One of my clubmates (not into computers) has scratchbuilt a 1/4 scale WW1 Bristol FB2 biplane and now needs to know how to work out the C. of G. position.
Anyone out there got a practical formula or guide on working out CG for Bipes, taking into account stagger, differing chords etc?

Brian

Tall Paul had one of the simplest and most obvious rules of thumb. Look at the top view and do the CG as if it's only one wing with the area being only what you can see. Don't worry about the extra area hidden under the top wing. Only what is visible in a top view is the outline of the equivalent single wing. Assuming it's not an overly small tail it'll work as well as anything.

Besides, the FB2 had same sized wings with no stagger so the CG is the same as if it was a single wing.

This has worked for me: From Lennon on Bipes.

Les

Thanks guys

I've printed those off and passed them on, looking forward to seeing the model turn up at our field. It is, of course, an F2B not an FB2 - I'm a WW2 warbird fan and don't know my bipes very well.

cheers
Brian

Full-Scale NACA wind tunnel tests conducted in the 1920s and early 1930s indicated that the center of pressure of a biplane wing combination was located at 22% to 23% of the MAC (mean aerodynamic chord), compared to 25% to 26% for most monoplanes. The reason for this is not clear, as far as I know, but bipes need their CGs slightly farther forward.

I wonder if this is why my Lazy Ace wants to baloon on landings. I am balanced at 25% MAC according to the plans but still wants to rise on flair.
Incidence is 0 - 0 all wings and hor. stab.

Big Al,

If the craft balloons on landing then the air speed is still to high at the point of rotation or round out.

I have often reduced the prop pitch to as low a pitch as possible in order that the idle engine RPM thrust will not still overcome the drag factor. The craft will then slow as the nose is slowly raised, which of course increases angle of attack and adds more and more drag thus increasing sink rate.

I practice attempting to hold the craft off the runway at 2-3 inches for as long as it will fly. The idea is for the craft to no longer be flying at the touch down point. Air speed too low and angle of attack to high equals a stalled condition at touchdown.

Another hint, is to trim the craft for landing just as you would when flying a full scale craft. On the down wind leg, power is pulled back and the elevator is reset for a descending glide. If this is done correctly, dropping power will steepen the descent and not gain speed as the elevator is controlling the angle of attack and air speed. If the craft needs to land further up the field, add one notch of power and the glide slope will be at a reduced angle and lengthen the touch down point. When you round out just off the run way, reduce the engine to the idle setting.

Best regards,
THC

By "baloon on landing" I'm wondering if you mean that it wants to nose up after charging in to the landing. Any airplane will tend to nose up if it has extra airspeed. And having the CG on the forward side of things makes it worse because of the extra up trim needed to compensate for the forward CG.

If you can confirm that your model has a very strong nose up tendency when the airspeed comes up a bit you can try a couple of things.

First move the CG back and retrim for level flight. How far? Try some dives and release the stick. Watch how it recovers. For a model the size of the Lazy Ace I would suggest that a dive recovery from a 30 degree nose down dive at 1/2 throttle should take about 80 to 100 feet of height to recover between neutralizing of the stick to level at the bottom of the first pull up. If it happens any sooner than you're probably struggling with a too far forward CG.

Secondly learn to slow the model a bit more in the landing pattern by flying the model with some back stick pressure but not so much that it's on the verge of a stall. Many models will settle into a nice descent rate when flown at a slow speed like this. Practice a few mistakes high at first to get the hang of it. The idea is to avoid the stall by a wide margin but to not let the model drop into a shallow dive in the glide. Here again if your CG is quite far forward you're probably running a lot of down trim to avoid the nose high pitching when full throttle is added. That same down trim works against you at low throttle by forcing the model into a fast glide or even a shallow dive. Adding up elevator to flare before touchdown will turn that extra speed into a "balloon". The cure may be as simple as learning to add some up elevator trim to flatten the glide before you start your landing approach. But if you do be ready to push forward slightly on the stick if you decide to add power for a go around. That up trim will tend to make the model pitch up into a stronger climb than you're used to. Going back to the first trick to move the CG back and retrim the elevator will help to minimize this power off to power on difference but it won't eliminate it entirely.