Can helicopters do loops and rolls?

Cecil's Columns/Staff Reports
41

Re: Structural difference when used upside down.

This isn’t intended to imply anything regarding the ability of a helicopter to FLY upside down.

Many things are easily strong enough to function one way, yet would break when inverted.

For example, the plastic ‘foot’ on my computer (presumable to keep it out of any spilled water) will hold up my case, even when I sit on it, for a combined weight of around 100kg or so. But, if I place the computer upside down and try to lift it by the foot, it creaks ominously and those four little screws don’t seem like they’d hold if I was to suspend the computer by the ‘foot’ and then try to pick myself up while hanging from the computer.

In a similar way, many devices are strong only in the direction of anticipated force. If I was to invert my chair while holding onto the legs the seat would fall off, but because I rarely use the chair while not firmly resting on the floor, this isn’t a design problem.

The wings on an airplane could be strong enough to hold 20x the weight of the airplane, in regular flight, but significantly weaker if used the other way. (ie, put blocks under the wings and jump on the fusilage, and it’s fine. Put blocks under the fusilage and jump on the wings, and they might break.)

It is not hard to imagine many similar problems that MAY prevent any specific helicopter from flying upside down.

  1. The rotors may flex one way easier than another, and in inverted flight, be too close to the body.

  2. The bearings might not work well with all of the force coming in the opposite direction from what is expected.

  3. The airfoil shape of the rotors doesn’t change, only the pitch. Perhaps the pitch doesn’t have the full range of motion on both sides of ‘neutral’.

I’m sure a helicopter COULD be made that would properly fly upside down, as the model helicopters can, but in a world of lowest bids and the like, and lacking a good reason to want to do this for extended periods (not just a quick loop) I doubt that many have been made that can.

42

Strength shouldn’t be a problem since certified aircraft have to be able to withstand negative G forces. How much depends on the type of aircraft and the certification category. However, it’s probably 2 G’s negative or more.

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Re: Logical flaws in White Knight’s post.

The plastic foot on your computer could very well hold in the way you described, it is the screws that would fail. Since the screws play no part in the support of the case in the “proper” position why do you include them in the inverse?

Inverting your chair is another way of stating, I can place a glass on top of my table, but it falls off if I try to place it on the underside. Again this is irrelevant to the strenghth of the device.

In points one and two of your post you use the words “may” and “might”. Would it be correct to assume you are guessing here? Likewise in point three you use the word “perhaps” a guess too? Granted you also use the phrase “I’m sure”, this infers some knowledge of the subject right? Or are you simply expressing faith in the aerospace industry?

Shall we take this to the absurd as an acid test?

Ok lets assume that by some herculean feat I am able to lift the Golden Gate Bridge and rest it upside down on its foundation. It would collapse under its own weight. This would not provide evidence that bridges are weaker inverted. All the components of the bridge maintained their full load bearing capacity. What changed was the structure of the bridge itself. It went from hanging on the end of a rope to sitting on top of one. As I pointed out before this changes the nature of the machine and is therefore an unfair test.

Remember the discussion was aircraft wings? Specificly rotary ones. My inverted strength comment even included the word roter. How did you conclude I was talking about your chair by that simple statement?

I already adressed this with my response to Konrads post, but his “whirlyness of the whirler” comments were witty enough to warrent only a single barreled comeback.

Finally, were the heck do the “bearing not working” questions arise from? Is there really anone out there that assumes a rotor is mounted like the front wheel of a bicycle?

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Let’s not be too harsh. It’s entirely possible that a rotor can withstand more positive G’s than negative. In fact, I’d consider it highly probable. The question is simply whether the rotor system can withstand one negative G, which it certainly can.

There are lots and lots of ways to build something that is stronger in one direction than the other. 99% of the aircraft flying in the world can tolerate much more positive G than negative. Even aerobatic planes have to be able to handle 6 positive G’s but only 3 negative.

A trivial example would be a wing with wire braces. Put six wire braces on the bottom of the wing and three on the top, and the wing will be much weaker inverted. Don’t put any on the top, and the airplane will be unable to sustain inverted flight.

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Harsh wasn’t really the tone I meant to take at the onset of my post, really.

I did feel compelled to respond since it was my post that was being challenged. The way I saw it I had two options.
A) Restate my position as to enforce its validity
B) Refute the conflicting argument.

Since you handled A rather deftly I was left with no choice but B. Sorry about the tone, wrenching on a Harley for the last two days has left me a tad abrupt.

On the plus side, the bike is running now with no more damn oil leaks so I am likely to be a little more tactfull.

46

Re: EvilGhandi…

(btw, the nick is WhiteNight, not Knight…)

To the contrary, the screws are necessary in the proper orientation because they keep the foot connected to the computer. I would have to move both seperately if it weren’t for some sort of mechanism performing this role. And the point is that the screws aren’t expected to perform the role of holding the 20kg computer, only the 250g foot. (guessing)
The whole discussion is about how the whole system (body, rotors, motors, bolts, etc) of a helicopter would have to be perfectly capable of working inverted for the helicopter to fly properly in an inverted manner. Something as trivial as a gravity-feed on the fuel (intake at the bottom of the tank) could keep the whole thing from working.

Well, yes. I do have faith that the aerospace industry could make a helicopter that could fly upside down, if someone was to give them a good (financial) reason to do so. But I also have as much knowledge of the subject, in general, as I think is needed short of actually being able to construct the thing.

Given that model helicopters can fly upside down, and the basic rotor design has no problem working upside down (I’ve even flown those rotor-on-a-stick things you spin with your hands in an inverted position, and they work fine that way.) then it’s ‘merely’ a problem of removing any elements of the design that assume one vertical orientation. An example would be the bottom-feeding fuel line, or the later mentioned (by dhanson) wing bracing issue.

I’ve often been given a problem, either mechanical, software, or hardware, and had to modify it so that it’ll work in different circumstances than originally intended. I think I have a pretty good grasp of the process.

I fail to see how this test is any more unfair than flipping any other mechanical device upside down and expecting it to work. A bridge is simply, as you said, the most obvious example of this absurdity.

I didn’t. But I thought it would simplify to the discussion to use an example with only one moving part that still illustrated the problem.

It seems likely that it is. I honestly doubt they’d hold everything in with a few teflon cuffs.

Not only are we getting hung up on specific examples, but which mechanism is being used doesn’t change that it is still being used ‘improperly’.

In regular flight, the helicopter hangs suspended from the rotor shaft. Force is exerted in such a manner to pull the shaft up out of the helicopter. The maximum downward force would usually be the resting weight of the rotor assembly. If you inverted the helicopter, it would have to rest on the rotor shaft and the weight applied to that end of the retaining mechanism would be the weight of the helicopter, which in models I’ve seen, is ‘slightly’ greater than the weight of the rotor.
(Is there a preview option for posts? This is my first attempt to use UBB…)

47

Aircraft including helicopters will almost always be able to withstand more positive G’s than negative, for a simple reason: aircraft often pull fairly significant positive G’s, but very rarely pull negative G’s. Certification standards reflect this. If I recall correctly, the certification standard for normal category aircraft is +4.4, -1.6 G’s So you engineer for that. There is no point in having the aircraft be able to sustain a lot of negative G’s, and it adds weight. Every material has a different strength in compression than tension, so the ability to sustain positive G’s will never be the same as the ability to sustain negative G’s unless the aircraft is specifically overbuilt to do that.

The inverted fuel system is another example of a requirement for inverted flight. You may also need an inverted oil system. An aircraft without an inverted fuel system will usually lose the engine after about 20 seconds of inverted flight. All competition aerobatic aircraft have their engines modified with inverted fuel and oil systems.

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Sorry to revive this thread when it was so close to disappearing on to page two. I have been indisposed and unable to respond until now. I am reluctant to bring it back to the top of the list but unfortunately, it begs reply.

First, sorry about the misspelling of your handle White, it was unintentional.

After reading your reply to my “bridge” test, it occurs to me we are arguing the same point. Perhaps a better phrasing would have been, “A wheel placed on its side becomes a dinner plate”. My point being that in many cases, changing the aspect changes the nature of the machine. One could easily state that a car placed on its roof will no longer function as a car, it is now a roadblock. My reasoning here is, machines function by differing fundamentals, comparing an inverted car to a wire supported biplane wing is not a fair analogy, nothing more, nothing less.

My confusion was most likely the result of an earlier discussion between DH and myself as to whether a prop becomes a wing by changing its aspect. Of course it became apparent quickly that our disagreement was not that at all but rather, whether the underlying mechanism of lift could best be attributed to Bournnelli’s theorem or to Newton’s laws. My side being they weren’t the same machine, his, that they were.

Bearing all that in mind, I will attempt to reply to your post by briefly describing the mechanism in question and regrettably, contradicting DH again. (I hate doing that, he knows his stuff and makes good arguments)

Understand that this information is from years of experience in rotary wing aviation, not from my observation of various office equipment. It is not a guess and to my knowledge is completely factual.

The main rotor blade is of homogenous construction, consisting of a metal skin bonded to an underlying substrate, this is attached to the hub via the blade spar, an integral part of the blade. (Composite rotors composed of mainly synthetic materials are fast replacing these with advances in technology but feature the same uniform construction) In either vertical or horizontal cross section the blade will appear damn near symmetrical, the most notable difference being the different shape of the top surface as compared to the bottom.

The spar is attached to a clevis in the hub, in most cases frighteningly enough, by a single bolt. The hub assembly, though mechanically more complex than the blade, shares this symmetry. Note this, you will find it quite common on rotating machines.

Hopefully this will answer once and for all whether the rotor itself is up to the task of supporting the weight of the aircraft.

The main rotor assembly is attached by lowering it onto the splined rotor shaft and secured with a single “Main rotor retaining cap” known in the biz as the “Jesus nut”, ostensibly because in the event of its failure the pilot and crew will be making the acquaintance of its namesake shortly. The pitch change tubes are then attached to the rotor.

The main rotor shaft then passes down through the swashplate assembly into the transmission (on single rotor craft). Here’s where I contradict DH. Since 100% of the flight control is achieved by varying the pitch of the main rotor (on a fully articulate rotor), the shaft and transmission are stressed against a fair amount of shear force. Since a shear would place strain on all bearing surfaces in a rotating assembly equally we can safely assume that the transmission is engineered accordingly. That is, no amount of force generated by the aircraft itself is likely to cause a transmission failure.

The airframe is a different matter. As DH correctly pointed out these aircraft are unlikely to encounter much in the way of negative G. In fact they are not likely to encounter much in the way of positive G either. Obviously, due to the nature of the craft, the primary force acting on the airframe is torque. Additionally in gunships, (my specialty) serious consideration is given to the action of weapons fire. But that is neither here nor there, I just felt like adding it.

You did state that you had a general understanding of the subject just short of being able to build one. Am I to take it you are a self-studied “aero-phille”? Your insight in pointing out the fuel problem shows good lateral thought and does add to the thread. If it is of any interest, the models I am familiar with partially solve this problem by using a collapsible fuel tank. The gas gauge on the other hand is an absurdly complex device.

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Evil: I wasn’t talking specifically about the design of the rotor hub, but there are lots of other components that may be stressed to higher positive G’s than negative. Engine mounts, etc.

You’re not really contradicting me, because I was attempting to speak a little more generally than what you are doing. Just as there are airplanes that can withstand just as many negative G’s as positive, the same may be true for helicopters.

I’ll see if I can dig up the certification standards for commercial helicopters and see if they shed any light on this.

50

I would like to summarize and make one correction
1)No current helicopers can sustain inverted flight.
2)They could if we wanted to make them that way.
3)From way back in the beginning, rigid (Hingeless) vs. flexible (articulated) hubs have some effect on rolls and loops, but are not the final word.
4)The Apache (an articulated helicopter) can do a loop.

Thus spake the rotorhead. I have the backing of the Rotorcraft Center of Excellence at Penn State behind me on this one.