Sanzuwu SSTO

[pa1983]

Because I wasn't sure, I thought I'd go test this with a purpose-built plane. Behold, the LowHigh:

The only difference between these two craft is that the body was rotated exactly 180°. The wing was placed before placing the landing gear (because as we know, gear is massless and does not induce drag outside of the SPH) so that the direction of lift passes through the CoM ball but slightly infront.

I allowed both aircraft to get airborne before trying any manoeuvres. There was a difference of about 20m/s between taking flight as the gear is placed slightly differently. I waited until they were both doing 130m/s in level flight and then kept my finger on the S key to perform a hard pitch up without fine controls on. The high wing initially pitched up fast but then the rate of turn slowed as the direction of travel lagged behind where the nose was pointed. At no point did it flip.

I tried exactly the same thing in the low wing. After getting past about 40° it flipped over uncontrollably.

This problem is clearly caused by not having a separate stabiliser. With the addition of this T-tail, the low wing test aircraft became controllable and was able to pull much tighter turns than the high wing (with or without the tail). Note also that the engines had to be moved to bring to CoM further back as I didn't want to have to move the wing section.

Lastly, I tried a low stabiliser placed on a length of massless cubic struts. I also removed the front elevators that can be seen embedded in the wing in the first two pictures in order to bring the direction of lift further back so that I could compare it to the previous T-Tail version without moving the main wing. The aircraft performed well, and although it didn't turn as fast as the low wing with high T-tail, it did not flip.

In conclusion, I was correct that low wings are inherently unstable, but clearly this is only true when they don't have a separate stabiliser. Originally I thought that having a pair of elevators on the leading edge of the wing as well as the trailing edge would act the same as having trailing edge elevators and a separate pair of stabilisers, but they obviously don't. This then is what's causing some instability at high rates of turn in the fully fuelled Sanzuwu. I don't think it's much of a problem though as long as you don't pull any fast turns after taking off

Your correct that flipping that craft was unstable. That do not mean others cant make perfectly stable low profile winged craft. I never sad your craft was stable just that low profile winged craft can be perfectly stable.

I can dive at full power strait down and pull up hard and never let go of the S key and still get out of the dive with out flipping. If you dont believe me I can make a video of it. Its a Delta winged craft.

All my latest craft are low profile based because its overall better concept if you ask me. Spreads the load better on the ground and in the air.

For the first 6 months that I did spaceplanes I made them with mid profile wings because low profile wings percent more challenges then your typical sandwich mid mounted wing. But doing one test and conclude that they are bad because they might be hard is not a good idea.

There perfectly doable. I can keep holding S and do loop after loop and it will never flip.

Crafts flipping is caused by many factors not just one. If the craft is to rear heavy it will flip because the rear end is just waiting for an opportunity to travel first. Make the craft better balanced and a low profile wing will not be a problem.

Most beginners also use control surfaces inappropriately. To many are often used because they are trying to compensate for ASAS behaviour or the simple fact that CoM is moving or out of place relative to CoL etc etc. More control surfaces are not better to solve misbehaving crafts. Theres a fine line that offers just the right amount of authority with out compromising stability or responsiveness. Placement of control surfaces are extremely important and if you ask me almost always never placed optimally.

In the end it do not mater if a high mounted wing in theory is more stable if one can create a perfectly stable low mounted winged craft. End result is what mater.

So like I sad low profile winged craft are perfectly stable and doable if done right.

Edited October 11, 2013 by pa1983