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Sep 22, 2016, 08:08 PM
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Advice for building 5 ft wing span/12 lbm model airplane?

Hi. I'm modeling an RC aircraft in my class project. The design of the aircraft is very simple. Rectangular wing with 5 ft span and 8 - 10 aspect ratio, weight is about 12 lbm, and the plane will look like cessna. It doesn't have to do aerobatic flight. Just fly up to around 300 ft with some load for more than 30 min. Since we just started the preliminary design, there are still many things to decide.

First thing I want to know is, in order to estimate the lift coefficient we need to know the average take-off/free stream velocity of the plane with this scale. My guess is about 30 mph at take-off and 50 at trimmed flight, but is it a good guess?

Second, what airfoil shape is ideal for this type of plane? I'm thinking semi-symmetric or flat bottom is suitable, but I'm not exactly sure. Do you have any suggestion for this? (from NACA 4 digit airfoil if possible)

And third, how to determine the design/size of horizontal/vertical tail? I think they are important for stability and control, but I have no idea where to start...

I would appreciate any kind of advice.
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Sep 22, 2016, 09:06 PM
Sink stinks
Montag DP's Avatar
It sounds like you are asking us to do your homework for you. If you are in a design class, I'm sure you are supposed to think critically about these questions yourself and come up with solutions that you can justify based on your understanding of the course material. So I suggest you review that if you don't know where to start.
Sep 23, 2016, 01:18 AM
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Tritone FPV's Avatar
Question 1 and 3 can be answered just by searching the web, and i think some of those questions have been asked in this forum already. But as for the airfoil that would depend on what flight envelope you'd like to achieve. e.g. under-cambered airfoil for slow flight
Sep 23, 2016, 08:20 AM
You can learn a lot from "Model Aircraft Aerodynamics" by Martin Simons. I think it is availabe from Amazon. Have fun. Learn a lot !!
Last edited by Marion; Sep 23, 2016 at 08:28 AM. Reason: Corrected book name
Sep 23, 2016, 04:26 PM
B for Bruce
BMatthews's Avatar
While such a model will fly it won't be fun or easy to fly. With that span and aspect ratio the wing area will be very low. And that means that the wing loading will be extremely high. Higher than is at all practical to fly. And even if it were built with an aspect ratio of as low as 6 the planned weight will still make this a very difficult and touchy airframe to fly by anyone but an expert. And frankly I'm not sure they would be able to fly it without some disaster either. Certainly at the very least a paved takeoff and landing area would be required.

Keep in mind too that while lift charts show results up to around 12 to 15 degrees angle of attack to reach the maximum lift coefficients that at the Reynolds numbers on models it is difficult for a model of this size to reach an angle of much over 8 AoA before separation and stall occurs. So to be safe don't look at data that is either way over the Rn you will be operating at or at data that is roughly in your Rn range but which extends to over 8. You are very unlikely to see your wing reach those values.

A suitable weight to produce a wing loading that is still overly heavy but which should be at least flyable on that size of model would be more in the range of 5 to 6 lbs if you keep the aspect ratio as high as you're considering. If you reduce the aspect ratio to more like 6 to gain more wing area then you could tolerate up to maybe 7 lbs. Even at these 5 or 7 lb scenario the model would still not be at all friendly for an average skill RC pilot. The takeoff and landing speeds will be very high. I suspect it'll be every bit as much as your 30mph speed guess. Even flaps won't alter that by enough to make a big difference. And while in flight care will be needed to keep the speed high and avoid a stalling into some form of spin and probable crash. A far better goal to produce a flyable model would be 5 lbs takeoff weight for the AR=8 scenario or up to around 6 lbs for an AR=6 version. Much over that and you really need a skilled and practiced expert pilot if you expect to have a chance of survival. And that long paved runway area to allow for a shallow and deliberate liftoff and climb out.

30 minutes of flight will also require an extensive size battery pack or a larger than normal fuel tank. And if it is glow powered that means a main tank and small hopper tank with some sort of float valve. Or a fuel pump with a pressure relief and bypass return such that you maintain a constant but low pressure at the needle valve.

Clearly you have no one on your team at the moment which is a practiced RC flyer or they would have already commented on this issue. And you're going to require a pilot at some point anyway. It would be a good move for your team to locate someone with a good background of flying heavier than typical models locally to act as a resource and eventually your team pilot.

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