Lengthening Fuselage

[Stephen Bennett]

Hi,

I,m currently in the process of drawing up a Biplane in Autocad. I've traced around the three view and scaled it up to the wingspan i want.

I now want to lengthen the fuselage as the plane as a short moment arm, but also as a short nose moment.

I,m planning on powering the model by electric so a longer nose would make balancing for the C of G easier.

I want to know, is it ok to lenghten the fuselage both a the front and rear of the wings. And what effect if any will this have on the wings being in a different position, and the flight characteristics.

Hope this makes sense,

Thanks

Steve.

[Brandano]

More inertia on pitch and jaw. Should be fine for moderate increases in length, but if you end up in an F104 or X3 layout you might experience inertial coupling issues. I think it will also affect the spin behaviour. My "gut feeling" is that it will be more prone to spinning, but I have no hard technical background for this, other than the odd distribution of mass and the inertial coupling effects. Another way to think of this is: I have a biplane and I want to trim all wingtips. How will this affect it?

[BMatthews]

What design is it and what are you hoping to gain from lengthening it? If it's a model already and it was a succesful flyer then you're messing with success and the mods may ruin the very thing about it that drew you to the design in the first place.

But if you do go ahead with it then lengthening it to equal %'s is the right way. Just don't get carried away or the stuff Brandano mentioned wil

[Stephen Bennett]

The plan that i,m drawing up is a Pitts. I've built quite a few models from kits and plans and now want to attempt something totally from scratch.

Since many models of the Pitts seem quite twitchy ( i know this is their character anyway) i thought lengthening the fuselage slightly would make for a more controllable model.

Having read in threads here that people have extended the fuse on other models to help this and get better aerobatic performance. But there is no mention whether is was extended for or aft of the wing.

Steve.

[CloudyIFR]

In what axis are you trying to reduce the so called twitchiness?

If it's in the pitch axis you'd reduce the elevator effectiveness by increasing the distance of the tail from behind the wing, but in so doing you'll need to decrease the size of the horizontal and even perhaps the vertical tail. What percentage I'm not certain of. Also, you'll of course need more nose weight to balance properly as well, as you'll in effect be increasing the weight in the tail.

If you're trying to reduce the stability in roll then you'll either need more weight in the wingtips, more wingspan or reduced aileron area.

You stated that you're vying for better aerobatic performance, if you are meaning for more stable aerobatics you may achieve that in the Pitts design, but if you wish for more wild throw around the sky aerobatics you're going the wrong direction with the design.

For ground stability during taxi/takeoff and landing look at the gear spread on my old Gee Bee Dreamer, it was a great all around flying bipe.

Curtis
Montana

http://h1.ripway.com/cloudyifr/images/geebee.jpg

[Brandano]

Lengthening the tail arm might make the aircraft more stable, but it will also increase the control authority since the elevator leverage arm will be increased. I don't see why the plane should be any less "twitchy"

[Stephen Bennett]

Curtis,

Can i have both stable aerobatics and something a little wilder (3D) perhaps ?

What direction should i be going in for something a little more aggresive.

The Pitts i,m drawing up is the Python so the aircraft shape is slightly different from the earlier Pitts designs.

Brandano,

You think the same way as me about the increase in fuselage length.

At the moment i have increased the length of the fuselage by 1" in front of the wings and 1" aft of the wings.

I think it will be a case of finishing the drawing, building the model and just seeing how well (or badly) it flies.

Steve.

[Texas Buzzard]

If you want to dampen out the quickness to control inputs of a Pitts bipe:

1. Yes you can lengthen the tail moment arm or;
2. Keep original fuse length but enlarge the Area of the tail surfaces. Most kit models of full scale planes do have enlarged tail surfaces. This makes for a simple fix.

Usually a place to start when scratch designing is to have the distance from the CG to the hinge line of the elevator equal to between a ratio of 2:1 to 2.2:1 relative to the distance from the CG to the rear of the prop. OR
The tail moment arm is 2.2 times the nose moment....you can make the tail moment even larger for more stability. But this may force you to have to add weight to the nose. Ha, Ha it's said the design of an airplane is a comedy of compromises. :) :rolleyes: :)