A very silly question!
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A very silly question!
Just wondering, do helicopters get affected by turbulence or does the flexibility of the rotor head / gubbins damp it out as the rotating "wing" appears much more flexible than the literally "fixed" wing.
Cheers
Oddly interested of Haslington
Cheers
Oddly interested of Haslington
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They are ultimately affected by turbulence, but differently to fixed wing and arguably less than for an equivalent fixed wing, but read below.
The airframe hangs from the rotor system: it is pretty close to being a pendulum setup with the airframe being the conker/weight, but it has a complex suspension point: the rotor system. "Turbulence" or varying wind applies varying forces intermittently to the airframe. Like any pendulum where the weight is pushed, it will swing. Unlike a simple pendulum, the helicopter will apply force to the rotor system, which will respond and absorb some of the force, but also flex and alter its position a bit.
For most 2 blade rotor systems there is an extreme situation which can occur in severe turbulence, which can lead to fatal accidents. If the turbulence is sufficient and the helicopter flown so as to allow it to happen, the effect of the turbulence can "unweight" the rotor system: a condition known as low or negative G. By "unweight" I mean that the mass of the airframe is no longer applying itself under gravity to apply a strong positive force downwards to the rotor system through the MR shaft. When this happens the MR system and airframe "fly" separately. Unless luck +/or pilot corrections are applied correctly, the divergence between the MR system and airframe can cause the 2 to come into contact with dire results.
Multi blade rotor systems are much less likely to be affected by "low-G", largely because of a different way in which the rotors are connected up and controlled in a multi blade system.
The airframe hangs from the rotor system: it is pretty close to being a pendulum setup with the airframe being the conker/weight, but it has a complex suspension point: the rotor system. "Turbulence" or varying wind applies varying forces intermittently to the airframe. Like any pendulum where the weight is pushed, it will swing. Unlike a simple pendulum, the helicopter will apply force to the rotor system, which will respond and absorb some of the force, but also flex and alter its position a bit.
For most 2 blade rotor systems there is an extreme situation which can occur in severe turbulence, which can lead to fatal accidents. If the turbulence is sufficient and the helicopter flown so as to allow it to happen, the effect of the turbulence can "unweight" the rotor system: a condition known as low or negative G. By "unweight" I mean that the mass of the airframe is no longer applying itself under gravity to apply a strong positive force downwards to the rotor system through the MR shaft. When this happens the MR system and airframe "fly" separately. Unless luck +/or pilot corrections are applied correctly, the divergence between the MR system and airframe can cause the 2 to come into contact with dire results.
Multi blade rotor systems are much less likely to be affected by "low-G", largely because of a different way in which the rotors are connected up and controlled in a multi blade system.
Last edited by Helinut; 24th Jul 2012 at 16:53.
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Thanks for that. So the airframe gets swung around under the disc by turbulence rather than the "bumpy road" type experience of the fixed wing airframe?
The -ve G bit I get having read a bit about tail boom stries and blade underswing etc.
Thanks again. I can sleep tonight now!!
The -ve G bit I get having read a bit about tail boom stries and blade underswing etc.
Thanks again. I can sleep tonight now!!
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When I did scenics in an R44 once upon a time, I hardly had anyone sick in the helicopter. Maybe one person the whole time. The fixed wing passengers on the same scenic route were different. They had at least 5-6 every week or so (2 hour scenics)
Semi-rigid rotor heads (where some or all of the conventional hinges are replaced by elastomeric bearings or similar) can give a rougher ride for the same reasons that they give greater control power.
The Lynx, with a titanium rotor hub, is bounced around far more than it would be if it had a fully articulated rotor head.
The Lynx, with a titanium rotor hub, is bounced around far more than it would be if it had a fully articulated rotor head.
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rotating "wing" appears much more flexible than the literally "fixed" wing.
Most of the lift on most helicopters occurs on the outer end of the blades where they are quite fast.
The difference is that even though the speed of our airfoil where it intercepts the relative airflow is much faster, than the interception speed of the average light F/W airfoil, our airfoil only bites off small chunks at a time and of course can flap a bit in the process.
Therefore the interception of a mass of air moving in a strange direction is not quite as harsh. However if the mass of air is large enough then we all get to move together in the same manner.
An average light F/W will seem very harsh when it is operated at a fast airspeed in turbulence penetration because it bites off the one big 'chunk' straight up and its wing doesn't flap independantly of the airframe, make sense?
Mostly in a helicopter I feel secure even though some days I feel like a cork in a bottle at low level, wind and air currents rolling over trees and other surface intereferences. Gets tiresome after a few hours, that cup of coffee and out of wind sunny spot easily becomes beckoning.
Other days I have been in F/W all alone with just me and myself and maybe a well strapped down helicopter engine in the back, in severe thermal turbulence and it scares the living be jeesus out of me.
Except for a rigid rotor system, which reacts more like a FW
My observation as a low-time heli pilot, but a reasonable amount of fixed wing... in lt-mod turb, the kind that makes things a bit uncomfortable in f/w but not unpleasant and absolutely no control issues, the heli (R44 in my case) kind of twizzles around a bit under the rotor. There is no up-and-down movement to speak of. A passenger probably wouldn't notice much. It is much less uncomfortable than f/w. This is what happens for example in the turbulent air coming over the mountains and heading directly for San Carlos airport in the late afternoon.
Yesterday I happened to do a longish flight (by heli standards - KPAO-KLSN, about 70 miles) on a hot afternoon (30-35 C) over dry country, so lots of thermal turbulence. This is the kind of thing that is seriously unpleasant for the pax in f/w, though perfectly safe - if tiring for the pilot. The main movement of the heli was swinging under the rotor. It was certainly noticeable by a pax, but I think much less nausea inducing than in f/w.
I'm a pilot, not an aerodynamicist, so I'm afraid I can't explain exactly why this is.
Yesterday I happened to do a longish flight (by heli standards - KPAO-KLSN, about 70 miles) on a hot afternoon (30-35 C) over dry country, so lots of thermal turbulence. This is the kind of thing that is seriously unpleasant for the pax in f/w, though perfectly safe - if tiring for the pilot. The main movement of the heli was swinging under the rotor. It was certainly noticeable by a pax, but I think much less nausea inducing than in f/w.
I'm a pilot, not an aerodynamicist, so I'm afraid I can't explain exactly why this is.
I fly helicopters to pay the bills and small fixed-wings for fun. It is true that lightly-loaded wings (e.g. Piper Cub) make for a much more uncomfortable ride in turbulence than more highly-loaded wings.
In fact the toy I fly most often has quite highly-loaded wings and doesn't seem to give a much more bumpy ride than a similar-sized 2 seat helicopter if I slow the cruise from 120 KIAS to a more helicopter-ish 90.
It has occurred to me that if a light aircraft with high wings were to have flapping hinges at the roots and the lift struts were to be telescopic with internal springs and hydraulic dampers, then this suspension system would make the aeroplane similarly less prone to bouncing in air currents.
However there would be a weight penalty and increased maintenance costs.
In fact the toy I fly most often has quite highly-loaded wings and doesn't seem to give a much more bumpy ride than a similar-sized 2 seat helicopter if I slow the cruise from 120 KIAS to a more helicopter-ish 90.
It has occurred to me that if a light aircraft with high wings were to have flapping hinges at the roots and the lift struts were to be telescopic with internal springs and hydraulic dampers, then this suspension system would make the aeroplane similarly less prone to bouncing in air currents.
However there would be a weight penalty and increased maintenance costs.
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I have 1000 fixed wing and 600 rotary (added recently).
I know the helicopter has added gyroscopic stability but
I still think if I was presented with seriously turbulent conditions enroute I would rather be in a Cessna 172 than a 2 blade helicopter of similar MTOW/capacity.
Cessna 172 worst case = flipped onto back in midair, few too many Gs, panels show sign of overstress, so what, recover and send overstressed aircraft to engineer for assessment.
2 blade helicopter in same turbulence = not survivable
Please convince me I am wrong.
OOW
I know the helicopter has added gyroscopic stability but
I still think if I was presented with seriously turbulent conditions enroute I would rather be in a Cessna 172 than a 2 blade helicopter of similar MTOW/capacity.
Cessna 172 worst case = flipped onto back in midair, few too many Gs, panels show sign of overstress, so what, recover and send overstressed aircraft to engineer for assessment.
2 blade helicopter in same turbulence = not survivable
Please convince me I am wrong.
OOW
