Translational lift flying backwards
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Translational lift flying backwards
As an interested outsider in the world of helicopters, I was wondering if it is possible to enter translational lift whilst flying backwards, and if so is that a bad place to be, I assume the back end would want to swap places with the front and a whole host of other nasties.
Thanks for any replies
Wigglyamps
Thanks for any replies
Wigglyamps
You can get it forwards, sideways, backwards, whenever the airflow across the disc is about 12 kt.
The directional stability will become a problem when you get towards the limits of tail rotor authority, and it will want to weathercock into wind. Also when going backwards the horizontal stabiliser will cause you to tuck the cyclic back into your intestines, and again you will run out of cyclic authority and have a windscreen full of ground, somewhat scary.
The directional stability will become a problem when you get towards the limits of tail rotor authority, and it will want to weathercock into wind. Also when going backwards the horizontal stabiliser will cause you to tuck the cyclic back into your intestines, and again you will run out of cyclic authority and have a windscreen full of ground, somewhat scary.
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I must admit to never having considered the question that there might be a good reason to try this kind of manoeuvre. Backward departure through translational lift? Military skill to depart-from-dodge quickly?
Anyway, the backward manoeuvre can be even more interesting, if you are not careful. I have not tried to fly that profile myself, but someone did demo it to me recently (very windy day).
When you increase speed and pass through translational lift, as the disk flaps back the direction of travel means that it lifts the tail (it would have been the nose in fwd flight, and we simply push through with the cyclic). Going backwards, as the tail lifts the air-flow then gets under the horizontal stabilizer (if you have one) and lifts the tail further. That creates a positive-feedback loop as the angle of attack on the stabilizer increases. The ship will rapidly adopt a "very severe" nose-down attitude. If the pilot is not anticipating this event there is a natural tendency for him / her to pull-back on the cyclic which - particularly in a teetering head rotor machine - creates a severe risk of amputating the tail. The control input reactions are therefore not intuitive.
Glad to have been in the demo, and even more sure than before that I will not be trying to effect a departure backwards.
Anyway, the backward manoeuvre can be even more interesting, if you are not careful. I have not tried to fly that profile myself, but someone did demo it to me recently (very windy day).
When you increase speed and pass through translational lift, as the disk flaps back the direction of travel means that it lifts the tail (it would have been the nose in fwd flight, and we simply push through with the cyclic). Going backwards, as the tail lifts the air-flow then gets under the horizontal stabilizer (if you have one) and lifts the tail further. That creates a positive-feedback loop as the angle of attack on the stabilizer increases. The ship will rapidly adopt a "very severe" nose-down attitude. If the pilot is not anticipating this event there is a natural tendency for him / her to pull-back on the cyclic which - particularly in a teetering head rotor machine - creates a severe risk of amputating the tail. The control input reactions are therefore not intuitive.
Glad to have been in the demo, and even more sure than before that I will not be trying to effect a departure backwards.
"Just a pilot"
As to why, how about turning the tail away
from the pad entry area? Used to be a common situation in the Gulf of Mexico after a cold front. Pax exiting and loading the aircraft, especially on break day weren't always completely 'in the moment' and lots of pads with only a single access point and it could well be on the downwind. A pilot feels helpless watching somebody chasing something downwind towards towards a low tail rotor...
Allowed to routinely operate in winds up to 40 knots, being above ETL was common in the hover. As mentioned, one might have the cyclic well aft, even bouncing against the aft stop, depending on CG and aircraft being flown. Technique was slow turn, evaluating control authority all the way. The real trick was getting the nose slowly back into the wind after loading.
Allowed to routinely operate in winds up to 40 knots, being above ETL was common in the hover. As mentioned, one might have the cyclic well aft, even bouncing against the aft stop, depending on CG and aircraft being flown. Technique was slow turn, evaluating control authority all the way. The real trick was getting the nose slowly back into the wind after loading.
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Rearwards flying
Not sure if it adds much but some might like to know I frequently demonstrate various Enstrom models in rearward flight at speed ... (28/280/480) In fact with a 15 knot following breeze, I've recorded a 70 knots groundspeed on the GPS. A good example can be seen on my North Weald Enstrom 28F display sequence. (video on the opening page of my web site which includes the loop) ... I've often thought it would be nice to have a rearward reading ASI fitted for display flying. There is an open secret.
To obtain anything over 50 knots the airframe needs to be flown fairly rapidly through the translation lift and associated flap-forward area using aft cyclic to achieve a steep disc angle relative to the surface. The technique is to replace a high percentage of conventional translation air flow with the good old induced airflow. Not sure what it proves though! Perhaps to say the usual ... "Don't try this without a good FI/DAE on board." Happy flying to all pps. Dennis K.
To obtain anything over 50 knots the airframe needs to be flown fairly rapidly through the translation lift and associated flap-forward area using aft cyclic to achieve a steep disc angle relative to the surface. The technique is to replace a high percentage of conventional translation air flow with the good old induced airflow. Not sure what it proves though! Perhaps to say the usual ... "Don't try this without a good FI/DAE on board." Happy flying to all pps. Dennis K.
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I remember , it used to be part of a check test flight on the Sea King for tail rotor. When you want to stop going backwards and you push the cyclic forwards, you really get a windscreen full of ground as you get the flapping disk action plus the forward cyclic. Even expecting it, it makes your bum start eating the seat!
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Hit 82kts with a Notar during flight testing in Mesa years ago...fun it was
During the Whirlwind course we were given a QHI demo of the effect of recovery from fast rearwards flight, in a backwards hover taxi. The old bird used to kick up her heels quite rapidly (about the only time it did anything rapidly).
I used rearwards translational lift during my time displaying the Puma HC1. One manoeuvre was to climb at full power from the hover whilst flying backwards at the fastest controllable speed. On reaching 450 feet agl, leveling off allowed the airflow to suddenly build under the horizontal stabiliser. Applying forward cyclic allowed the tail to rise (rather than the nose to drop) until the pitch attitude reached 90 degrees nose down. This was followed by a downwards vertical roll through 180 degrees, allowing the aircraft to be flown away in the opposite direction. A full 90 degrees nose down was needed, otherwise after the 180 degree roll the aircraft came out more nose down than vertical.
During one display practice the nose tucked right under to an estimated 120 degrees nose down at the top of the manoeuvre (estimated by the ground observer, my Squadron Boss) That one was bit exciting (you can't see the sky at all until you've rolled through and the ground is very rapidly approaching!) but at least after the 180 roll we were away from the vertical.
I used rearwards translational lift during my time displaying the Puma HC1. One manoeuvre was to climb at full power from the hover whilst flying backwards at the fastest controllable speed. On reaching 450 feet agl, leveling off allowed the airflow to suddenly build under the horizontal stabiliser. Applying forward cyclic allowed the tail to rise (rather than the nose to drop) until the pitch attitude reached 90 degrees nose down. This was followed by a downwards vertical roll through 180 degrees, allowing the aircraft to be flown away in the opposite direction. A full 90 degrees nose down was needed, otherwise after the 180 degree roll the aircraft came out more nose down than vertical.
During one display practice the nose tucked right under to an estimated 120 degrees nose down at the top of the manoeuvre (estimated by the ground observer, my Squadron Boss) That one was bit exciting (you can't see the sky at all until you've rolled through and the ground is very rapidly approaching!) but at least after the 180 roll we were away from the vertical.
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Be careful of a downwind departure, which is sometimes necessary. Especially if you're near max gross weight. You can go from translational lift from the rear to out of it on takeoff as you accelerate, and suddenly you can be out of ground effect, out of lift, out of power, and out of ideas simultaneously. It's not that you should never do it, it's that you should always think and plan carefully before you do it.
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When I was younger and quite stupid, I tried rearward translational lift in a 365C. The rapid flip to a 90 degree nose down attitude certainly caught my attention, and reinforced the fact that even the most docile of aircraft will bite you if mishandled.
And life gets even more interesting on a Bell with a "sync elevator". (47, 204/205, 212 etc)
Hence the placard "Protracted rearward flight prohibited".
It works against you when flying backwards like a canard in reverse.
Then again on a Bell Medium translating rearwards can be beneficial.
Forwards, backwards, sideways is of no consequence to the main rotor, its more about the pieces hanging below it that becomes a "piloting" issue.
Hence the placard "Protracted rearward flight prohibited".
It works against you when flying backwards like a canard in reverse.
Then again on a Bell Medium translating rearwards can be beneficial.
Forwards, backwards, sideways is of no consequence to the main rotor, its more about the pieces hanging below it that becomes a "piloting" issue.
.
Most advantageous wind direction - to make maximum use of translational lift - in the Chinook was taught as from the 9-10 o'clock position.
Main problem with rearwards flight at speed in that aircraft wasn't the aerodynamic effects, it was the reversal of airflow through the engines. Folk watching the Chinook display may occasionally have heard the rapid popping sound associated with the early stages of compressor surge/ stall.
Main problem with rearwards flight at speed in that aircraft wasn't the aerodynamic effects, it was the reversal of airflow through the engines. Folk watching the Chinook display may occasionally have heard the rapid popping sound associated with the early stages of compressor surge/ stall.
John R81 - a much more useful manoeuvre (especially in a strongish wind) is - starting from an into wind hover, climb slightly as you move backwards keeping straight initially with pedals then, at a groundspeed that retains translational lift, yaw the aircraft gently round, maintaining the cyclic moving in the desired direction of travel (so it starts in the 6 o'clock position and gradually comes round to the 12 o'clock) until you are now flying in the opposite direction to the initial hover.
This is useful in mountain valleys, areas where there are obstacles ahead or just for handling practice.
You could just turn downwind and do a downwind transition but you might not have enough TR control in 30 to 40 knots for that.
The skill is to maintain translational lift throughout so you don't get any Tq spikes or run out of power.
Best practised on a 15-20 knot day before attempting it in a stronger wind.
The yaw is best done with power pedal so that any weathercocking doesn't cause Tq spikes (shouldn't happen if you keep ETL).
This is useful in mountain valleys, areas where there are obstacles ahead or just for handling practice.
You could just turn downwind and do a downwind transition but you might not have enough TR control in 30 to 40 knots for that.
The skill is to maintain translational lift throughout so you don't get any Tq spikes or run out of power.
Best practised on a 15-20 knot day before attempting it in a stronger wind.
The yaw is best done with power pedal so that any weathercocking doesn't cause Tq spikes (shouldn't happen if you keep ETL).
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Rearward autorotation
Ref the above ... it doesn't have to be somewhere.
I've actually sat alongside a highly type experienced and especially competent UK pilot who demonstrated a rearward entry into autorotation from 400 feet and the following descent terminating with a full skids-on touch down while remaining in rearward flight. The type was a Robinson! DRK.
I've actually sat alongside a highly type experienced and especially competent UK pilot who demonstrated a rearward entry into autorotation from 400 feet and the following descent terminating with a full skids-on touch down while remaining in rearward flight. The type was a Robinson! DRK.