Sojourner

I just finished a very short coupled model of a WWI monoplane. I drew the plans up about 5 years ago and I've been working on it off and on ever since. Yesterday I tried to fly it for the first time. I had the CG set at 2.5" from the leading edge on a 8" chord so it was between 25% and 30% of the chord. It has a fixed landing gear, tail dragger using 3.5" WWI type wheels. The rudder is full flying as in, there isn't a fixed verticle fin.

The first thing I noticed was taxing. It was terrible. It took almost full left rudder to get it to go straight. If you eased off just a little it would start making sharp right turns and try to ground loop if you were going to fast. I've got tow-in on the wheels, and both spin freely. I just have a skid on the back. Basicly, it was impossible to taxi in a straight line. I'm looking for options to check to improve the ground handling, to see if I can get it to handle as it should.

Second, on take off, it poped up about 18" in the air, snapped to the right, the right tip hit the ground, and the wing folded. An easy repair, after going back to the drawing board to beef up the center section of the wing. The last time I had a plane do this, it was tail heavy. As I said earlier, it is short coupled. Can anyone verify that on short coupled planes that the CG needs to be 20 to 25%? or what the flight charictaristics are when the CG is too far back?

Thanks,
Soj

Rodney

As to CG, yes I'd move it forward a bit, especially for first flights and be sure to have adequate takeoff speed. As to the full moveable fin, where is it hinged at? What percentage of the area is ahead of the hinge line? This could be the problem if more than 10 percent of the area is ahead of the hinge line.

FHHuber

Look for a wing warp first.

Often, especially when operating on pavement, a tailskid doesn't do much good for taxiing. You get full torque-turn (or as some would call it... P-factor) effect with no effective compensation from the rudder until you are well above stall speed. Because I operate from a paved runway, I had the same problem with ground handling. I put a small steerable tailwheel on the skid of my Fokker Dr1. Problem cured. If you are careful about how you put the tailwheel on, it can be almost invisible while the plane is in flight, and it can come off easilly for operation from grass at scale competitions.

I also avoid any toe-in or toe-out on my taildraggers... If you can see the toe angle... its too much. (measureable in any way toe-out is bad...) However up to 5 deg of chamber (top further apart than bottom of wheel) helps... If your lg mounting allws that adjustment.

Your take-off is typical of a tailheavy plane. You may be short coupled enough to need to balance it as a flying wing... at 15% MAC. (you can fly anything at that CG... it just might be detrimental to aerobatics. I've flown planes at 5% MAC CG.)

Sojourner

THanks for the tips. I can't put a steerable tail wheel on this critter. After I get my scanner working I will put a picture on and you will see why. The rudder is above the fuselage, and nothing sticks out below the elevator to hook on to. The shaft for the rudder stays inside the fuselage and runs down through the horiz. stab then the bell crank hooks onto the bottom. If I had known this was gonna be a problem, I could have redesigned the tail of the airplane and run the shaft on through the bottom to hook on to the tail wheel. Hind sight ya know... I built the landing gear to scale dimensions, which has a spread of about 8", after doing a little research, I have found that 25% of the span would be better, that is about 10.5". I can easily bend the gear out which will provide more stability, but it may not give better ground handling. I will build a new wing, and get the CG set to 15% to 20% instead of the 25% to 30% it was set to and hopefully it will fly. Thanks, this makes the second airplane I've built from scratch that has suffered from Aft CG problems. If nothing else, I have learned that it is MUCH better to start a little nose heavy than trusting the 25% rule on the cg..

Thanks,
Soj

FHHuber

build a small, solid balsa glider with the same incidences and halfway decent representation of the airfoils (if symetrical... symetrical, thickness not a big factor at the tiny scale)
Balance that with modeling clay... glide. When it balances for a smooth glide... find its CG. Use that % MAC (or just a bit forward) on the big model. Should fly fine. Works 90% + of the time.

There's other ways to get the LG fixed... but one aspec of your problem could be the aft CG. On many planes if the CG is too far aft of the LG, they are terribly "squirrley" on the ground. I've fixed a lot of squirrley taildraggers by moving the wheels back 1/4 to 1/2 inch. (when the CG was correct for flight) I've also seen planes with aft CG's act badly on the ground and be fine when the CG was corrected. So the CG fix might fix BOTH.

Sojourner

A glider... I hadn't thought of doing that. It has a symetrical airfoil NA0012 or something around there. It's been so long since I built the wing and I forgot to record what it was. Someone suggested I check the incidence, I did that just before I flew it (that morning) and it had 1 degree of washout on each tip. I bought one of the laser incidence meters, they work a lot nicer than the one I made from balsa with string, weights, and degree charts.

I'm going to build another wing, beef up the middle section a little and get the CG moved forward, widen the gear about 2" and try it again. I didn't know that CG would effect taxing. That is interesting to learn. I can test that by adding weight to the nose of the fuselage until it balances at where I think the CG should be and then taxi it around without the wing.

I just love building my own airplanes. Now I have a great resource to figure out what might be wrong when they don't work right, it is even better...

THANKS!
Soj

RaceCity

Good to see somebody actually BUILD a model!

Seems as though most of the suggestions thus far have been at least "valid"...with the exception of the extreme forward CG idea which is not. It may improve ground handling, but...raises the stall speed of the aircraft to the point where you WILL crash the plane hauling it off the deck.

Bear in mind that WW1 aircraft were/are the antithesis of stability. Their full size counterparts were linen clad coffins! Very few pilots lived beyond a hundred or so hours in those....they were that bad.

A squirrely WW1 model?....that's about normal.

You may find it helpful to research the "settings" for some other,similar WW1 models. What worked for them?

Having the wheel centerline at or just slightly ahead of the wing LE is sometimes advantageous as well.

Hope this plane works out for you, but don't expect it to have all the mild mannered characteristics of a trainer, or a sport plane...it most likely won't.

Good Luck! Post a picture of the plane!

Sojourner

Thanks for your input. It isn't a "SCALE" plane, I've taken a few liberties. The center line of the axil is about 1/4" in front of the leading edge of the wing.

I've flown an extremly nose heavy plane before. It took 3/4ths of the up elevater to keep it straight and level. I wish the moment it took off and I realized this i whould have chopped the throttle and let it land. I didn't, but made it to the base leg, and let up just a fraction to let it descend. It descended alright, verticly, and no amount of up would pull it out.

As for the wing, it is flat and without taper. I've flown wings like this before, so once I get the CGs right, it should fly like the others.

I took a Ukie combat wing and made a fuselage, stuck a .40 on the nose and flew it several times. It was a Hoot to fly. It would excellerate going straight up. Had a very short tail moment, no rudder, and a full flying elevator, and ailerons. The wing was 32" span, and about 1" thick. I think it was a Voodoo, designed for a .25 to .35. I let a few others fly it and they enjoyed it. I had the ailerons set a little fast and it went into a knife edge, I over controled and ended up in another knife edge, and it hit the ground. The wing was splinters, but the fuselages survived. I made another wing, but it didn't fly as well. I think I didn't get the cg far enough forward. It would fly at about 1/4 throttle, but if I gave it any more throttle, it would stop all forward movement and just spin like a dog chasing it's tail. Pull the throttle back and it would stop, straighten out, and start flying again. I've still got it, but it needs some minor repairs.

Oh, one other thing, from what I've read, the reason WWI planes were flying coffins wasn't because they were unstable (some were, others were not, they needed a stable gun platform to work as a fighter). The alies didn't allow the pilots to use parachutes, since the cotten fabric was covered in nitrate dope (very very flamable). The gas tanks weren't self sealing, and when they sprung an oil or gas leak would catch on fire. Without a parachute, the pilot was in a terrible fix. According to Eddie Rickenbacker (not sure of spelling) in his autobiography, the alies were afraid the pilots would jump at the first sign of trouble rather than try to fight and get the planes down safely. Also, flying was very new then. They didn't have 40 hours of flight school with a certified instructor. They didnt' have combat manouvers being taught. You had kids with as little was 8 or 10 hours of flight training, bearly soleing by todays standards, flying combat against experienced pilots. Who do you think will win? Experience and craftiness will often defeat youth and strength...

Thanks again,
Soj

RaceCity

Heh Heh....neat stories about the "scratch" projects. That's OK. Scratchin' your own is where the real fun is at. Folks really miss out on the real thrill of R/C with ARF's and all the rest.

LG position sounds good at about 1/4" ahead of the LE. There's no magic in that position in and of itself...it's a happy compromise between having the gear too far back (nose over) and too far forward...(like pushing a rope).

4000 flight hours (fullsize) and 25 years R/C gives me the authority to say that WW1 aircraft are unstable. They are.

All in all...it would seem once you get the "details" ironed out in your latest project...it'll be a lot of fun, and you'll have a model to be proud of. YOU designed it, and that's better than anything somebody can sell ya.

Good Luck...Keep Hammering away at it!