parking lot on take off no less.....lack of judgment seems to be the overall consensus. Just wouldn't back off when things started to go wrong. I do have to say that reading here has boosted my confidence as many seem to fly this type of plane with great success and most of the warnings I hear are to look out for its ground handling and be careful of dropping the speed too much. Anyway, I think I am going to use exponential to soften the center and see what happens. The other difference between mine and the ones discussed here is that mine only has a 4 ft wingspan and I would hate to have it too twitchy on its maiden flight.

This is my first bipe....I have flown a Kadet MarkII(long ago), Global American Flyer and a GP .40 size 300 Extra S arf. Just hoping this plane isn't over my head.

Jeff
Old 05-10-2002, 06:19 PM
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Default Balancing and Control Throws

If you have access to RealFlight or other simulator, I would pick an appropriate bipe model and fly it on the simulator, fool around with the control throws and have all the virtual crashes you want until you are happy with the throws (which will be in degrees, not inches). Then translate those throws to your Pitts, and go conservative for the trimming out flights. Add to the throws as you like when you find the conservative throws too limiting.

For test flights I would skip the dual rates, just fly on conservative "low" rate. Add the high rates later, as you determine how much more you want.

A little expo is always nice, but you don't need much if any for conservative throws. As you set up high rates later, then expo becomes more desirable.

As to CG, if you are still framing or at least not covered, my favorite method for setting CG is to locate the plan-recommended CG on the top wing (for a bipe or high-wing model). Normally at that point the structure will have at least a double center wing rib, and perhaps the main spar will intersect at that point also. Whatever structure is there, beef it up a little bit through the thickness of the wing with small pieces of balsa block or triangle stock, and drill a vertical hole 1/4" or 5/16" through that spot. Take a dowel of the appropriate diameter and drill a vertical pilot hole down its center, to receive an appropriate sized screw eye from your local hardware store. Epoxy the dowel into the wing. When the model is covered and you are ready to position the battery, hang the bugger by the screw eye with a line from the ceiling, light fixture, whatever, and hang just a few inches above whatever surface (floor, bench, table) you're working with. With a little luck, you will be pretty close to balance both front to back and side to side, and you can fine tune the balance just by putting the battery to one side or another of the fuselage, a little forward or aft of receiver. I like to take my first flight with the nose down just a degree or two, then adjust later on after flight testing.

If CG is off by more than what the battery placement can correct, you can add weight to nose by going from wooden to APC prop, or lighten the nose by going from APC to wooden prop. (My preference is for wood, because I like a light prop for the engine to turn, but I will use an APC if I need more noseweight.)

If battery placement plus prop selection still leaves you off, then the next thing I would do is add a redundant battery. (Ask Red Scholefield about how to set this up.) I'd a lot rather have another few ounces of battery on standby than a useless piece of lead somewhere.

If two batteries won't solve your CG problem, then radical measures are called for. Now you may have to swap engines for something a bit lighter or heavier, or remount the engine a little further in or out, maybe move servos to tail, etc. All this is of course best done with framed up but not covered fuselage.

Other measures: for less tailweight, take off your sheet balsa tailfeathers and build up sticks; for added tailweight, balsa sheet your tailfeathers, cover with fiberglass and epoxy.. If you're going to have to add weight, you might as well get some benefit from it.

By the way, I am often the only one at the field when I fly, so it is helpful for me to be able to pick up my plane in one hand and transmitter in the other. If your plane is not too big (less than 1/4 scale) you can handle it easily if you make up something like a little stevedore's hook to keep in your pocket, then slip it through the screw-eye. By picking it up at CG you can walk with it easily in one hand, lift it over fence or barrier, etc. If I am going to leave my CG jig in permanently, I normally run a piece of 8-32 threaded rod all the way through the thickness of the wing, fasten it on the bottom with fender washer & nylon locking nut, another washer and locking bolt on top, then thread on a drawer pull knob selected for low drag finish. This doesn't look very scale like, but I like the convenience of it.
Old 05-10-2002, 06:26 PM
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Default Balancing and Control Throws

Originally posted by Jeff396
We just had one of our members put his in the parking lot on take off no less.....lack of judgment seems to be the overall consensus
A good point to make here too is that these types of airplanes depend a LOT on airspeed and since they tend to have a lot of torque with a relatively short wingspan, many fliers get in trouble by taking off before they have sufficient airspeed resulting in a (sometimes uncontrollable) roll to the left.
Old 05-10-2002, 07:19 PM
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Default Balancing and Control Throws

Yep, that's what happened.....rolled to the left and came very close to a family sitting at one of our picnic tables. Scary!!! I know that the guy who sold him the plane, who has nothing good to say about the Pitt's which is why he sold it, has warned me repeatedly to make sure I have what I probably think to too much airspeed on take off. I have also been practicing higher that normal speed landings on my other two planes. I actually now prefer this method of landing though I get made fun of a lot. Anyway, they rarely bounce that way.

I have already done a pre balance on the plane and am very happy with it. I did it by tipping it upside down on the top wing using one of those GP CG balancers. I then put on of the little levels that comes with it on the horizontal stab to be sure the cg point was directly above the pivot point and let it go. It consistently went nose down so at least I'm close with no weight. Of course that was before covering and painting the ABS.

Jeff
Old 05-10-2002, 07:25 PM
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Default Balancing and Control Throws

I will give the simulator another shot. I have the G2. I took the Super Skybolt and modified it, as best I could, to be my Pitt's. The problem I have with it is that all of the planes seem to be far too stable compared to real life. I would have though that I should have gotten a pretty hard pull to the left after creating the Pitt's but there was very little. I just wonder about the accuracy.

Jeff
Old 05-10-2002, 08:43 PM
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Default control Throws

Jeff,

The controls I had recommended would be in each direction, as I had said I would use duel rates if you use Expo start with 10-15%. And by all means keep the speed up on landing, You will find this plane has more drag than you might think and will snap at the blink of an eye.

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