Help Design a Human-Powered Helicopter

[Cyrus]

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.

 

[FredGarvin]

Didn't you like my last post Fred? This thread is about a contest with no practical application, isn't it?

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:

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

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.

 

[Phrak]

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]

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]

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]

"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]

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]

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]

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]

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]

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]

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]

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]

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]

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:

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

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.

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

 

[Phrak]

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]

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? It’s difficult to expect much from one's fellow posters, without some prodding, after such a blank reception.

Do you have anything yourself?

 

[Cyrus]

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? It’s 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]

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]

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?

 

[Cyrus]

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?

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]

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.

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]

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?

It's called coning. All rotors, including regular helicopters, do this.

 

[Phrak]

Sorry to be criptic. All I know is that torsional rigidity of a tube is to the forth power of the radius.

 

[Phrak]

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?

 

[Phrak]

You are under the false premise that they were not made of carbon fiber - they were.]

carbon-carbon

 

[Cyrus]

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?

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).

 

[Phrak]

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).

Then we are talking about the same thing, where the limiting factor upon weight considerations in rididity over strength.

Materials to compare are carbon fiber/carbon from mesophase pitch that has higher Youngs modulus compared to carbon fiber derived from the more common polyacrylonitrile--popular for it's strength.

However the carbon fiber/carbon may be prohibitively expensive. It is carbon fiber composite that undergoes a second and sometimes third process of reheating then reintroduction of matrix material.

 

[dr dodge]

"4.1.4 No devices for storing energy either for takeoff or for use in flight shall be permitted. Rotating aerodynamic components, such as rotor blades, used for lift and/or control are exempt from consideration as energy storing devices"

its not against the rules
I can see how the added mass and complexity would start you down the road of diminishing returns
thanks for the explain

dr

 

[rplatter]

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.

 

[Cyrus]

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.

How is a gyrocopter going to hover?

If you could get the blade moving fast enough, you could achieve lift off.

That's fairly obvious...

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.

Err...okay. Try getting a person to pedal around 1 HP and see if they are also able to shift their body weight around (This isn't going to happen).

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.

Errr...okay. I'd like to see some calculations as to why you think this would work.

 

[FredGarvin]

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.

It's not an interesting problem because it is fundamentally flawed. It is a dead issue. Period.

You can throw terms like "laminar flow" or Tesla turbine but I don't think you really have any clue as to what you are talking about. A gyrocopter, while using less power, still requires ORDERS OF MAGNITUDE more power than a single human can provide (and those are olympian athletes). On top of it, they can't hover.

Let's stick to actual engineering discussions and not turn this into a thread that belongs in Skepticism & Debunking.

 

[Phrak]

Interesting problem.

I appreciate your input rplatter. You're thinking far afield, and outside what at first blush appears unworkable. Nothing wrong with that, that I can see. It has a chance of lending inspirational direction.

 

[dr dodge]

I would like to "refine" my previous input.
I am still sticking with the dual rotor assembly. if the inside "stability" rotor has more mass than the outside one it should work as an inertial gyro that should give some added stability to the system. you would drive the outer rotor off the inside one.
as far as the angle of attack problem, how about this. the outer blade is completely flat to start and then, when "critical rotation" is reached you inflate a series of tubes, one at a time. the "tube bladder" takes the flat rotor blade, and gently makes it an air foil. no blade angle changes. Using pressurized internal structural members would add rigidity making the material used potentially thinner than if just stand alone. the hardest part I can see is evenly inflating both rotors evenly to avoid instability. then use compressed air or air water "retro-rockets" for the lateral corrections, drive the compressor off of the inner disc. and use 2 people, that way one person is the "engine" and the second is "second engine" until run up is completed, then they assist for power and pilot the craft. I can see an advantage in 2 persons because it would be very hard to concentrate on control while pedaling your butt off.

and a side note, if the rotors were on the ground you'd get an "extension" of ground effect

you may fire at will...lol

dr

 

[Phrak]

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.

This may have some traction if it can stay within the bounds of mutable rules.

Though these so-called laminar flow lifting things don't appear to be very efficient, it may be possible to produce an efficient version powered by a central propeller.

The more air one can grab per unit load, the greater efficiency to maintain altitude--less dv/dt is required for each parcel of air. This is accomplished on fixed wing aircraft by increasing the wing span.

If the skirt can be made large and self supporting, or nearly so, it would have a small overhead in weight penalty, and support the total load upon a larger volume of air.