View Full Version : Flight endurance of a model and a full-size rotorcraft.
09-13-2006, 07:45 PM
A theoretical/hypothetical question.
Assuming that there are two absolutely identical helicopters; the only difference being that one was an exact 1/4-scale version of the other. They are both required to operating in the same medium (air) and atmospheric conditions. In addition, their absolute likeness means that no adjustments are made for differences in Reynolds Number etc.
Which craft would have the greatest flight endurance?
09-14-2006, 09:28 AM
the full scale
09-14-2006, 12:16 PM
The question is related to the viability of an electric UAV helicopter, based on today's level of electro-mechanical technology.
I understand that an electric RC helicopter has a very limited flight time of 15 - 20 minutes. The extremely generalized question was to find out if scaling up an electric UAV helicopter would improve the flight time and/or range over its smaller sibling.
09-14-2006, 06:53 PM
I think you should figure in power density and weight to get a realistic answer.
09-19-2006, 07:03 PM
You cannot get an acceptable rotor figure-of-merit with a symmetrical airfoil below a Re of 1000K. That's why all RC helicopters have disk loadings that are at or below 1 pound per square foot. You can, however, get FM up in the .8 region or higher if you are very careful with your airfoil selection, but trimming the pitching moment in the real world (even with profile reflex) is a dark art at best.
You can increase chord to get the Re up, but then you hit the induced drag factor of increased solidarity ratio.
Paul Pounds is a acquaintance of mine:
His problem is Re < 100K, but the point is that air just doesn't scale the way we'd like it to. He avoided the pitching moment dilemma by using fixed pitch. ;-)
Remember, it's all about Cl ^1.5 / Cd. Lift squares with velocity, and induced power CUBES. So you can't just twist the small rotors faster to get there.
To answer your question, Dave, my money is on the big chopper.
09-20-2006, 12:29 PM
Thanks for your comments on scaling, plus the link to the paper by Paul Pounds, et al. The paper presents some interesting information, plus it raises some questions.
Their application of a hub spring to a teetering rotor is of particular interest. The ability to have greater control over a teetering rotor has 'bugged' me for years. Bell's idea of using a hub spring (http://www.unicopter.com/B329.html#Hub_Spring) will result in a 2P vibration, of course.
Last week, when contemplating electric helicopters, a potential solution came to mind. It consists of locating a mass, such as the electric motor or batteries, above and attached to, the rotor disk. A preliminary recording of the idea is here. (http://www.unicopter.com/1535.html)
The following related thesis might be of interest, assuming that you are not already aware of it. A SMALL SEMI-AUTONOMOUS ROTARY-WING UNMANNED AIR VEHICLE. (http://www.personal.psu.edu/lnl/theses/HanfordThesis2005.pdf#search=%22%22Paul%20Pounds%2 2%20(helicopter%20OR%20rotorcraft)%22)
Having posted a link to this hub spring idea, perhaps it might be a good idea to do a little thinking about it. http://www.unicopter.com/Duh.gif
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