when inverted flight is there changement of helicopter behaviour? character

[450man]

I can see that being so Tom. How are you saying that relates to inverted flight? Are you saying that because of that the air displacement aspect has less effect than we might otherwise think?

[trillian]

So here's a thought ... The blades generate most of their lift/thrust (not going to argue on that one anymore ) towards the outside of the disc - since that's where the blades have the highest speed.

Near the root, the lift generated is much smaller ... and it's this airflow that would be near the body of the helicopter. so perhaps the body of the heli is only affecting this part - where the effect is potentially smaller?

I think what turbulance there may be due to downdraft around the body is fairly insignificant because, as you have said, the tips are moving many times faster than the root and perportionately only a small amount of the rotor discs lift is generated near the grips. But that's just a guess.

It may also be different depending on if it's a flybarred or FBL heli because a FBL system will work exactly the same inverted but a flybar is always operating relative to the body / main shaft, so the forces at work may not be quite the same since when inverted the body does not 'hang' beneath and instead it's balancing in a different way.

[450man]

This is an incredibly complicated issue and a great topic that the OP has raised. I feel that all my assertions are really only educated guesses. It would be good if someone who truly understands the physics of this would put us straight. The aerodynamics of helis is SOOO much more involved than fixed-wing. I find the theory side to flying helis absolutely engrossing... just wish I understood it more.

[Planehazza]

I thought the whole idea of low pressure on the top sucking the model into the air had been dis-proven? Is it not the angle of attack effectively deflecting airflow downwards creating lift?

[tomatwalden]

I can see that being so Tom. How are you saying that relates to inverted flight? Are you saying that because of that the air displacement aspect has less effect than we might otherwise think?

As Trillian says below. I'm thinking that the disturbance due to the body being below the disc might be negligible since the thrust/lift around it has less significance than at the outside edges of the disc.

I'll stick my neck out even further and say I suspect that helis are probably much more unstable inverted purely due to the pendulum effect, but that many/most of us don't notice as we have flybarless unit compensating for un-commanded roll.

I'm going to guess that the difference would be far more noticable on a flybarred model.

[Planehazza]

As Trillian says below. I'm thinking that the disturbance due to the body being below the disc might be negligible since the thrust/lift around it has less significance than at the outside edges of the disc.

I'll stick my neck out even further and say I suspect that helis are probably much more unstable inverted purely due to the pendulum effect, but that many/most of us don't notice as we have flybarless unit compensating for un-commanded roll.

I'm going to guess that the difference would be far more noticable on a flybarred model.

The rest of what you said is beyond my knowledge, but it's true what you said about the blades being more effective on the outside. This is shown by modern propellers having twists in them - more pitch in the middle as they're going slower than the tips are.

[tomatwalden]

I thought the whole idea of low pressure on the top sucking the model into the air had been dis-proven? Is it not the angle of attack effectively deflecting airflow downwards creating lift?

I'd be amazed! Lift from a wing with positive angle of attack comes from two things - direct reaction lift (air pushing the underside of the wing) and from the Bernoulli effect (pressure differential below and above the wing). This is certainly the case for fixed-wing and I can't see any reason why it wouldn't be the case for rotary flight ...

[trillian]

I'd be amazed! Lift from a wing with positive angle of attack comes from two things - direct reaction lift (air pushing the underside of the wing) and from the Bernoulli effect (pressure differential below and above the wing). This is certainly the case for fixed-wing and I can't see any reason why it wouldn't be the case for rotary flight ...

You have two somewhat different scenarios on a helicopter depending on whether it is hovering or in motion. In a hover it is airborne by virtue of higher pressure under the rotor disc than above it due to some pitch being applied. The Bernouli effect will never get you off the ground if you have a symetrical wing or rotor airfoil and keep it at zero pitch. The same thing applies to fixed wing, if you have zero angle of attack a symetrical airfoil will never generate 'lift' because the pressure will be equal on both sides, it's only when you pull the nose up a bit that you'll get off the ground.

On a heli, once you're in forward flight the whole rotor disc starts to behave like a wing so you then have complicated combinations of pressure changes with different pitch and then angle of attack of the whole rotor disc relative to the heli's movement through the air.

[tomatwalden]

On a heli, once you're in forward flight the whole rotor disc starts to behave like a wing so you then have complicated combinations of pressure changes with different pitch and then angle of attack of the whole rotor disc relative to the heli's movement through the air.

Yup, dissymmetry of lift and flapping/teetering does my head in ... still don't really get how flapping works ....

[sp4rky-m4rky]

I would say since you (I do) feel that it should be less stable you expect it to be and therefore is feels very stable because it is more stable than you expect. yes?

Regarding the lift, most seems to have already been said, A symmetrical blade is not going to produce lift on its own, the lift comes from the angle of attack which effectively directs the air down, in a hover the blades shift the same mass times velocity as the models weight times gravity. In forward flight the physics are different so why not just compare the two in hover?

Having the mass above or below the disk should be just as stable when they are in line, the center of mass falls through the center of lift. Upright or inverted when the model rolls the center of mass moves from the center of lift and a change of pitch on one side of the disk is required to restore balance. It is surely just effective at restoring the balance inverted or normal? The model revolves around the center of mass and not the center of lift? or somewhere in between?

Also compared to the width/diameter of the blades the offset of the mass is fairly small. Imagining the blade disk as a solid fixed disk you wouldn't be surprised that the model would be very stable sat on a surface up side down.

[450man]

Regarding the lift, most seems to have already been said, A symmetrical blade is not going to produce lift on its own, the lift comes from the angle of attack which effectively directs the air down, in a hover the blades shift the same mass times velocity as the models weight times gravity. In forward flight the physics are different so why not just compare the two in hover?

You are suggesting that a symmetrical aerofoil cannot produce lift apart from directing the air down. That is not actually true. A sysmmetrical aerofoil will not produce as much lift as a non-symmetrical one but it does act like any other aerofoil in all respects including creating lift in the normal way... it is just not as efficient.

[450man]

Having the mass above or below the disk should be just as stable when they are in line, the center of mass falls through the center of lift. Upright or inverted when the model rolls the center of mass moves from the center of lift and a change of pitch on one side of the disk is required to restore balance. It is surely just effective at restoring the balance inverted or normal?

You are ignoring the fact that when upright the moment created when the model rolls creates a moment that is self-righting... it is call pendulous stability and it is employed to great effect in high-wing fixed-wing planes. High-wing planes often have no dihedral because there is sufficient stability from the pendulum effect.

Whereas when inverted the mass of the body is above the rotor and so the moment created is trying to always upright the model and so is unstable. Left to its own devices, with enough height and with no other inputs an inverted the model will always try to put itself the right way up and because of this pendulum effect there is built in to every heli a pendulous stability that is not there when inverted.

I can demonstrate this if you imagine a piece of string attached to the rotor head and another attached to the centre of the body. Suspend the heli by the lift string and pull down on the weight string. This then represents a heli in a stable hover with lift and weight equal and opposite. If you now tilt the heli over so the strings are no longer in line and continue pulling on the strings the strings will just line up and the heli will return to upright.

If you leave the srings attached and invert the heli the lift still goes up and its string now goes up from the rotor head through the body and the weight still goes down and its string now goes down through the rotor head. Tilt the heli and pull the strings and what happens. The lift one wants to pull the rotor head up and the weight one wants to pull the body down and the heli uprights itself.

In both cases the heli will want to go the right way up and so it is stable when upright and unstable when inverted. The heli stability system, either the flybar system or the FBL controller, has to work harder, as does the pilot, to maintain the inverted attitude with regard to the pendulum effect because when upright there is help from the pundulous stability and when inverted it works against.

[unwind-protect]

I thought the whole idea of low pressure on the top sucking the model into the air had been dis-proven? Is it not the angle of attack effectively deflecting airflow downwards creating lift?

So, the Bernoulli model does account for lift correctly, however I always think that it is a far more complicated model for understanding what it happening than is necessary.

We've all see the demonstration of blowing over the top of a curved sheet of paper and seeing it rise. However, if the oft-chanted mantra of "faster air=lower pressure=lift" is to be believed, surely the paper should rise even when I blow 10cm above the sheet of paper?

In reality, the lift is only generated when the air stream attaches to the surface and is _deflected_downward_ by the boundary layer effect. This makes, for me at least, the Newtonian model of conservation of momentum to give a far more natural model, and explain phenomena such as downwash far more intuitively than the Bernoulli model.

[450man]

In reality, the lift is only generated when the air stream attaches to the surface and is _deflected_downward_ by the boundary layer effect. This makes, for me at least, the Newtonian model of conservation of momentum to give a far more natural model, and explain phenomena such as downwash far more intuitively than the Bernoulli model.

No, not really. If you look at airflow models of aerofoil wing sections in fast level flight you will see that typically the angle of attack is about 2 degrees or less. This amount of downward deflection is insufficient to account for the amount of lift generated. I don't know about heli blades because there might be issues at work there that I am unfamiliar with, but certainly as far as aeroplane wings are concerned the bulk of the lift at cruising speeds is created by the Bernoulli effect. We know this too because of the strong wing-tip vortices. These are clear indicators that the lift is generated by the Bernoulli effect being the result of the two airflows, one over the wing and one under, meeting at differing angles thereby creating a vortex off the wing tip. This fortex increases significantly as the AofA and wing loading increases showing that there is a very significant percentage of the lift is being generated via Bernoulli.

I am not discounting air deflection and Newton completely but that tends to only be significant at very high AofA and/or slow airspeeds.


I am pretty sure of my details here but how that relates to helis and blade aerodynamics I do not know.

Edit: In fact there is significant further evidence of the huge significance of Bernoulli generated lift and that is when an aerofoil stalls. The upper airflow breaks away from the laminar flow over the top and forms eddies and at that point the lift reduces massively and the center of lift generally moves backwards creating a nose down moment causing the aircraft to pitch down.

[sp4rky-m4rky]

Yes an unsymmetrical airfoil such as a plane wing will produce lift with no AofA. A symmetrical wing with no angle of attack such as a symmetrical heli blade at zero pitch will produce no lift, the pressure top and bottom will the same as the air speed top and bottom will be the same.

The model will also only self level and pendulum if the center of mass is below the axis it is rotating on. What I was suggesting is that perhaps the model rotates on an axis that crosses through its center of mass therefore it wont pendulum when right way up and will have the same characteristics inverted. I would imagine this axis of rotation will likely be offset but not in the line of the blades, the pendulum effect may still be reduced making it fairly insignificant.