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.

Tuesday, August 9, 2011

Smoking Ace Pt.1

Ok, this is not the most verbose of howto's .I will start explaining the most important parts of the pics belo.w.
These are the basic structural elements of the Smoking Ace fuselage.

One thing that I have to say is that it is going together quickly. There are some unforeseen technical bits trying to assemble the fuselage. Mainly around the stiffness of the joints between the sections. The fuselage is 42" long at the moment. A tail and nose cone have not been added, I hope to have them and the lid done soon.

Special thanks to Bill Sheffer at Unique Cutting & Metalworks for his help and support on this project!

Saturday, July 2, 2011

Coming Soon: How to Build a Smoking Ace

Hey everyone,

Next month I will start a simple construction project, demonstrating how to build a "Smoking Ace". It is not always pretty but the goal is to be straight. Please check back in the next few days or weeks to see our progress.

Friday, March 25, 2011

Adventures in Aircraft Model Validation

After several months of working on the Smoking Ace, I have learned one thing. Balsa is not your friend.
Aluminum is not so bad, but it is what it is and you need to make sure you bolt it up.

We have been working on conreol systems for the plane. It works like a normal pusher. The simulator models that we use suggest that it is pretty predictable to fly. I hope that is the design and not the simulator dumbing things down, so that my feelings are not hurt. Our primary model is X-Plane 9.3. It was pretty easy to build a model in the system, for what it is worth. I could plot out a simple airfoil and plot out the weights and measures of the plane as we are predicting them. I had to use a custom airfoil, our low moment  airfoil is well understood, but is not super common in general libraries. We use the e204 with a 16" chord.

We chose 16" because it allows us to get 2000 square inches of wing area in a reasonable span. No, there is
nothing reasonable about wings that are longer than I am tall by a ways. That would be fine, but there are two of them. Which means we have to take it outside to assemble the plane. As a reference, this is gives us a do not exceed gross vehicle take off wing loading of 50.2 oz/square foot.This is a little high, but for a 14' wingspan it is not out of bed.This is all in the batteries. This is an electric plane, with a 65cc equivalent motor driving a 22" fan.

This configuration should give us around 30 pounds of thrust. Hopefully, this will be more than enough to keep it flying. The plane's primary mission is to fly straight and level and take pictures and instrument readings. I think that the configuration will be successful at this. The Smoking Ace turns like a school bus, so it would be unexpected for us to try and snap roll it.The sim says behaves like a normal pusher configuration.

I mean that unlike a puller plane, when you put on the power you have to put on elevator or the nose dives. To compensate for this a bit our elevator is set at 4 degrees nose down. This us effective at counteracting the
offset of the line of thrust to the center of gravity. There is no vertical tail in this configration. So we have a soft feeling rear end. Combined with our shortish, blended fuselage, means that we have to use the inner  ailerons sometimes in flight as poor man's elevator. Long wings and an aggressive sweep makes this an effective technique. Double, mid-span control surfaces help to give us an effective rudder. Differential roll inputs counter-act the other roll inputs and change the drag profiles of the wings and developing a yaw moment with little roll.

In the next generation of the aircraft, there will be duckerons on the wing tips. Yeah, I know this is not an
approved, aerospace term. Which was discussed in an earlier posting. Properly termed, they are drag rudders.

Using X-Plane sizing them was pretty easy. Sure I can estimate that a drag force at the tip of the wing will
 result in a given yaw moment. However, it is hard to estimate the tip effects on approach when the drag rudders are too close to the wing tips. We found that they should be one chord or so from the wing tip. Farther out made them more effective, but it also made them impractical during low altitude maneuvers. They were so effective that they burned off airspeed and slammed the plane into the ground. Watching your model cartwheel wing tip over tip is exciting, but depressing.

NOTE: You have to release the brake when you are in X-Plane. Otherwise it has some crazy effects... I would have  thought that if you kept making the same mistake, they could figure that out and say... release the brake... or if you throttled up and the plane was not moving... some other visual cue may help.

On the other hand, I kind of thought that most of the planes were kind of lackluster to fly. The differences
between a Piper Cub and a 747 were not as striking as I would have hoped. A Cub should have felt a bit under powered and then feel kind of flippy. When it got into its acrobatic range. Where the tail and rudder became effective nearstall conditions.

If you want the latest revision of our model, you can get it here. If you find something interesting, or if we made a mistake please feel free to comment at info@fatmanflying.com .

Sunday, January 23, 2011

The Smoking Ace is in the jig

I got the parts for the Smoking Ace this weekend from Bill at "Unique Cutting and Metal Works". Bill is great to work with and help us on this project.

The parts are coming together well. They were bracketed this weekend and got a coat of enamel. The issue that I see at the moment, is that small variations in a straight line make a big difference now that the spars are weight reduced. Lots more small radii to worry about. I do not think it is a big deal, but I lost a bunch of holes because they were too close to other penetrations.

The best thing, that has come out so far, is that the battery carriers really look like they fit as designed. That is cool, I hope that I can get the heat pipe design to work too. One of my coups of engineering will be to steer some of the motor, esc and battery heat to the same spot across a Peltier device and out to the free stream. I am not sure it will be worth more than a few volts, but something is better than nothing. Peltier recovery is one of several technologies that we hope will get our electronics power-management efficiency up.

I will get some pictures up as soon as the fuselage gets a bit further along. One of the most interesting things about this design, is that it is based on triangles. Any third year structures student will remind you that triangles are the only self supporting shape. Self-supporting makes the air-frame easier to construct because it tends to stiffen up as you assemble it. This means that you can break the assembly up a bit more than normal and things self-support and then support when assembled. Anything that makes this easier to do is good.

More to come.