Help Design a Human-Powered Helicopter


by jzvonek
Tags: design, helicopter, humanpowered
Phrak
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#73
Dec3-09, 09:51 PM
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Quote Quote by Cyrus View Post
This is outside the spirit of the rules, and would not be allowed, for obvious reasons.
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
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#74
Dec3-09, 10:32 PM
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Quote Quote by Phrak View Post
But to be fair, all these attempts would have exploited ground effect.
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
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#75
Dec4-09, 12:06 AM
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Quote Quote by Mech_Engineer View Post
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.
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
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#76
Dec4-09, 06:02 AM
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"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
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#77
Dec4-09, 08:58 AM
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Quote Quote by Cyrus View Post
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.
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
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#78
Dec4-09, 11:56 AM
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Quote Quote by Mech_Engineer View Post
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.
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
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#79
Dec4-09, 12:04 PM
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Quote Quote by dr dodge View Post
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.
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.

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
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
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#80
Dec4-09, 01:16 PM
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Quote Quote by Cyrus View Post
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.
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
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#81
Dec4-09, 02:01 PM
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Quote Quote by Phrak View Post
So it's within the rules, but outside some unstated rules?
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
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#82
Dec4-09, 02:03 PM
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Quote Quote by mheslep View Post
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.
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
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#83
Dec4-09, 03:16 PM
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Quote Quote by mheslep View Post
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.
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
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#84
Dec4-09, 03:41 PM
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Quote Quote by Cyrus View Post
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.
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
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#85
Dec4-09, 10:25 PM
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Quote Quote by FredGarvin View Post
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.
Thanks Fred, that would help a great deal, but I'm afraid I can't figure out the variables.
Phrak
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#86
Dec4-09, 10:30 PM
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There is a good and obvious reason storing energy in the angular momentum of the rotors is not significantly useful. The energy storage increases as omega squared. The drag of the rotors also increases as omega squared and (delited) [directly proportional to the excess mass deleted, as this is in error, as pointed out by Cyrus]. The power required to keep the excess mass aloft is proportional to the excess mass. The best one can do is weight the ends of the rotors and hope you still have excess stored energy after 5 minutes to stay aloft.

Without doing any heavy mental lifting, this means that you have a short time to use the energy to give you the pop up to the 3 meter requirement You are then are required to keep the excess mass aloft in ground effect.
Phrak
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#87
Dec4-09, 10:57 PM
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Quote Quote by russ_watters View Post
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.
Didn't I already note that in a past thread?

I haven't come across a great deal of creative thinking on this thread. Two serious-money attempts at this have been made, without success, using fairly common approaches. Since then somewhat more rigid materials have become more commonly available. (How does the specific modulus of commercially fabricable carbon-carbon compare to aluminum or steel? The last two are equal.)

I offered a very feasible helicopter that no one seems to given noticed. Why is that? Itís difficult to expect much from one's fellow posters, without some prodding, after such a blank reception.

Do you have anything yourself?
Cyrus
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#88
Dec4-09, 11:14 PM
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Quote Quote by Phrak View Post
Didn't I already note that in a past thread?

I haven't come across a great deal of creative thinking on this thread. Two serious-money attempts at this have been made, without success, using fairly common approaches. Since then somewhat more rigid materials have become more commonly available. (How does the specific modulus of commercially fabricable carbon-carbon compare to aluminum or steel? The last two are equal.)
You are under the false premise that they were not made of carbon fiber - they were.

I offered a very feasible helicopter that no one seems to given noticed. Why is that? Itís difficult to expect much from one's fellow posters, without some prodding, after such a blank reception.
Did you mean two hang gliders 900' apart?
Cyrus
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#89
Dec4-09, 11:18 PM
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Quote Quote by Phrak View Post
The drag of the rotors also increases as omega squared and directly proportional to the excess mass.
Come again? Drag has nothing to do with mass.
Phrak
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#90
Dec4-09, 11:35 PM
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Quote Quote by Cyrus View Post
Come again? Drag has nothing to do with mass.
My mistake. Drag increases as omega squared, but not proportional to the mass.

What do you think of wing tips on the rotors? I didn't seen any on the attempted craft. Would they contribute to adverse to the individual rotors around their axiis? Or result in flutter?


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