cress

Remake of vintage freeflight plane. Fringing to figure what it is and where to start to balance it. It has a very under chambered wing and a flat bottom thick airfoil elevator stab. Thanks

Lifer

My Feedback: (1)

With a lifting stab the c/g will be somewhere between 50 to 60% of the chord measured from the leading edge. I have no idea what the name of the aircraft might be.

rgburrill

A nose heavy plane flies poorly, a tail heavy plane flies once. You put the CG at 50-60% of chord and you won't fly long.

The CG should be at or forward of the CL, center of lift. Now you can go through all the calculations to determine the center of lift or you can do the simple thing like has been done since the beginning of flight, experiment. Start with the CG at 1/4 chord and move it back until the plane stops flying better each time. With that big tail you can likely get away with start at 30%.

Over 100 years of aviation has shown that aerodynamics is all about experimentation - all the design in the world just gets you a stating point. I used to get paid to do this stuff. My boss and I at Boeing disproved some "hard and fast" rules during wind tunnel testing.

Lifer

My Feedback: (1)

Check out some Free Flight designs that have similar moments and lifting stabs. Note the C/G on those designs and then decide for yourself.

UStik

Very good advice to start with more forward balance, no doubt. In this case, I wouldn't just start at 25% from leading edge. The "lifting stab" will not lift before the C/G is set far backwards, but its camber and especially the wing's camber make for a lot of nose-down pitch (airfoil moment). That's why such models have a lot of decalage (say 4° incidence difference between wing and stab) and a rearward C/G. This model has a very small elevator, which is undoubtedly enough if balance is OK but may be not powerful enough in case the C/G is very far forward.

The attached pictures show a glider I built in the sixties. It has similar configuration and undercambered wing airfoil. Unfortunately, the plan had to be repaired with tape but I drew a red arrow where the C/G should be. The instructions specify 69% from leading edge. Another well-known example is the Telemaster with (not even undercambered but) flat-bottom airfoil where the neutral point (aft limit of C/G) is at 50%. I would use at least that as a starting point.

wnewbury

Dragonfly by Tex Newman. Bill Northrop's Model Builder Magazine. Published 1971 and October 1996. Mine flew well - super easy. Radio influenced rather than radio controlled.

wnewbury

CG on plans looks like 55% or 60%.

speedracerntrixie

My Feedback: (29)

This should help.


Looks to be in the 50%-60% range to me.

rgburrill

Well I'll be

Lifer

My Feedback: (1)

It's always a good idea to advise based on knowledge or experience. Those requesting help depend upon accuracy rather than a guess or an adage. Free flight is an excellent training ground that unfortunately isn't as widespread as it once was.

Truckracer

My Feedback: (19)

You apparently don't have any free flight experience. Many of the old designs did indeed fly with the CG at 50 - 60% MAC. With that said though, if this plane is being flown with RC assist, I would move the CG to a more normal location though still rearward of what one would call normal for a traditional airframe. Reason being this plane is meant to fly very slowly and any excessive use of elevator (which could mean almost no movement) could result in an instant stall that will be followed by a series of scallops that could be very hard to correct unless you have experience with this sort of plane. Other than during powered climb when down elevator will be necessary, most of a flight including the landing can be performed with little or no elevator use. Even rudder inputs should be kept to a minimum. Design for a very slow glide, not aerobatics. Can be fun but challenging if horsed around.