Hey guy's,

When designing a twin boom aircraft, what can be expected in performance in horizontal (perpendicular to the fuse ) area
vs lateral (in line with the fuse and between the booms ) area?

I understand that an area close to 25% of the wing area is required on conventional aircraft, but if the area between the booms stays with this theory, will the aircraft retain stability?

Lets say the distance between the booms is fairly narrow,
12 3/4". The wing span is approximately 68". Will extending the area between the booms in a lateral direction give any additional stability if you extend it to the 25% mark in relation to the wing area? This would also include the area of the elevator.

I am in the process of building a turbine powered model of a flying car. I have built and successfully flown several pusher prop versions and a ducted fan version of this aircraft .

I have flown one with no over hang on the stab, but it needed a gyro for pitch stability.....I will try to attach some pictures.

Thanks
Rod Springer

There are three ways to increase stability:
1. Move the CG forward.
2. Increase the tail area.
3. Increase the tail moment arm length.
The stability is measured by the distance the CG is ahead of the Neutral Point (NP). This distance is called the Static Margin (SM). The first method leaves the NP in place and increases the SM by moving the CG forward. The second and thrird methods increase the SM by moving the NP aft.

In tailless aircraft the tail area and moment arm are zero and the NP is at about 25% of the Mean Aerodynamic Chord (MAC) of the wing. The flying wing becomes stable when the CG is ahead of 25%of the MAC.

For conventional configurations a number called the tail volume coefficient (TVC) comes into play. TVC's in the range of 0.4 to 0.6 can be made to work well with CG locations between about 28% and 40% of the MAC. The larger the TVC the more aft the NP will be. Trainers want a more forward CG (larger SM of about 7% of the MAC) and aerobats want a more aft CG (smaller SM at about 2% of the MAC). The TVC is the tail moment arm length exprssed in multiples of the wing MAC times the tail area expressed as a fraction of wing area. Planes with similar TVC and CG locations will have similar stability. Armed with basic aerodynamic principles and a fundamental understanding of stability, you can throw out all the rules of thumb that sometimes lead to wrong answers.

Ollie, Thanks for the insight.......

I assume from what you are saying that by extending the tail moment arm, which I have done, and increasing the area of the stabilizer area, which I have done, and by keeping the SM at around the,say, 5% of the MAC would/should be a fairly safe bet?

By increasing the area of the stab, does it matter if it is an increase in width vs length, or are we just talking " area ", not looking at shape?

Rod

Increasing the height of the surface, and thereby it's aspect ratio, will be more efficient but I'm not sure the difference will be noticeable on an R/C model.

Still I'd increase height as long as you don't encounter any structural issues. It will most likely get more of the stab up into cleaner air and provide a little more efficiency that way as well.