- #71
Cyrus
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Phrak said:The fence or duct is attached to the ground, not the helicopter.
This is outside the spirit of the rules, and would not be allowed, for obvious reasons.
Phrak said:The fence or duct is attached to the ground, not the helicopter.
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.Phrak said:Didn't you like my last post Fred? This thread is about a contest with no practical application, isn't it?
Cyrus said:This is outside the spirit of the rules, and would not be allowed, for obvious reasons.
Phrak said:But to be fair, all these attempts would have exploited ground effect.
Mech_Engineer said: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 said: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.
Mech_Engineer said: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.
dr dodge said: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
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.Cyrus said: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.
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.Phrak said:So it's within the rules, but outside some unstated 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.mheslep said: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.
mheslep said: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.
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.Cyrus said: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.
FredGarvin said: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/reports/1941/naca-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.
russ_watters said: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.
Phrak said: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? It’s difficult to expect much from one's fellow posters, without some prodding, after such a blank reception.
Phrak said:The drag of the rotors also increases as omega squared and directly proportional to the excess mass.
Cyrus said:Come again? Drag has nothing to do with mass.
Phrak said: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?
Cyrus said:To be formally correct, it increases with the tangential velocity, r*omega^2. I thought about wingtips but there was a reason why they were not justified. I can't remember right now, but I'll look up why and post later. Aerodynamically, there is only so much you can do here. In my mind, the key to getting this to work is a very clever structural design that is extremely light weight while meeting the stress requirements. This is much easier said that done. Quad anything means huge weight penalties, but inherent stability. A tip driven rotor, or coaxial means *significant* weight savings, but an unstable monster. Pick your poison.
Phrak said:I have concern over the upward bending of each rotor as a result of lift. This is forth order, isn't it? Do you number for this?
Cyrus said:You are under the false premise that they were not made of carbon fiber - they were.]
carbon-carbon
Phrak said:We've gotten out of sync. But now that I know you are talking about a hinged rotor rather than rigid, I think you will have too much coning, won't you? Is there a way to suppress it?
Cyrus said:I didn't say anything specific to a hinged rotor. The rotor will cone no matter what the hub attachment. The only way to minimize this is to increase stiffness, which will inevitably come from a heavier blade - unless you can find a material that is stiff in the direction you need for the same weight (good luck).
rplatter said:Interesting problem.
I would suggest starting with something that already exists, like a gyrocopter.
And yes, I know they require forward momentum to get moving, but they are light and run on low power.
If you could get the blade moving fast enough, you could achieve lift off.
Stability is another factor. Some Gyrocopters use gravity similar to the way a hanglider does. Shifting the weight of the pilot angles the collective.
Height would be controlled by speed of the blade. Rotation becomes the difficult part here. Both of the blades and of the craft. Possibly a counter rotating blade unit, or an angled fin projecting into the down draft. attaching power to the blade unit becomes touchy if you are using a free hanging pilot compartment. maybe a belt mechanism or a universal joint. I would offset the drive shaft from the blade hub so you could use some type of gearing at that point and to minimize the difficulty of construction.
Another approach would be to use laminar air flow and instead of a bunch of blades just have a saucer. If you spin a smooth plate it will force air out from the center. (Tesla turbine) shape it in a dome shape and the air going out will be directed down also.
The dome/saucer will also provide structural support to the entire structure.
I would imagine that a fairly soft but strong material could be used, a large mylar sheet or something. With a belt around the outside edge to provice rigidity. Just stretch it tight and smooth. You still have to deal with the craft rotation, but that may be simple.
rplatter said:Interesting problem.
I would suggest starting with something that already exists, like a gyrocopter.
And yes, I know they require forward momentum to get moving, but they are light and run on low power.
If you could get the blade moving fast enough, you could achieve lift off.
Stability is another factor. Some Gyrocopters use gravity similar to the way a hanglider does. Shifting the weight of the pilot angles the collective.
Height would be controlled by speed of the blade.
Rotation becomes the difficult part here. Both of the blades and of the craft.
Possibly a counter rotating blade unit, or an angled fin projecting into the down draft.
attaching power to the blade unit becomes touchy if you are using a free hanging pilot compartment. maybe a belt mechanism or a universal joint. I would offset the drive shaft from the blade hub so you could use some type of gearing at that point and to minimize the difficulty of construction.
Another approach would be to use laminar air flow and instead of a bunch of blades just have a saucer.
If you spin a smooth plate it will force air out from the center. (Tesla turbine)
shape it in a dome shape and the air going out will be directed down also.
The dome/saucer will also provide structural support to the entire structure.
I would imagine that a fairly soft but strong material could be used, a large mylar sheet or something. With a belt around the outside edge to provice rigidity. Just stretch it tight and smooth.
You still have to deal with the craft rotation, but that may be simple.
rplatter said:Interesting problem.
rplatter said:Another approach would be to use laminar air flow and instead of a bunch of blades just have a saucer.
If you spin a smooth plate it will force air out from the center. (Tesla turbine) shape it in a dome shape and the air going out will be directed down also. The dome/saucer will also provide structural support to the entire structure. I would imagine that a fairly soft but strong material could be used, a large mylar sheet or something. With a belt around the outside edge to provice rigidity. Just stretch it tight and smooth. You still have to deal with the craft rotation, but that may be simple.