|Feb 27, 2004, 01:56 AM|
A Few Questions--SAE Aero Design
I have a few questions relating to the SAE Aero Design competition. Here's a thread discussing other aspects of the competition. I'm the team captain for Gonzaga University, and there's been a few things I've been pondering, but have not found solid answers for yet.
1. I've been told that a tapered wing with straight leading edge (like an Edge 540) has better low-speed characteristics. Is this true, and why/why not? And would it matter for our plane in the competition.
2. Besides finding the thrust of the engine/prop combination, what else can one get from static thrust measurements? We would also be measuring the RPM.
3. We are considering winglets, but do not have the capability to run software or test in a wind tunnel. Are there any wingtip devices (tip plates, winglets, straked wingtips, etc.) that seem to generally work better than others? And could these be used on our aircraft with little customization?
Feel free to ask for clarification.
Thanks in advance,
|Feb 27, 2004, 05:33 AM|
Punta Gorda, FL
Joined Apr 2002
1. The aircraft will be operating near the stall when carrying maximum weight. There are several interesting trade offs involved. On the one hand an untapered, untwisted wing is quick and simple to build accurately. On the other hand a wing that is designed for minimum induced drag will have a highly tapered planform as will a wing that is designed for minimum spar weight. However wings that are highly tapered have vicious tip stall characteristics that put the plane at serious risk of crashing when flown at low speeds and low altitudes.
2. No comment since I am a glider guy.
3.Winglets are hard to design and hard to build accurately but you might get extra points for design sophistication. With unlimited wing span beyond 10 feet the simplest way to decrease induced drag is to increase aspect ratio. You should look at the trade off betwen a simple, easy to build wing of high aspect ratio with good tip stall margin versus a more complex planform with tiplets and the associated tip stall tendencies. The higher the maximum lift coefficient of the airfoil,the more importance has to be assigned to induced drag reduction. So, airfoil(s) selection involves a trade off with the rest of the wing design. A drag budget is a useful tool in weighing design alternatives and establishing project priorities.
Most development projects take at least twice as long as planned so there is a trade off between complexity and simplicity when there is a deadline involved.
The less experienced the design team the more safety factors are appropriate in all aspects of the project.
|Feb 27, 2004, 08:19 PM|
The 200 foot takeoff requirement puts most of the performance capability on the motor-prop.
The plane MUST be in the air at 200 feet.
The less rolling resistance, the better.
The more static thrust, the better, as thrust diminishes with speed.
Figure what load you expect to lift.
Figure the acceleration capaibility of your motor-prop with that load.
Adjust the angle of attack so a minimum amount of elevator will lift the plane off.
Accelerating rapidly, then a large change in angle to take off does NOT get the plane off. it must kinda cruise off.
|Feb 27, 2004, 08:34 PM|
The objective of the SAE Aero Design competition is to design an RC airplane powered by an OS .61 FX engine to lift at least 8 pounds in payload weight, taking off within 200 feet and landing safely within 400 feet. The rules this year dictate that the wingspan be NO LESS than 10 feet, and the payload bay be able to hold a 4" by 5" by 6" box. Besides the flying part, there is a design part, consisting of a written report and oral presentation, as well as submittal of plans.
The official website:
|Feb 28, 2004, 12:05 AM|
We haven't gotten to any sort of payload prediction yet, but we're shooting for at least 20 pounds of payload weight. We want the airframe to weigh 8 pounds, but it will probably end up being closer to 9 pounds, which is reasonably acceptable.
|Feb 28, 2004, 01:01 PM|
A Lockheed Martin Technical Fellow, Dr. Leland M. Nicolai wrote a technical paper on the SAE-AIAA airplane design parameters.
It was on the SAE site, but I see it's vanished..
I uploaded it to one of my sites:
"Estimating R/C Model Aerodynamics and Performance"
2,010 KB.. Sorry 'bout that...
It takes the design from the first pencil scratch to takeoff.
|Feb 28, 2004, 02:26 PM|
Interesting, a minimum wingspan limit this year. Those are going to be heavy models compared to a couple of years ago, since the high Cl(max) airfoils that have been popular are Reynolds restricted (min 200,000 for the 1223). That would mean a min chord of about 12" @ 25mph...and a min wing area of 1400 sq" (12"x10'). Bring out the composites folks!
That paper is cheating...it explains most of the hard parts about designing these models (at least for the report)
1) Planform is only one part of the equation, but it is important. The situation you describe leads to a wing that usually stalls with a nose-down pitch, which is fairly safe. Beware that the taper ratio doesn't put your max local Cl too far out towards the tips, however.
Stall behavior is very important for these models, since they are operating at highCl/high alpha most of the time. Any attempt to reduce the load on the pilot (not worrying about a spin, for ex) will be very valuable...and very appreciated by your pilot!
I have a spreadsheet that might be helpful if you want to play with planform. It's an extension of LiftRoll, among other things. Post an email address if you're interested.
2) Be careful with these measurements, and remember that engine performance will vary widely from place to place. Compensating the measurements was tricky for our team.
3) Winglets are more useful when your span or planform is restricted. The best winglet is a bit more wingspan!
|Feb 28, 2004, 06:03 PM|
I recommend that your team develpe Thrust required (drag) and thrust available data for you aircraft and engine/prop combination using payload combinitions from 0 to 25 lbs. I wrote a paper in 1991 called "Model Airplane Performance Analysis" and published it in the NFFS sympo of that year. You may want to get a copy for some of the equations and methods. I will be sending you 4 files from that paper illustrating my basis of computation.
|Feb 28, 2004, 07:44 PM|
After the 2000 SAE Design West competition, where several of the MeCoA constructed K&B .61s melted parts, SAE investigated alternative engines.
I did the testing, using a K&B and OS 61 FX which I already had, and the Thunder Tiger and Magnum 61s, which they supplied.
All of them were quite close to each other in performance, but none of them exceeded 8 pounds of static thrust.
We decided on the OS as the best suited alternative, although any of the others would have just as good.
A careful break-in makes all the difference.
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