Autorotation technique.
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Some good thoughts here. Of course, the right answer is to not put yourself into a bad situation, if you can avoid it.
In general, I think of the autorotation in three phases: entry, sustained, landing. Each has its own peculiarities that we could all talk about for days. When you get into any low level situation, then you may have to omit the sustained portion.
For example, zero speed 500' AGL, lower collective and dive for airspeed, you should be able to attain your normal sustained parameters, so all is simple.
The entry and the landing both have airspeed changes and height changes. I think of the landing phase as a graph of speed and height, start high and fast, end at 0/0. If the entry speed and height approaches the landing speed/height line you just switch from entry phase to landing phase, allowing time and energy for the transition. Sounds complicated, but its really not.
Change the example to 200' and zero speed, you lower collective and dive. At 100' you have ~40knots (made up numbers), you haven't attained your sustained parameters, but your speed/height combination looks just like the middle of your normal landing phase, so you raise the nose to your decelerating attitude, allow the rpm to build a bit and continue with the landing you've trained.
topendtorque,
#1 I agree you don't want to lose all rpm or collective at the tree tops, but you also don't want to hit the trees at max speed. Seen the effects of that. I'd rather take my chances on a 35' foot fall, with a small cushion available than a 2000fpm descent into lumber. My plan if I don't think foliage will cushion me is to zero speed and minimize descent just above the tree tops, then dump collective to try and recover rpm on the way down, cushion at the bottom. Of course, that would be pointless if I thought the trees were going to destroy the blades so this is all situationally dependant.
This gets a bit more difficult on the West Coast of Canada. 200' trees that are greater than 5' diameter at the base, and with foliage not always dense enough to stop or slow your descent. Reliable engines and/or prayer is the best alternative we could come up with.
#2 Trade airspeed for altitude. Same as above, you start in the entry phase, but in this case you approach the landing phase from below. When you flare on entry, you made need to raise collective to control rpm, but that is type and situation dependant. When your altitude & height intercept the landing phase, it gets easy again.
In general, I think of the autorotation in three phases: entry, sustained, landing. Each has its own peculiarities that we could all talk about for days. When you get into any low level situation, then you may have to omit the sustained portion.
For example, zero speed 500' AGL, lower collective and dive for airspeed, you should be able to attain your normal sustained parameters, so all is simple.
The entry and the landing both have airspeed changes and height changes. I think of the landing phase as a graph of speed and height, start high and fast, end at 0/0. If the entry speed and height approaches the landing speed/height line you just switch from entry phase to landing phase, allowing time and energy for the transition. Sounds complicated, but its really not.
Change the example to 200' and zero speed, you lower collective and dive. At 100' you have ~40knots (made up numbers), you haven't attained your sustained parameters, but your speed/height combination looks just like the middle of your normal landing phase, so you raise the nose to your decelerating attitude, allow the rpm to build a bit and continue with the landing you've trained.
topendtorque,
#1 I agree you don't want to lose all rpm or collective at the tree tops, but you also don't want to hit the trees at max speed. Seen the effects of that. I'd rather take my chances on a 35' foot fall, with a small cushion available than a 2000fpm descent into lumber. My plan if I don't think foliage will cushion me is to zero speed and minimize descent just above the tree tops, then dump collective to try and recover rpm on the way down, cushion at the bottom. Of course, that would be pointless if I thought the trees were going to destroy the blades so this is all situationally dependant.
This gets a bit more difficult on the West Coast of Canada. 200' trees that are greater than 5' diameter at the base, and with foliage not always dense enough to stop or slow your descent. Reliable engines and/or prayer is the best alternative we could come up with.
#2 Trade airspeed for altitude. Same as above, you start in the entry phase, but in this case you approach the landing phase from below. When you flare on entry, you made need to raise collective to control rpm, but that is type and situation dependant. When your altitude & height intercept the landing phase, it gets easy again.
Last edited by Matthew Parsons; 29th Oct 2011 at 16:29.
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Bear in mind the helicopter rotor system takes about 1000 feet of descent to aerodynamically establish autorotation
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Eagle 86
"Tourist,
I guess your public profile says a lot about you"
Nice.
Always good to play the man not the argument.
(anyway, what's wrong with being a wafu?!)
Come to Predannack any day of the week and you will see a variety of autos.
My personal favourite was always the
3000' 90kts into wind Seaking comedy auto invented by a certain very grumpy pilot who now flys for Loganair.
On top the target (15kts of wind was nice), enter auto and rapidly ramp it up to hit zero speed without height loss.
As soon as zero airspeed, level the cab and spot turn in auto right.
After one complete turn, bury the nose to get 70kts and continue the turn into a 360 auto right.
As you approach the into wind heading dump the nose further to get into a range auto up near Vne.
As the wind has drifted you backwards, the two stage flare should bring you nicely back to a spot landing on the H.
I still remember my large ginger instructor on 810sqn demonstrating that for a real double engine failure, you can do whatever necessary to get you back to the spot. His party trick was to fly over directly the spot into wind 90kts 2000' and get the student to say when he thought it was no longer possible to get back to the spot in auto. When you eventually said "now" some 20 seconds past the spot he would wait another 10 seconds before entering auto then straight into a wingover in auto, dive back towards the spot before another wingover to bring him back into wind for the flare/check/level. As he said. "if you control the Nr, then the rest is just normal flying"
"Tourist,
I guess your public profile says a lot about you"
Nice.
Always good to play the man not the argument.
(anyway, what's wrong with being a wafu?!)
Come to Predannack any day of the week and you will see a variety of autos.
My personal favourite was always the
3000' 90kts into wind Seaking comedy auto invented by a certain very grumpy pilot who now flys for Loganair.
On top the target (15kts of wind was nice), enter auto and rapidly ramp it up to hit zero speed without height loss.
As soon as zero airspeed, level the cab and spot turn in auto right.
After one complete turn, bury the nose to get 70kts and continue the turn into a 360 auto right.
As you approach the into wind heading dump the nose further to get into a range auto up near Vne.
As the wind has drifted you backwards, the two stage flare should bring you nicely back to a spot landing on the H.
I still remember my large ginger instructor on 810sqn demonstrating that for a real double engine failure, you can do whatever necessary to get you back to the spot. His party trick was to fly over directly the spot into wind 90kts 2000' and get the student to say when he thought it was no longer possible to get back to the spot in auto. When you eventually said "now" some 20 seconds past the spot he would wait another 10 seconds before entering auto then straight into a wingover in auto, dive back towards the spot before another wingover to bring him back into wind for the flare/check/level. As he said. "if you control the Nr, then the rest is just normal flying"
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R22 autorotation
Hi guys
One thing i am having an hard time to understand and visualize is the relationship between cyclic movements and rrpms? Why rrpms change as the helicopter pitch is changed during the autorotation?
Many thanks
Baobab72
One thing i am having an hard time to understand and visualize is the relationship between cyclic movements and rrpms? Why rrpms change as the helicopter pitch is changed during the autorotation?
Many thanks
Baobab72
Think of it as a windmill. If you blow towards the windmill perpendicular to the windmill rotor plane (directly into the windmill rotor), you get a lot of rotation in the system. That same windmill, if you blow from the side, you won't get anything out of it.
Cyclic alters your attitude towards the airflow or if you will alters the relative direction of the airflow in relation to the rotor disc. Flare - increase of area-airflow, cyclic forward, decrease in area through which the airflow is going (just like blowing the windmill from the side.
Cyclic alters your attitude towards the airflow or if you will alters the relative direction of the airflow in relation to the rotor disc. Flare - increase of area-airflow, cyclic forward, decrease in area through which the airflow is going (just like blowing the windmill from the side.
Baobab
When you bring the cyclic back, you are raising the nose and changing the angle the air meets the blades - this will give more lift (and drag) but the extra lift will make the blades try to fly up (cone up). As the blades cone up, their centres of gravity will move towards the rotorhub (only a small amount) enough to increase the RRPM because of the conservation of angular momentum (think of a spinning ice skater who brings her arms in to spin faster and out to spin slower).
The opposite is true when you move the cyclic forward (this can be very dangerous in a teetering head helicopter if you do it quickly as it can lead to mast bumping).
In summary - bringing the cyclic back increases RRPM in auto (and reduces speed) and moving it forward decreases RRPM (and increases speed). It is the rate of change of the airflow that determines how quickly the RRPM rises or falls.
When you bring the cyclic back, you are raising the nose and changing the angle the air meets the blades - this will give more lift (and drag) but the extra lift will make the blades try to fly up (cone up). As the blades cone up, their centres of gravity will move towards the rotorhub (only a small amount) enough to increase the RRPM because of the conservation of angular momentum (think of a spinning ice skater who brings her arms in to spin faster and out to spin slower).
The opposite is true when you move the cyclic forward (this can be very dangerous in a teetering head helicopter if you do it quickly as it can lead to mast bumping).
In summary - bringing the cyclic back increases RRPM in auto (and reduces speed) and moving it forward decreases RRPM (and increases speed). It is the rate of change of the airflow that determines how quickly the RRPM rises or falls.
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coning 'cause' doubtful - taught but doubtful
Ice skaters can't keep moving their arms in forever - the coning explanation at best is only a (transient) minor contributory effect - likely to be the dominant effect is the Total Reaction becoming 'more forward' (causing more 'autorotative force') corresponding to the increased angle of attack derived from increased negative induced flow achieved by the flare.
You can see this by holding an increased loading through bank in auto where the transient coning effect does not manifest itself above the effect of the ongoing continuous bank.
You can see this by holding an increased loading through bank in auto where the transient coning effect does not manifest itself above the effect of the ongoing continuous bank.
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or even more simply
like Phoinix says but more simply:
the thing that makes the blades go around in auto rotation is the air passing up through the disc.
when you pull the nose up more air goes through the disc faster, so it'll get more of what makes the blades go around - that will make the blades go faster.
the thing that makes the blades go around in auto rotation is the air passing up through the disc.
when you pull the nose up more air goes through the disc faster, so it'll get more of what makes the blades go around - that will make the blades go faster.
Another thing to throw into the mix - a fixed wing in a fully developed spin is in autorotation - after the stall and subsequent prospin control inputs a considerable amount of height is lost until stable autorotation is established. GAGS E86
I've taught spins in FW, and autorotations in helicopters, and I'd never take the position that they are the same sort of maneuver. Not sure what you are after there.
A helicopter autorotation is a different critter from a spin. The rotating wings are not stalled during the evolution, and the rest of the aircraft, once the torque is reduced/removed by reducing pitch, doesn't flop about. You remain in controlled flight.
About autorotations to check control rigging:
I agree that it takes some altitude to establish a steady state auto, steady state ROD, and your test airspeed right on the money to take the precise data point needed to check control rigging. I think it's a bit much to use such a requirement to assert that you aren't in a stable auto in less than a thousand feet. Depending on rotor inertia, you can get stable enough in light helicopters to make a complete auto in less altitude. In the big and heavy ones, maybe not ... haven't done on in a while, so I'll not go any further on that score.
eagle, I very much liked the list of cases you provided for reference, since engine out and auto fit into a lot of different conditions, and one needs to be prepared for any of them.
Great thread, by the way.
Thanks to all who have contributed.
Ice skaters can't keep moving their arms in forever
increased angle of attack derived from increased negative induced flow achieved by the flare.
when you pull the nose up more air goes through the disc faster, so it'll get more of what makes the blades go around - that will make the blades go faster.
