Tuesday, April 24, 2012

If the question is to use forced induction on a uav...

The answer is probably, "Hell Yeah!". I have been learning everything there is to know about practical, turbocharged-systems. So over the next few posts, I will hopefully generate a reasonable technical approach to safely turbocharge a 4-stroke Briggs and Stratton Animal engine or a Raptor. It depends on which one I can find and afford here in Central Europe.

We have found a few 1.5# turbos good for 3-4psi of boost. The mad scientist in me says that I think we can find some light super chargers and run them on a small electric motor for less weight. I know that the whole point of a turbocharger is more about recovering wasted energy in the exhaust system. However, I wonder if the weight/power generated ratio would be better served with a high efficiency electric motor. Since they are pretty amazing these days.

As I come up with some numbers for weights, and possible concepts, I will keep everyone in the loop.

Friday, April 20, 2012

Smoking Ace Update

We are still alive. Just working on some other projects at the moment.

The Smoking Aces are still under construction. After flying the planes for hours on simulators and finally the real thing, we realized that you can't cheat physics. It makes for some interesting behaviors in short-coupled tail designs. What is kind of cool is that deeply-swept wings actually set the maximum angle of attack for the aircraft. We surmise that it is due to over-wing airflow. As the aircraft pitches, the airflow over the wing obscures the clean flow over the tail and finally completely blocks the flow.

Not so much block, but the distance between the tail and the wing is shorter than the mean turbulence. So the dirty air is not laminar enough to make a well-circulating flow around the tail. As the tail loses laminar flow and finally gets only turbulent flow it loses lift and finally stalls. This reduces the effectiveness of the tail and the plane rotates back to balance the tail input. This seems reasonable enough an explanation to the pitch wobbles, that are only partially-tamed by gyro input.

The first time we saw this behavior, it was exciting. Our pilot certainly thought so.

There is even a slight regression in our projects. Not so much as a negative thing, but more an artistic reawakening. I have to finish working up some drawings so we can discuss some design requirement changes. Building the Smoking Aces, we discovered that what we thought was a lot of space, was not. We also found out that 1000 in*lbs is a tough thing to take out in such a small space. Considering each wing applies this, we found some strange deflections in the air frame when the wings support large flight loads. These were certainly not unexpected, but we did not realize that our load paths were not as well controlled as we though.

Looking back through some old documentation, I found some of our original "Fat Man" designs. What we learned with the "Fat Man" was that fuselage depth or in its case root thickness was our friend. If you have healthy wing roots, there is a lot more space to manage payloads. More so, there is a trade off with larger thickness designs. The trade-off that is important is that often their curvatures do not change much.

When you build metallic structures, you see the need for structural caps. Without them, it is hard to fix skin to the structure without some heroic measures for the most part. Next time I will talk about some of the things that did not work for us.