Helicopters B

[chalker7]

I like and encourage the curiosity, exploration and research! I was honestly just curious, not saying if it is necessarily correct or incorrect. I suspect the true optimal angle of attack is highly dependent on the reynolds number of a lifting surface (e.g. 15 degrees would be extremely high for a supersonic aircraft.)

One point to think about is that on most helicopters (and propellers,) the angle of attack changes from the root hub to the tip. This is for a practical reason. If we ignore the impact of the size of the lifting surface (specifically the chord,) drag effects and shape of the airfoil (big things to ignore, I know!) lift can be summarized (up to a limit) as: lift = angle of attack * airspeed across surface. In our small models, we can assume the airspeed is how fast the lifting surface is moving through the air (wind isn't usually a factor.) In a wing this is easy since it the whole thing travels at the same speed.

On a rotor or propeller it's a bit different since they are rotating. When something is spinning around, even though all parts of the radius are moving at the same rotational/angular velocity, different parts are moving at different linear velocities. Specifically, the tips are moving faster than the hub. That means that if the whole lifting surface is at the same angle, the tips would be producing more lift than the hub. That's a problem for a handful of reasons, fundamentally that the tips will be generating more lift than the hub and this will put a lot of strain on the airframe and possibly even break things. To address the efficiency question from above, it might not work very well if the tip is at 15 degrees and the rest of the rotor is at some angle greater than 15 degrees (which would be the situation in the majority of layouts) since the various distances of the rotor would be producing different amounts of lift.

[chalker7]

Also, just to add something to think about, the caption for that image says "a typical lift coefficient curve," with typical being the key word (meaning there may be other curves for other conditions.)

[jander14indoor]

Let me repeat again, angle of attack is angle of surface to relative wind, NOT physical angle of blades.

The shape of that lift curve is VERY dependant on the airfoil shape, including the optimal angle of attack vs drag!!! It is NOT always at 15 degrees. In fact, reading that curve I can tell you the airfoil behind it had a high camber (indicated by zero lift at NEGATIVE angles of attack. A symetrical airfoil will have zero lift at zero degrees!!

See the following reference for another lift curve where the peak is at 20 degrees! More important, note that the max lift to drag ratio is at SIX!! degrees.
http://www.lets-go-fly.com/Lift%20and%2 ... 20Wing.pdf

For an indepth look at airfoils, in sizes and Reynolds numbers appropriate to model aviation I suggest the following site:
http://www.mh-aerotools.de/
Dr Hepperle is a world renowned expert on airfoils and aerodynamics, especially in areas related to model aviation, where believe it or not, things DO work differently than full scale airplanes. Yes the underlying principles are the same, but turns out, aerodynamics is tricky to scale since size actually DOES matter.
Some really good stuff there if you are interested. He even models typical model wings made with ribs and spars to show the effect of those shape distortions on the wings performance!
And an in depth analysis of propellors (remember, a rotor is just a propellor pointing straight up).

And contrary to an earlier post, drag is ABSOLUTELY CRITICAL in these events. Yes it is low, but guess what, so is your energy, and in this duration event its all about drag vs stored energy!!

To contradict my sometimes partner in crime, chalker7, you actually DO want the angle of attack constant along the radius. But like he said, you can't get there by having a constant physical angle along the radius due to different speeds at different radius.

Finally, unless you are messing about in a wind tunnel, or have a hellacious powerful fluid flow simulation, you really can't measure the angle of attack on these things. So don't worry about it, its not very useful to the problem we've posed to you.

Instead, worry about things like reasonable blade design. Helical foils, which fall out of the x-design props automatically. Appropriate surface areas. Appropriate aspect ratios. Low drag. How many blades. etc

THEN, VERY IMPORTANT, you must match your motor to your rotor design for optimum performance. And it will be unique for each design.

Frankly, I suspect the rotor design is pretty forgiving as I've seen a number of variations with similar performance once the rubber is matched to the rotor.

Regards,
Jeff Anderson
Livonia, MI

[chalker7]

Ha, ok, I'll differ to Jeff in this situation. The more important point is that this stuff is complicated and there isn't necessarily a single correct answer for everyone. Experiment and figure out what works best for you! The only data point that ultimately matters is the time on the watch at the end of a flight....

[Bazinga+]

Thank you very much, the links really helped...

[Somicvs]

Yay going to MI state again. I hope this year they are more strict in following the rules for helicopters. Disk or other stuff fell off & they did not stop the time. Another ? How do we find out the times for other regions?

[1nxtmonster]

Shaving off .4 grams and increasing rubber length to 1.5x body weight added 20 seconds to my time. How will 1:10 do at states?

[Bazinga+]

I dont think it would place at NYS... but idk about pennsylvania. Last year our team got 2:20 i think and we got 3rd. Best time was 2:50... pretty competitive, but 2:50 could have gotten like 2nd at nats, but the team didnt go to nats. But i think penn should be a little less competitive.You should do fine, but i think you should try to get around a minute 30 and then you can relax.

[AlexNo]

Hey everyone, I need a helicopter kit fast. What are some websites I can buy from? My price range right now is $20-$100

[hogger]

Hey everyone, I need a helicopter kit fast. What are some websites I can buy from? My price range right now is $20-$100

http://www.freedomflightmodels.com/