# Help Design a Human-Powered Helicopter

by jzvonek
Tags: design, helicopter, humanpowered
P: 595
 Quote by Phrak Really, I'm not sure how this works out. Can you give details?
It's precisely as you noted: Volume and mass of an aicraft increase as the cube of a scaled length, as does the induced drag. However, their parasitic drag, which is most of the drag while at cruise, increases only as the square of a scaled length.

 But there is still stress and strain to consider for human powered flight, in general.
Yes. However, all other factors being equal, you're more likely to be successful if you design it to be powers by eight cyclists than you are if it's designed to be powered by four, due to the cube-square issue, above.

 Do you recall something called the Square-Cube rule, or square-cube law as applied to the strength of a bone or beam, or even a wing as it scales in length only? The idea is to keep material density unchanged, and the shapes of everything stay the same. It's just scaled up in size. Latently I found that the amazing Wikipedia provides it. http://en.wikipedia.org/wiki/Square-cube_law For Aerodynamic Forces: "When a physical object maintains the same density and is scaled up, its mass is increased by the cube of the multiplier while its surface area only increases by the square of said multiplier. This would mean that when the larger version of the object is accelerated at the same rate as the original, more pressure would be exerted on the surface of the larger object."
As this applies to a wing, you're going to have higher wing loading with a scaled up version. This is why we can build scaled models of fighters that required steel and aluminum alloys while we can use balsa and plywood build a scaled R/C version that'll pull 20 Gs.

 Under the hot summer midday sun, you might expect one horse power per square yard of solar insulation. The best Solar electric panels are about 15% efficient, I think...
Market leader's SunPower's panels conversion ratio is 19.3%. However, we're concerned less with electrical power output per square foot than we are with power per lb. The current world record is 41.6%, achieved on August 26, 2009. For thin-films, which are much lighter than crystalline, it's expected to range from 30% to more than 35% over the next decade.

 Well, I did too, but no one took me seriously. It would significantly reduce the weight-per-pilot requirement of the airframe--by as much as 50%. But there's a catch. There must be at least one crew member that cannot rotate but face the same direction throughout the flight.
Yes, the pilot! Well, he can darn well pedal, too!

Thanks!
P: 336
 Quote by Cyrus Internal pressure does not change material properties. And the rotation of the blades will cause all the air to............(think about it).
pressure does change the properties of a sealed container. take a piece of plastic tubing good to 200 psi. no positive pressure differential, its limp as a ....whatever
put 200 psi positive pressure, you can hold one end and it will stand erect. rocket tanks have no structural rigidity without positive pressure.

and do you think all the air will fly out to the ends???

dr
P: 4,777
 Quote by dr dodge pressure does change the properties of a sealed container. take a piece of plastic tubing good to 200 psi. no positive pressure differential, its limp as a ....whatever put 200 psi positive pressure, you can hold one end and it will stand erect. rocket tanks have no structural rigidity without positive pressure. and do you think all the air will fly out to the ends??? dr
My point was that it will change the rigidity of the structure, but not its strength. The strength is a material property inherent to the plastic. It will yield at some sigma stress value, air or no air.

As for the air going to the ends. Not "fly out", but it will "pile up" due to pressure gradient. There is going to be a radial acceleration. My friend is doing pneumatic trailing edge flap actuation on rotor blades and has this very problem. The centrifugal force is:

$$F_{cf}= \frac{M \Omega^2 R}{2}$$
 P: 336 the added rigidity would decrease the structure needed. obviously it does not change the strength this then potentially would allow less mass of the rotors. as far as "pile up", are you saying that the rotation would significantly increase the pressure inside the rotor at the ends? dr
P: 4,777
 Quote by dr dodge the added rigidity would decrease the structure needed. obviously it does not change the strength
Perhaps, if you could resolve the centrifugal force problem. I suspect you would find the airfoil sections to be 'bulging' near the tips, and under inflated at the root.

 this then potentially would allow less mass of the rotors. as far as "pile up", are you saying that the rotation would significantly increase the pressure inside the rotor at the ends? dr
It's certainly possible.
P: 595
 Quote by Cyrus It's certainly possible.
It's reality, and the fundemental basis behind radial-flow compressors.
 P: 336 that means potentially, as its rotation increases, if a check valve at the root allows only air in then the rotor will self compress the inside air charge, adding rigidity as RPM's increase. If properly designed, this pressure increase could then changing its shape as speed increases. this would allow it to "free spin" at slow speeds then change its shape when up to critical rotation maybe this gas pressure could also be applied to control lateral movement dr
P: 4,777
 Quote by dr dodge that means potentially, as its rotation increases, if a check valve at the root allows only air in then the rotor will self compress the inside air charge, adding rigidity as RPM's increase. If properly designed, this pressure increase could then changing its shape as speed increases. this would allow it to "free spin" at slow speeds then change its shape when up to critical rotation maybe this gas pressure could also be applied to control lateral movement dr
This sounds too complicated. I don't think it will work. Why go trough all this trouble when you can just make it out of ribs, stringers and a spar? You need to do a feasibility study on this idea and flesh it out more.
 P: 595 Agreed, Cyrus. All squirrel-cage blowers found in central a/c and heating units throughout homes and businesses are radial-compressors. Dr. Dodge: "this then potentially would allow less mass of the rotors" Assuming the internal pressure would be enough to counteract that from the external airflow, then yes. However, I don't know if this is the case, and would have to run the calcs to be sure. I suspect, however, that it will not be, and that you'll have to ensure a rigid structure with shrunk skin, much like monokote over balsa ribs for R/C aircraft (or doped silk used on WWI aircraft).
P: 8
 Quote by mugaliens Agreed, Cyrus. All squirrel-cage blowers found in central a/c and heating units throughout homes and businesses are radial-compressors. Dr. Dodge: "this then potentially would allow less mass of the rotors" Assuming the internal pressure would be enough to counteract that from the external airflow, then yes. However, I don't know if this is the case, and would have to run the calcs to be sure. I suspect, however, that it will not be, and that you'll have to ensure a rigid structure with shrunk skin, much like monokote over balsa ribs for R/C aircraft (or doped silk used on WWI aircraft).
when you talk of all this does it end up to energy storing devices which are illegal by the rules.....according to me if the fixed wing was easily achieved why don't we just make these rotors just like a wing but not fixed i.e by incorporating an extended flap like trailing edge n with a built in pitch angle n make them coaxial blade settings this saves on weight n with efficient driving mechanism then we are definitely airborne rather than complicated mechanisms with more weight
P: 4,777
 Quote by jeff kimathi when you talk of all this does it end up to energy storing devices which are illegal by the rules.....according to me if the fixed wing was easily achieved why don't we just make these rotors just like a wing but not fixed i.e by incorporating an extended flap like trailing edge n with a built in pitch angle n make them coaxial blade settings this saves on weight n with efficient driving mechanism then we are definitely airborne rather than complicated mechanisms with more weight
First and foremost, the fixed wing flight was not "easily achieved" by any stretch. But to your second comment, I fail to see the point of incorporating trailing edge flaps. You will have to justify this design, because doesn't make sense for this application. Moving on to a coaxial design, this will indeed save structural weight; however, and more importantly, it will be unstable (if you don't believe me, google the coaxial designs to see the common problem that plagued all of them). "then we are definitely airborne" ......sure, whatever you say . Honestly though, stability of a coaxial is a poor, and the actuation lag time constants are high. In short, you save weight but gain significant stability and control problems.
 P: 336 if the design of the component does nothing but store energy, and nothing more, then it does IMHO violate the rules. I am not saying that, what I am saying is the flight has specific time goals, but if it takes 2 men (or women) 4 hours to bring the whole machine up to speed, then it is not an energy storage device. the idea is that you can only apply x amount of work/time so if you can not change the power you (as a person) can put out, then the only way to get the power you need is more time. because the counter rotating rotors are critical to the aircraft, and do the lifting once proper rotational speed is reached, they are not storing the energy, it is being used. The counter rotating rotors will add a natural "gyro stability" by not needing to offset the rotation with a tail rotor. You control the amount of lift by the amount of power you split between the two, but both are doing the work. simpler example: 2 dc electric motors connected together by their leads together. if you spin one, the other will turn. no energy storage. add a battery in between, charge it with one motor, and run the other. energy storage dr
P: 4,777
 Quote by dr dodge if the design of the component does nothing but store energy, and nothing more, then it does IMHO violate the rules. I am not saying that, what I am saying is the flight has specific time goals, but if it takes 2 men (or women) 4 hours to bring the whole machine up to speed, then it is not an energy storage device. the idea is that you can only apply x amount of work/time so if you can not change the power you (as a person) can put out, then the only way to get the power you need is more time. because the counter rotating rotors are critical to the aircraft, and do the lifting once proper rotational speed is reached, they are not storing the energy, it is being used. The counter rotating rotors will add a natural "gyro stability" by not needing to offset the rotation with a tail rotor. You control the amount of lift by the amount of power you split between the two, but both are doing the work. simpler example: 2 dc electric motors connected together by their leads together. if you spin one, the other will turn. no energy storage. add a battery in between, charge it with one motor, and run the other. energy storage dr
I think you should reconsider this statement in light of the extremely low rotor rpm. There is no such stability, as I stated previously.
 P: 336 ???? a little less cryptic response would sure help action-reaction works the same regardless of rpm dr
P: 4,777
 Quote by dr dodge ???? a little less cryptic response would sure help action-reaction works the same regardless of rpm dr
The pole-zero structure of the open loop A stability matrix of the coaxial human powered helicopter at low RPM has right half plane poles. It is unstable, as was found out by the CalPoly HPH team, and the associated NASA TN.
 P: 336 my discussion and input was mainly geared towards "energy storage" but if coaxial rotors are so unstable, how come they work fine in rc toys? much easier to fly than single rotor rc's dr
PF Gold
P: 3,098
 Quote by Cyrus The pole-zero structure of the open loop A stability matrix of the coaxial human powered helicopter at low RPM has right half plane poles. It is unstable, as was found out by the CalPoly HPH team, and the associated NASA TN.
Several modern aircraft frames are inherently unstable, but made stable with computer controlled fly-by-wire in the loop. Is that a possibility here?
P: 4,777
 Quote by dr dodge my discussion and input was mainly geared towards "energy storage" but if coaxial rotors are so unstable, how come they work fine in rc toys? much easier to fly than single rotor rc's dr
Because they spin much, much faster. When your HPH rotors are spinning 12-15 rpm, good luck getting gyroscopic anything.

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