Twin 1

Hi All,

Here is a question for you;

I have been considering projects for my final year of University and have been thinking of doing something on the theory of flight. I have looked at pictures of Radio Controlled Helicopters and this got me thinking,

Can Helicopters fly inverted for a sustained period?

I would appreciate your technical expertise on this subject and reasons.

P.S. I am aware that Apaches can loop and roll, but not sustained inverted flight

Hummingfrog

Hold on I'll just go and try

Upside down now

No

Tried in the RN Seaking sim and it doesn't.

HF

aspinwing

Think about it for a while.

When a plank is flying inverted, what is the angle of the longitudinal axis of the a/c compared to 'normal' flight.

What do you think would happen if 'inverted' blades of a helicopter developed 'negative' lift? Hints: coning angle, tail boom.

Twin 1

Aspinwing,

I am no helicopter technical expert, we do all that in the final year, but is the jist of what your saying that the blades will take out the tail rotor?

If so could a helicopter be made to prevent this?

And how can Remote controlled Helicopters fligh inverted?

Cheers

Moneyshot

Maybe it's all to do with the weight of the machine, the rigidity of the rotor head and the lesser(inverted)coning angle thus produced with a much greater rrpm. I would think it possible to design and fly such a real helicopter but let me ask you a question. Why would you want to?
Also.. Why can a man withstand a drop of 10 feet and an elephant not? only joking..

Twin 1

Moneyshot:

My lawn mower is broken!!!!!



Seriously, good points!
Cheers

nickp

Having flown R/C helicopters inverted, there are two differences that are obvious. Firstly the blades do not cone to anything like the same extent, and secondly the clearance between the blades and the tail boom is relatively greater.
They also have about ten degrees of negative collective pitch which helps, but the setup of the collective would be interesting on a full size - it would be in the middle and if you raised it you would lift off, but I hate to think what would happen if you lowered it!

RW-1

My Concept flies inverted with 4.5 deg of neg pitch in hover, up to 9 at forward flight inverted.

But basically I agree with NickP and MoneyShot. The model rotor is doing about 1800 RPM and while the coning angle is less comparatively, the room between the rotor and boom is greater (comparatively).

When discussing inverted forward flight, or even an inverted hover for full scale, I also tend to wonder "Why?"

No practical use for either maneuver, certainly would be hard to maintain references while hanging upside down.

Or maybe just as a last ditch combat tactic? To quesinart your opponant with the mains ?

CRAN

Twin 1,

Yes, theoretically helicopters can fly inverted for sustained periods, but lot's but lot's of systems would have to be designed differently from the outset to accomplish this. Such as - hub would need to produce massive control moments to counter static instability, you would need a high performance stability augmentation system, the fuel system would need to be pressurised & and the engines modified to be intolerant to attitude. To mention but a few!

Incidentally, on the subject of negative pitch and coning - the Lynx has some negative collective pitch travel and consequent negative coning to pin it to the rolling decks of ships in rough seas prior to being lashed down.

What were you intending to do for your project and where are you studying?

CRAN

Off road

I think this would be an interesting question for Nick Lappos.

I wonder whether the Commanche can fly inverted for a sustained period?

Fly safe

sling load

for a given airspeed and collective pitch setting in normal flight, the whole rotor is rigged with the fuselague hanging beneath it, in the theoretical case of inverted flight, you would need substantial negative pitch over what is required for normal flight, plus as cran said, a stab aug system that would allow a fuselague to remain above it IMHO.


That in itself would be something probably more expensive than the helo. Yes, the RCs can do it, but I would like to see the hp/weight ratio compared to a helo.


It would be a most unnatural thing to lower the collective when approaching the inverted, ouch!

Robbo Jock

I've seen pictures of RC helicopters inverted. I've often wondered how they manage it.

Normal situation, disk-a-spinning, body hanging beneath it, swinging gently in the hover. Disk tilts forward and starts sliding in that direction The body, subject to drag and inertia, slides slowly backwards, rotating about the pivot - the rotor hub. At some point all the forces will end up in equilibrium (drag, weight of body, horizontal and vertical components of total reaction) and the body will end up hanging at some angle, swinging gently in relation to the disk. This angle can be varied by adding horizontal stabilisers and such, but basically the body hangs back from the disk and is held from moving further back by its weight. A stable situation.

In the inverted state, the disk is spinning away and the body is 'hanging' above it. Balancing a broomstick vertically on the end of one's finger springs to mind. This is an innately unstable state, any movement is sure to be divergent. Assume the CoG is absolutely vertically above the hub and we're stable. Tilt the disk forwards and the disk will start sliding in that direction. The body, subject to drag and inertia, slides slowly backwards, rotating about the pivot. We're now balancing that broomstick whilst standing on a skateboard that's started downhill. (I know, as Helicopter pilots, this is all in a days work and part of the fun !) But once the CoG has moved from vertically above the pivot point, there's nothing to stop it continuing to move. Weight's still pulling it back, drag's still pulling it back, there's nowt to hold it up. NOT a stable situation.

Forget coning, forget boom clearance, forget pitch, if the CoG is above the pivot, it's unstable. How do even RC helicopters manage to control this ? Are their hubs absolutely rigid in-plane ?

widgeon

Reminds we what one wag posted about best ROC for a helicopter could be obtained by flying inverted and autorotating up . I imagine biggest prob would be keeping fuel flowing to the engines unless you had a feed at the top and bottom of the tank.

CRAN

Robbo,

My understanding is that they do it in the following manner:

Firstly, the unstable moment created by the CG being out of line with the pivot point is balanced by hub moments. This is inherently dependant on the pilot/RC Operator having the reactions to correct the divergence before the body rotates to such a degree that the maximum moment achievable by the hub is exceeded and the aircraft crashes.

I agree with your analogy with the up-turned pendulum, this configuration is statically unstable - but that's not to say it couldn't be designed to work if the need arose. Blades don't need to be rigid and theoretically any hub will work though the more control power the better!



CRAN

heedm

I guess to be pedantic, we're talking about sustained LEVEL inverted flight. Or better yet, can a helicopter maintain -gz?

Theoretically has been discussed, but practically I'd like to hear ideas why we'd want to do this. Other than airshows and kicks, the only purpose I can think of would be an agressive bunt for someone getting attacked. Maybe the Comanche can do this?



Interesting aside. 1996 International Helicopter Championships in Salem, Oregon (great time, outstanding hospitality). National title holding RC Helicopter aerobatics guy put on a show that I still can't figure out. Inverted flight quite common, but vertical dives with extremely rapid yaw rates, pointing nose down at ~10' AGL then pushing into an inverted hover at inches. WOW. Slow level flight ~6' AGL starts pitching 360 degrees while maintaining altitude and slow forward flight. No idea how this one works, must be some sort of illusion thrown in with exceptional flying skill.

Second show of day was cut short when 6" inverted hover turned into 0" inverted hover and $5000 helicopter turned into 5000 pieces. Talked to the guy afterwards. His attitude "part of the job". My attitude, "I'm not flying real helicopter inverted". especially at 6".

Dave Jackson

Twin 1,

A helicopter with a really really rigid rotor located below the 'fuselage', is extremely stable. In fact, it is too stable. As the forward speed increases the drag of the fuselage resists the necessary forward tip of the disk.

Two examples are:- http://www.flying-platform.com/ and http://www.hiller.org/exhibits/onlin...-platform.html


RW-1 & heedm

>When discussing inverted forward flight, or even an inverted hover for full scale, I also tend to wonder "Why?" <

The UniCopter is intended to fly inverted. This feature is for use during combat. When attacking, the helicopter performs barrel rolls. This causes the enemies to laugh so hard that they can't hold their weapons steady.

Nick Lappos

Inverted flight in real helicopters is a neat parlor trick, and many modern helos are capable of brief encounters with negative G, but none are capable of continuous inverted flight due to a variety of reasons. Having flown and demonstrated negative G and inverted maneuvers, here is my read on it. (Caution, in many helicopters, negative G can result in loss of control or rotor failure) :

1) Engine and fuel systems are not designed to maintain proper fuel and oil flows, so the fuel pressure and oil pressure lights come on almost immediately. If positive G not restored in a few seconds, fuel or oil starvation will result in engine shutdown or internal failure.

2) Main rotor control is the sum of the thrust and the moment the rotor produces. As thrust is reduced, control effectiveness diminishes, to nil in many rotor designs. If there is sufficient hinge offset (the distance of the flapping hinge from the mast, expressed as a percentage of the rotor radius) the rotor can still be effective at negative G. For high offset rotors (such as the BO-105), -1 G is possible. For the Black Hawk and S-76, about -.5 to -.6 G is all you can reliably get to. I used to demonstrate -.5 G with snappy roll reversals in the armed S-76 and in Black Hawks.

For teetering rotors, like the older Bell family, the Tilt Rotors and the Robinson, fill out all your personal paperwork and settle all debts before trying anything like low G, because you won't have to worry about the landing.

Comanche can get very negative, because it has a high offset main rotor, but its fuel system is not designed for it. There really is no mission need to perform extremely low G for any length of time.

Dave Jackson

As mentioned, the idea of flying inverted is probably only a hypothetical situation, but it does raise interesting aerodynamic considerations.

I believe (subject to correction) that in right-side-up forward flight the speed stability is achieved by the positive pitch moment of the rotor (flap-back) being slightly more dominant than the negative pitch moment of the fuselage.

Assuming that the rotor is capable of providing negative pitch and that the helicopter is flying inverted, will not the pitch moment of the rotor (flap- back???) and that of the fuselage both be positive (in respect to the horizon, not the helicopter). This seems to imply that there will be excessive speed stability and that the maximum forward speed will be very limited.

Nick Lappos

In trimmed upside down flight, the rotor would behave exactly as it now does. At trimmed speed, an acceleration would require forward stick, and the rotor would back-flap in a stable manner. The fuselage would apply a resisting moment from the horizontal tail, as well.

I think the belief that the CG effects of the body's mass are dominent is not accurate, and overplays the effects of cg on the basic stability of the aircraft. In most helicopters in normal flight, the CG can be a positive or negative stability term, depending on the relationship to the mast. At aft cg in normal rightside-up flight, the mass is also an unstable term (an increase in G would cause an increase in nose up moment from the cg). This is a real effect, has the same magnitude whether upside down or not, and is easily accounted for in the basic design of the aircraft. In upside down flight, the aft cg would still be de-stabilizing, and a forward cg would be stabilizing.

Twin 1

Cheers Guys,

There are many good points raised, some of which I barely grasp, I will endeavour to further my understanding. However from the replies I can see that this is an interesting subject.

I am considering a final year project investigating the aerodynamics of inverted flight and inverted helo flight.
If I have any problems I now know where to come for answers!


As to the question Why bother?

No idea, I just thought why has it not been done, is it possible?

Question: If this helo was built, would you fly it???????????

Thanks for the replies,

Twin