iFLYrc_Vic

My Feedback: (6)

I have a GeeBee R3 Racer (Fantacy Racer) as shown in the attached photos. Many pilots have had major issues trying to fly this plane, especially this size model. here are the specs:
Wing Span: 59 inch
Length: 52.5
Wing area" 620 sq. In.
Weight: 8.5 lbs
Power: Electric motor with 6S Lipo producing 1200 watts
Prop: 16 x 12 APC
Flight Data: 500watts cruise, 50 mph

This issues I ran into were similar to reports by other pilots. The plane is very sensitive to rudder inputs - knife edge flight with very little rudder input. It "skids" on coordinated control turns. I had to resort to the "bank and yank" method of turning the plane. Hard to trim the rudder to achieve straight ahead flight. Landings are at least one bounce - hard to "grease" this one in. The larger models don't seem to have these issues.

1. I attribute the over-sensitive rudder to the short vertical stab and long rudder. Any thoughts on if this is correct?
2. Any suggestions on improving the flight characteristics of this model?

Thanks,
Vic

da Rock

Can't help with the flying problems, but have a suggestion about the landing problems.

The gear on that thing positions the tires way far forward. The tires should contact the ground just about directly below the wing's LE when the fuselage is parallel to the ground. The contact point is also called the footprint. When the footprints are too far forward and the airplane touches down, the plane's CG continues downward. The tires stop going down however. When the tires are positioned correctly, they have good leverage to resist the CG. When they're too far forward they don't.

When the CG keeps going it pushes the plane into nose up. The plane goes where it's pointing, nose up. It starts to porpoise.

If I were you, I'd run the model's measurements through Geistware's CG application and find out how far forward you can move the CG. Then make sure to keep the CG away from it's aft location. The CG range you get from Geistware is very good at keeping your plane safe. In this case, the forward location helps the landing as well.

The aft location won't help anything on that sucker. Especially as you've noticed how sensitive the surfaces are, which is an indication of an aft CG among other things.

rmh

A blind man can see that the drag profile is the real issue
The landing gear may look KOOL but it is an aerodynamic disaster.
If you plot lateral area and the moment arm of the gear - - the thing resembles an arrow shot tail first. These old designs were tricky at best - the pilot had to avoid any slip and any AOA over just what was needed to keep it going .

otrcman

Both da Rock and RMH have given you some good advice. So I'll only add this:

Most people think of Center of Gravity as strictly a pitch issue. But if you think of the CG as "the point about which the airplane pivots in all axes", then you will see that CG can be just as important in yaw as it is in pitch. If you have a lack of vertical area in the back, or an abundance of area in the front (you have both) then the aft CG limit for yaw may well be more critical than the aft CG limit for pitch.

So, just as the previous two posters have suggested, try moving the CG forward. I think you will see a big improvement not only in yaw, but also in the tendency to bounce on landing.

Dick

Rotaryphile

That airplane has very little vertical tail area. If you crank an airplane into a bank, and its nose does not drop much, it is yawing away from the roll, and would probably be inclined to snap roll at inopportune times. Might be worthwhile trying a temporary cardboard increase in the vertical tail, just for an experiment.

Noted 1930s era test pilot, Frank Courtney, said that he had never flown an airplane with enough vertical tail area. The importance of adequate vertical tail area took a while to catch designers attention. Witness all those WW1 fighters with their tiny vertical tails, and their horrendous stall/spin accident rate. Designers seemed to be slow to recognize this problem, but by WW2, vertical tails had grown enormously, particularly in bombers, where the vertical tail had to resist the heavy yawing forces caused by flight with one or more engines out.

For an experiment, I tried progressively, and ultimately, enormously increasing the vertical tail area of one of my own design aerobatic bipes by taping on cardboard chord extensions. I discovered that there seemed to be no upper limit to vertical tail (fin and rudder) area. The bigger the vertical tail, the deeper the airplane could be flown into stall without trying to snap roll. The only downside seemed to be additional drag and weight of the oversized vertical tail. I normally design vertical tails to have about 15 percent of the wing area, and even more helps handling, but starts to look a bit weird.

rmh

This turkey was developed for eye appeal with no regard for aerodynamic needs.
The original Granville full scale craft were not much better at first - being simply an attempt to streamline a big radial engine and ad enough flying bits n pieces enough to control it
later Granville models were compromised to provide better flying qualities .