Height/Velocity Curve: merged threads
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wg13:
There is a large body of evidence that just lowering the collective is not all that's needed - at least in most helicopters. Adding aft cyclic is a very important part of maintaining rotor RPM. Lowering the nose to get airspeed is a relatively minor part of the whole equation and should only be done if the rotor RPM is in the proper range.
Matthew:
For Part 29 helicopters, with more than 9 passengers, the HV curve is a limitation. and must be avoided (except in the US when operating off oil rigs).
There is a large body of evidence that just lowering the collective is not all that's needed - at least in most helicopters. Adding aft cyclic is a very important part of maintaining rotor RPM. Lowering the nose to get airspeed is a relatively minor part of the whole equation and should only be done if the rotor RPM is in the proper range.
Matthew:
For Part 29 helicopters, with more than 9 passengers, the HV curve is a limitation. and must be avoided (except in the US when operating off oil rigs).
I think its a 'given' that we flare as well as lower the coll. It appeared that Andy was suggesting that a pilot should concentrate on the cyclic only, to the detriment of lowering the collective. IA's for engine failure are of course lower collective, flare the aircraft. Brain time to reaction time will mean by the time you realise what has occured, NR may well by down or near to a non recoverable level. The flare will of course give a much needed increase in NR. Hopefully all heli pilots will instinctively lower coll and flare simultaneously. But, and the important part is, if the lever isn't lowered, a flare becomes rather pointless as the flare will only have effect for as long as the flare is held/increased. Ie G loading, C of G shift along the blade, et voila ice skater.
I think given the choice of only one control to move, I'd opt for the collective thanks.
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212 man said.
I believe that the lower part of the H/V curve shaded area on the left is calculated for takeoff and the upper part is calculated for the cruise although the join is not shown, and slightly different assumptions are made for the cruise part, i.e. a slightly longer delay before reacting etc.
also on many graphs (R22 is a good example) a recommended take off profile which remains outside the shaded area is shown.
also I am talking about single engine helicopters so Cat A, is not relevent.
maybe a clever chap could give us the definitive answer on this ?
regards
CF
is not true; the H/V curve is plotted for level flight, and the graph in the RFM will often state this. The published graph is only valid for one set of ambient and weight conditions whereas in reality it expands and contracts. It may even disappear given the right circumstances (how else can you do a vertial Cat A Take Off?)
also on many graphs (R22 is a good example) a recommended take off profile which remains outside the shaded area is shown.
also I am talking about single engine helicopters so Cat A, is not relevent.
maybe a clever chap could give us the definitive answer on this ?
regards
CF
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Shawn,
Thanks. I wasn't sure of all the operations in which it was a limitation, so I just said its "not usually prohibited". Is that the only situation where it is a limitation?
Matthew.
Thanks. I wasn't sure of all the operations in which it was a limitation, so I just said its "not usually prohibited". Is that the only situation where it is a limitation?
Matthew.
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SO..................
The general oppinion seems to be don't go into it if you can avoid it and if you do have to go into it use extreme caution and if you're unhappy about that.....stay in your bed!!!
For all you instructors out there, here's a way to win a pint off your students, I certainly lost when I was asked as a student many years ago: Following your first autorotation with a student ask him to guess within 10 knots the vertical speed of the heli just before you flare and tell him you'll buy the beer if he's right and he buys it if he's outwith the 10 knot limit......works every time!!! If you're feeling lucky you can extend the 10 knot limit. I have had answers that vary from 60 knots to 100 knots with nobody guessing correctly. Really doen't seem like 20 knots though does it?????
For all you instructors out there, here's a way to win a pint off your students, I certainly lost when I was asked as a student many years ago: Following your first autorotation with a student ask him to guess within 10 knots the vertical speed of the heli just before you flare and tell him you'll buy the beer if he's right and he buys it if he's outwith the 10 knot limit......works every time!!! If you're feeling lucky you can extend the 10 knot limit. I have had answers that vary from 60 knots to 100 knots with nobody guessing correctly. Really doen't seem like 20 knots though does it?????
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Rate of climb vs. Rate of Descent
A common thing among students being tested is that they try to keep a standard approach into confines, bombing into a footballfield-sized LZ at 65kts. This usually results in a quick-stop 20ft above the intended landing spot, and then a discussion about the height velocity diagram.
The diagram shows the alt/speed combinations from which a succesful flare can be made, landing safely. The extra caution needed when operating in the avoid area, means you have to be prepared to dump collective and try and recover some speed for a proper flare.
On approach into a confined area, I usually teach students to balance a comfortable speed, and a safe speed. Too safe/high speed means they might fly a perfectly serviceble chopper into trees, or do one of the other stupid things associated with non-practical flying. To slow and you lose translational lift, and once again its not practical, and even dangerous if they're descending.
Being at 150ft at 35kts is right in the curve, but you're collective will already be down, and a rate of descent will already be established. Very close to an auto.
My personal opinion is that a passenger safe landing can still be made if the engine fails, and lets face it, if the donkey quits, that is really all that matters. Even if you do a perfect auto, there is a lot of luck involved in putting a machine down without a scratch.
If I remember right, it's been mentioned in a previous thread that the 407 has a max safe climb of 2000ft/min, just so that the blades are still turning by the time autorotative rate of descent can be established.
The diagram shows the alt/speed combinations from which a succesful flare can be made, landing safely. The extra caution needed when operating in the avoid area, means you have to be prepared to dump collective and try and recover some speed for a proper flare.
On approach into a confined area, I usually teach students to balance a comfortable speed, and a safe speed. Too safe/high speed means they might fly a perfectly serviceble chopper into trees, or do one of the other stupid things associated with non-practical flying. To slow and you lose translational lift, and once again its not practical, and even dangerous if they're descending.
Being at 150ft at 35kts is right in the curve, but you're collective will already be down, and a rate of descent will already be established. Very close to an auto.
My personal opinion is that a passenger safe landing can still be made if the engine fails, and lets face it, if the donkey quits, that is really all that matters. Even if you do a perfect auto, there is a lot of luck involved in putting a machine down without a scratch.
If I remember right, it's been mentioned in a previous thread that the 407 has a max safe climb of 2000ft/min, just so that the blades are still turning by the time autorotative rate of descent can be established.
CF, you are correct in so far as there is a distinction between reaction times: the lower portion of thre curve is assumed to use take off power and normal reaction times whereas the upper portion is assumed to be the cruise and requires a 1 second delay befor intervening.
The definitive answers on what is required and how to demonstrate them are found in the two ACs for Part 29 and 27:
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/29e440234b4cc27586256e4300597dd7/$FILE/AC-29-2Cincorporated.pdf
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/6f95f1c7b8b1e89c86256e4300597919/$FILE/AC-27-1Bincorporated.pdf
The definitive answers on what is required and how to demonstrate them are found in the two ACs for Part 29 and 27:
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/29e440234b4cc27586256e4300597dd7/$FILE/AC-29-2Cincorporated.pdf
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/6f95f1c7b8b1e89c86256e4300597919/$FILE/AC-27-1Bincorporated.pdf
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I've often wondered why the N1 buzzer can't be fitted to a servo which drops collective, overcoming delayed response hence Nr drop. Problem would be when near enough to ground to be auto flaring. A low power/cost radar or laser sensor could detect ground proximity, to modify collective response.
Any thoughts on whether this system would improve safety?
Any thoughts on whether this system would improve safety?
Just imagine the scenario: as you are hovering sideways towards your refuelling spot your safety system decides to smartly lower your collective. D'oh!
Considering the relative "frailty" of ancilliary systems as I've encountered them I feel the proposed system would likely do more harm than good in the long run.
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Ask Airbus about automatic systems assuming you are landing!
Actually, the h/v curve should still be observed when flying a twin, in case an engine fails (it is critical for the Bell 212), but it is true to say that some machines (like the AS 355) do not have a height/velocity curve at all for some flight regimes.
Phil
Actually, the h/v curve should still be observed when flying a twin, in case an engine fails (it is critical for the Bell 212), but it is true to say that some machines (like the AS 355) do not have a height/velocity curve at all for some flight regimes.
Phil
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I've often wondered why the N1 buzzer can't be fitted to a servo which drops collective, overcoming delayed response hence Nr drop. Problem would be when near enough to ground to be auto flaring. A low power/cost radar or laser sensor could detect ground proximity, to modify collective response.
Any thoughts on whether this system would improve safety?
Graviman you must trust electronics more than most people!!
Any thoughts on whether this system would improve safety?
Graviman you must trust electronics more than most people!!
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Krobar:
There are many combinations of height and velocity where there is no use in attempting to do anything other than get the skids pointed in the direction of travel, level with the ground or slightly nose up and apply collective to stop the rate of descent. Trying to get airspeed to flare if you are less than 30 knots is a waste of time, and trying to teach a student to get airspeed to flare regardless of the initial conditions is not a good thing.
Nearly any time below 40 knots and 200 feet, there is nothing that will get you enough airspeed to flare. But you need to know that. A trip around the edge of the HV curve with an experienced instructor is worthwhile to show you the variables and procedures to use.
There are many combinations of height and velocity where there is no use in attempting to do anything other than get the skids pointed in the direction of travel, level with the ground or slightly nose up and apply collective to stop the rate of descent. Trying to get airspeed to flare if you are less than 30 knots is a waste of time, and trying to teach a student to get airspeed to flare regardless of the initial conditions is not a good thing.
Nearly any time below 40 knots and 200 feet, there is nothing that will get you enough airspeed to flare. But you need to know that. A trip around the edge of the HV curve with an experienced instructor is worthwhile to show you the variables and procedures to use.
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If you try top flare below 30, you will 'fall through', increasing your rate of descent. I teach my students autorotations from different entry points and below 30kts is a steady state. I do teach EOL/full downs, I'm a true believer in the real thing.
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AndyJB32,
what ever happened to "flying the aircraft first"? I'm inclined to agree more with wg13 when it comes to entering autorotation. Your harsh flare is not only going to bleed a fair amount of energy off (negating the flare effect you're after), but you are now limiting your options on range aswell. I would suggest that a coordinated entry into autorotation, whilst picking a field is going to give you more options and greatest time to pull off that peach of landing if, heaven forbid, it should all go quiet on you
. Remember, once it drops below that critical point, no amount of flare is going to get it back. Think "smooth" baby . Happy landings,
RotP
what ever happened to "flying the aircraft first"? I'm inclined to agree more with wg13 when it comes to entering autorotation. Your harsh flare is not only going to bleed a fair amount of energy off (negating the flare effect you're after), but you are now limiting your options on range aswell. I would suggest that a coordinated entry into autorotation, whilst picking a field is going to give you more options and greatest time to pull off that peach of landing if, heaven forbid, it should all go quiet on you
. Remember, once it drops below that critical point, no amount of flare is going to get it back. Think "smooth" baby . Happy landings,RotP
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. I am a little faster in a helicopter however - honest
Why won't the aircraft enter autorotation if you take pitch completely off the blades? Isn't that fundamental to our training and understanding of autorotation?
Intial decay in RRPM will occur almost certainly, and a flare will help to offset this problem. But the flare is no substitute or priority over lowering the collective surely? In an engine failire, blade pitch must be reduced as a priority, there no longer being power to overcome the drag penalty. Don't lower the collective in a timely fashion and the aerofoil will stall. Highly undesirable.
Simultaneous lowering and flare - sure. But prioritising the flare?
The flare will load the disk and increase RRM temporarily, as you say. In this "bought time" you lower the collective, the nose drops now and RRPM decays in pretty much the proportion you gained through a timely flare. Is that a safe?
TT
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My initial post was in response to the suggestion of an automatic collective lowering device to help out with engine failures. I think this is a very bad idea, and would cause more problems than it could possibly solve. I have never intended to suggest that an auto should be entered without lowering the collective, only trying to point out that lowering the collective mechanically without any cyclical input will not help much. Please carefully re-read my posts if you think this is what i'm implying Rotatingpart and Torquetalk. As for the the aircraft entering autorotation, as far as i understand it, the important part is the angle of attack rather than the actual pitch angle. Although these are closely related they are not the same thing. A blade can stall at pratically any pitch angle, but will only stall at specific angles of attack (i think these may be american terms as i trained out in the states) Look at chapter 16 in Helicopter Aerodynamics by R.W.Prouty for more detailed info.
Last edited by AndyJB32; 13th Dec 2006 at 22:16.
Well, being misunderstood can be very frustrating. and being misquoted can be too.
then again, if in a corrective post you write:
"If i were in level flight, and (purely hypothetically) had a choice of only moving one control in the immediate aftermath of an engine failure, i'd choose the cyclic"
..then not too surprising if a few people get involved in the misunderstanding...
less head banging and more post checkin methinks
yours in sport
TT
then again, if in a corrective post you write:
"If i were in level flight, and (purely hypothetically) had a choice of only moving one control in the immediate aftermath of an engine failure, i'd choose the cyclic"
..then not too surprising if a few people get involved in the misunderstanding...
less head banging and more post checkin methinks
yours in sport
TT