Help Design a Human-Powered Helicopter

Cyrus

Why on earth would you do such a thing? Think of it this way, what do you think will happen to the stresses at the hub when you suddenly dump the collective? The change in AoA will give a large impulsive pitching moment - bye bye blades.

Phrak

The fence or duct is attached to the ground, not the helicopter.

Cyrus

This is outside the spirit of the rules, and would not be allowed, for obvious reasons.

FredGarvin

I can't really comment on it basically because I'm not sure where you are going with it. The reason ground effect exists is because of the blockage of reingested vortices at the rotor tips. It's either blocking that reingestion or it's not. I doubt there is any exponential decay of ground effect, or however you want to put it.

[EDIT] OK. So I did some looking after writing that, and found some references that disprove what I wrote. Seddon shows a theoretical expression (making a few major assumptions) that seems to work well in most cases:

[tex]\left[\frac{T}{T_\inf}\right] = \frac{1}{1-\frac{R}{4Z}^2}[/tex]

This is supported by Figure 7 from Knight and Hefner:

http://naca.central.cranfield.ac.uk/...aca-tn-835.pdf

I also found this graphic which I am kicking myself because I have seen this before (a longggggg time ago)



So it's not exponential, but it does decrease with increasing Z/R ratio. I stand humbly corrected.

Phrak

So it's within the rules, but outside some unstated rules?

To continue with this unspirited concept, it occurs to me that it might be equally beneficial to have 'ceiling' effect to double things up. I'm not sure if this makes sense.

Edit: I've re-read the rules, and the spirit erules to eliminate hovercraft and other stuff outside the intended box). But to be fair, all these attempts would have exploited ground effect.

Mech_Engineer

The only way one of these things can get off the ground is to exploit the ground effect. Too much power will be required to fly outside the ground effect, that's why these things barely make it a foot or two off the ground with 100ft rotor diameters.

Cyrus

Additionally, the DaVinci III (with 100ft rotors) did not hover for very long because of stability. So you cannot say it was due to power issues. The Yuri had to stop its flight because it ran out of space due to drift. Again, a stability issue, not power. So sweeping statements about the power being too high are not strictly valid.

dr dodge

"Why on earth would you do such a thing? Think of it this way, what do you think will happen to the stresses at the hub when you suddenly dump the collective?"

the main reason is that the total accumulated work over time is "banked" in an hour or two of "run up" in order to have momentum help to keep the rotors going with the available power from the pilot. Isn't the changing angle of attack what causes a conventional "rotor'd craft" to fly? If so, why in this application it assembly would self destruct?

dr

Mech_Engineer

Stability or no, the guy is pedaling like a maniac and barely made it a foot off the ground. If power were not a major issue, the craft would have been able to take off and consistently gain altitude with time. Instead, it seems to be they lift off and stabilize in altitude at a very low height.

Cyrus

But they don't stabilize at a low height. The blades were so long that any small angular deflection results in a tip strike. My point is that they did not have the stability to try and get to any significant height.

Cyrus

This is fundamentally wrong if you look at the power output curve of a person and the power demands of the aircraft. You simply do not want to use such a method.

My understanding of what you have described is to spin up the rotors for an hour (and waste the pilots energy for no reason with a bunch of gearing that adds unnecessary weight) and suddenly change the collective on the blades. You would have to change the AoA of the blades quickly, otherwise they will slow back down. So you now need a larger, heavier blade hub to absorb the large transient stresses. This is idea gets worse and worse any way you slice it.

mheslep

Not only that, but Dodge is essentially suggesting an energy storage scheme, in this case storing energy in the blades angular momentum, which is against the rules.

russ_watters

Phrak, please drop this line of discussion, as it is distracting from the purpose of the thread. You're not the one who gets to interpret the rules of the contest, the people running it are. So it isn't useful to try an weasel around them for the purpose of discussing it in this forum, when it is obvious that such weaseling wouldn't fly with the organizers of the contest.

russ_watters

On that point, I'm not so sure the judges would agree. Yes, he's essentially saying to use the rotors as flywheels, but the judges may consider that acceptible. The rules certainly imply it where they give a specific exemption from that rule for rotors.

Cyrus

What he said is OK, it's not energy storage because the pilot put in his own energy to spin up the rotors and then went on with the flight. However, it's a useless endeavour.

What you could not do, is spin them up, and have someone else jump in and then take off. Or, store energy in a spring, and then come back an hour later and try to fly after you are refreshed, along with the help of the spring.

mheslep

You need not switch out the human. If they allow energy storage as long as it is 'the same continuous operator', then someone could leisurely store up 100 watt-hours in an hour of work/pedalling and then release it all via some mechanism (e.g. electric motor) at the rate of 6kw (8HP) for one minute of flight, collect $20k, thank you. But this is moot, storing energy is not the goal of this exercise.

Phrak

Thanks Fred, that would help a great deal, but I'm afraid I can't figure out the variables.