Konrad,
Your post makes assumptions that are not true. While rotor construction varies, they are not built to flex in one direction and not the other. Furthermore your rope analogy is flawed. To use a rope as a model, tie one end to something and climb up. Then tie the other end and repeat. Same strength both directions. To say a rope is not a pole is well, obvious. It isnt a fish either.
Finally the falling of the flying machine you describe and the resulting paniced livestock is known in aviation as a “Class A Mishap.” Just in case you were curious.
www.colheli.com
Photo of twin rotor craft. Note upward flex of blades
www.geocities.com/Hollywood/2307/misc/BellAH-1W.jpg
Photo of Ah-1 Cobra clearly in inverted flight.
If we go on to describe how it takes as much energy to climb down that rope as to climb up it, I’m gonna go get a scotch.
Most people have been focusing on the load on the rotor blades, but it is also likely that the rotor hub (and other drive components) may not be engineered for negative Gs. I know that airplanes typically have different limits for positive Gs then they do for negative Gs.
Propellor vs Rotor - all of the airfoils in a propellor have the same angle-of-attack throughout a revolution (ignoring transient changes in an adjustable pitch system), whereas in a rotor the angle-of-attack of the airfoils is constantly changing throughout a revolution, but identical at a given point in the revolution (again ignoring transient changes). I don’t know that this would qualify as a rigid definition in engineering terms, but it should help understand the differentiation.
EvilGhandi: I never said helicopters can’t fly inverted, just that not for the reasons you said. You want a simple example? Let’s say the rotor only has ball bearings on top of it. That would mean the ball bearings are meant to support the weight of the helicopter. The only time the rotor would not be on those bearings is when it’s resting on the helictopter and would only have to support it’s own weight, and not the weight of the entire chopper. This seems like a pretty reasonable assumption, I don’t know if they’re really built like that, but it seems possible. Either way, it doesn’t matter if they are, my point is that just because it can support the weight in one direction doesn’t mean it can do it in the other.
As for that picture how do I know that it was actually supporting the weight or whether it was just doing a quick loop and was actually pulling the chopper down while inverted?
I am growing weary of this debate.
First I will deal with Konrad.
Sir, in one breath you challenge the accuracy of my post then go on to say you know nothing of the design or construction of helicoptors.
I, on the other hand spent a considerable amount of time in school studying flight theory and mechanical design of those very aircraft. Then I went on to spend four years as a crewman on AH-1G/S mod Cobra gunships while assigned to the 25th Combat Aviation Division. You never mentioned your qualifications.
As far as the ball bearing paragraph? I havent a clue what you are talking about.
Ok on to the wing vs propeller nonsense.
I admit the rather long winded post on “times of yore” would probably serve to confuse more people than it would clarify.
In it I set out to demonstrate that while both devices are derived from the same basic machine, (the inclined plane) there is a fundamental difference in the theory behind each. Look Bournelli and Archemedies up in your encyclopedia, I refuse to elaborate more.
To Dhanson,
While in theory there is a fundamental difference between the two machines, In practice there are a myriad of forces acting on the flight surfaces of a given aircraft, so I will half concede to your point. Really I never meant to piss you off, Ok I did, but it was out of love.
On a mellower note, I would like to thank everone who noticed for not jumping on me for consistantly spelling Helicopter “helicoptor” gak I just noticed.
EvilGhandi: You seem to be confused. You could have invented the damn helicopter for all I care, I’m not talking about how they are really constructed. I was pointing out that someone (I don’t remember if it was you) said that since a helictoper is able to support itself right side up it MUST be able to do it upside down.
Now I’m not saying it can, and I’m not saying it can’t. All that I’m saying is that that statement is simply not true.
I’d be much obliged if you were to actually read my posts before replying to them.
Konrad. I don’t think it’s a matter of supporting the weight as it is of having rotor blades that can pitch around 360 degrees. As far as I can tell, that’s the only real issue with helicopter inverted flight. Again, the question arises, why would you want to? As long as the helicopter is maneuverable to get out of a situation, inversion isn’t important.
Jeff
Fair enough, Jeff, inversion is not imporant – but man, oh man, would it look coooooool in a James Bond flick.
Supporting the weight shouldn’t be a problem since helicopters are stressed for more than one negative ‘G’.
Konrad: Do helicopters fit into the same certification categories as aircraft? i.e. Normal, Utility, etc?
I really enjoyed the excerpt from chapter two of the History of Aerodynamics, but, for what it’s worth at this point, the Wright brothers were the first to recognize (as a result of an amazing series of wind tunnel investigations) that a propeller is not merely a screw, but is an airfoil that generates thrust through the same mechanism that a wing develops lift. This understanding led them to develop a a propeller with a high aspect ratio to increase L/D, and with washout to maintain favorable angle of attack over the length of the blade. This revolutionary device (check the pun) coupled to a remarkably lightweight gasoline engine of their own design and manufacture gave them a propulsion system with enough get up and go to make their airplane actually fly.
A wing is an airfoil used to provide lift. Fixed wings develop lift through the motion of the aircraft through the air. Rotary wings (also called rotors) develop lift by being rotated and thus moving through the air without the need to move the entire aircraft.
A propellor is an airfoil used to provide propulsion. The whirly things on a tilt-rotor aircraft serve for both lift-generation and propulsion depending on the regime of flight. They are therefore called “proprotors.” The physics of their lift/thrust generation mechanism does not change as they are tilted.
My credentials include a PhD in physics earned studying superfluid flow; five years building simulators for the Army Synthetic Flight Training System (including the AH-64, AH-1S, CH-47D); Commercial Pilot Certificate with Airplane Single Engine, Airplane Multi-Engine, and Instrument ratings; and Flight Instructor Certificate with Instrument-Airplane rating (no rotary wing ratings). So I’m just as ignorant as the rest of you.
By the way, it takes energy to climb in a helicopter, but you can descend gracefully without any further expenditure of energy. (Relax, Dex! I just threw that in so you’d have an excuse to get a drink.)
Dhanson: According to the official guidelines set out by the KDAWT [1] helicopters fit into 4 categories:
Military Helicopters: Have sharp pointy teeth.
Civilian helictopers: e.g. Traffic helicopters.
Mysterious black helicopters: Tinted windows, hover near your house emitting mind control beams and covering everything with a thick layer of strange dust.
Ambulance helictopers: These are the only ones allowed to use the reserved “car pool” lane when they have only 1 passenger.[2]
[1] KDAWT = Konrad’s Department of Avation and Whirly Things
[2] Only if they have their emergency lights flashing.
Thanks for adding that, Geezer. It’s basically what I’ve been trying to say all along. There are design problems that propellors have that wings don’t due to their size, rotary motion, angle of attack changes etc., but it doesn’t change the fact that they function using the same principles.
Now you can help me explain that wings create lift through moving a mass of air in the opposite direction, an application of Newton’s 3rd law.
Oooh. Tall order. It’s hard enough dealing with the fact that people (me included!) cling with religious fervor to their beliefs, but this one’s complicated by the fact that the competing theories are not, in themselves, wrong. Bernoulli’s theorem is valid and relevant to the question of lift production. Ditto for the Magnus effect. The problem (for me, at least) is one of logical explanation.
Yes, Fred runs faster than Ted (you know Fred and Ted?), and yes, the air flows faster over the upper surface of the wing, and yes, Bernoulli’s Theorem correctly asserts that there will be lower pressure in the region of increased velocity (pause for breath) but… the increased velocity does not cause the reduced pressure; rather, reduced pressure above the airfoil causes increased velocity. So how does the Bernoulli equation “explain” lift? I don’t think it does. And it surprises me that many people find this explanation satisfying.
It is also true (I think this is your point, dhanson), that in order for a wing to produce lift by working against a fluid like air, it must push the air downward–it must create downwash. The downwash is easy to see (or feel, rather) behind a propeller, but very hard to see (or feel) below a wing. So people invoke semi-magical explanations like the low pressure above a wing “sucks” the wing upward. Sure, the pressure difference between the bottom and top of the wing pushes the wing up, but that pressure difference is the result of accelerating air downward below the wing. It’s just that by the time the air has moved downward appreciably, the wing is long gone! (A propeller or a helicopter rotor keeps returning to push some more so you actually build up a wind. Hence the need to assert that props and wings are somehow different.)
It’s fairly easy to see how a wing with an appreciable angle of attack pushes air down with its bottom surface, but harder to see what goes on on the upper surface. Still, it is a basic fact of physics that in order for the wing to experience an upward force (lift), air must be pushed down below. And air being a fluid, it has to flow downward as a result of that push.
Somewhere in the world there is a really cool picture of a bizjet (a Citation, I think) climbing out of the top of a cloud deck, viewed head-on. You can see where the downwash has pushed the cloud away behind the jet, and you can see the wingtip vortices mapped out in the cloud trailing behind the wingtips. Fabulous! But it won’t change any minds.
I have tried to explain this to people of all types for a long time. I used to teach ground school to pilots, and when I’d explain this I’d get hauled aside by other instructors telling me I didn’t know what I was talking about.
The bottom line is that for an airplane to stay in the air the wing has to do work. The ‘classic’ Bernoulli explanation showing an undisturbed mass of air behind the wing depicts a situation where no work is done.
I wound up explaining that Newton’s 3rd law and the Bernoulli effect are explaining the same thing. The Bernoulli effect explains the mechanism, and the movement of the airmass explains the result. It’s like people having an argument saying, “A rocket produces thrust by moving mass in the opposite direction” “No it doesn’t! It produces thrust by burning fuel!” One leads to the other. The Bernoulli effect (and flate plate effect) produce the downward movement of the air mass that is required for lift to take place.
Does that make sense? It’s not strictly correct, but I felt it was close enough to get the point across without being too misleading. (Most aerodynamics texts for aviators get this stuff completely wrong. For instance, they often show a wing and claim that two particles that start at the leading edge both have to wind up at the trailing edge at the same time. Thus, the top one has to go farther, speeds up, density decreases, and a suction is created. This is demonstrably false, as one photo in a wind tunnel with a smoke trail will show.)
Oh my,
I’m gone for two days and I come back to find that not only do I have an engineer but now a friggin PhD arguing counterpoint?
That’s it, I give, Uncle.
That said I think this debate did serve to further the mailbag question by illustrating;
A) There is some contention even among experts as to what exactly are the underlying principles of flight.
B) Helicoptors could fly upside down if someone wanted to build them that way.
C) No technical post will go unchallenged on this board.
But do you believe?
But seriously, there are experts and there are experts. In aviation, I don’t really qualify. Having read Prandtl, Lamb, Batchelder, Ashley&Landahl, Langeweisch, and Raymer, however, I’m convinced that they are experts, that they and do not disagree with each other. Flight is well understood–by the experts.
Still, the mechanism of lift generation is very complex, and not at all obvious to those untrained in the details of fluid dynamics. How do you explain it to us mere mortals? I don’t know. I love to explain stuff (surprise!) but I don’t know how to do this one. All I know is that Fred and Ted don’t cut it.
What everyone seems to forget is the effects on the engines durring inverted flight. Loops, Rolls and Dives put excessive G forces on the engines forceing oil through the pistons into the heads, which leads to complete engine rebuilds after these stunts.
And you seem to have forgotton that these aircraft use turbine engines, therefore have neither pistons nor heads.