Hover Power + 10%
Joined: Mar 2003
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From: UK
PA42,
In answer to your 2 queries, I hope this may help:
1) British basic mil training is to initiate transition and maintain height with collective. After translational lift, apply hover power plus 10% (so if hovering at 70%, apply 80%). Once established, transfer to a type specific power (so for Sea King, apply 70% matched torques once established at 70 kts). I guess the basic principle is a mid-power setting at min power speed, giving greatest rate of climb for that power setting. That is the standard academic transition as I was taught. Willing to accept other variations by those more experienced than I. These procedures are laid out in the various British mil "Student's Study Guides" issued for various courses.
2) I guess the main reason for not applying max power in transition (except when you're in a hurry!) is so that in the event of an engine failure you haven't got a large amount of pitch applied at low speeds. I think this would make entry into autorotation or achieving a single engine flyaway that much more difficult. Again I defer to those with a greater knowledge.
SBW
In answer to your 2 queries, I hope this may help:
1) British basic mil training is to initiate transition and maintain height with collective. After translational lift, apply hover power plus 10% (so if hovering at 70%, apply 80%). Once established, transfer to a type specific power (so for Sea King, apply 70% matched torques once established at 70 kts). I guess the basic principle is a mid-power setting at min power speed, giving greatest rate of climb for that power setting. That is the standard academic transition as I was taught. Willing to accept other variations by those more experienced than I. These procedures are laid out in the various British mil "Student's Study Guides" issued for various courses.
2) I guess the main reason for not applying max power in transition (except when you're in a hurry!) is so that in the event of an engine failure you haven't got a large amount of pitch applied at low speeds. I think this would make entry into autorotation or achieving a single engine flyaway that much more difficult. Again I defer to those with a greater knowledge.
SBW
Joined: Dec 2001
Posts: 1,835
Likes: 3
From: Philadelphia PA
pa42 and I discussed this one night in one of Mojave's finer dining establishments (OK, so it was one of two non-fast food restraunts...)
One of the issues is that there is little (no?) standardization in the civil training world either for flight test requirements or for instructor training. Not helping this is the ludicrously low hours and relative lack of instructor training needed to be a helicopter instructor in the USA.
The military has perhaps a better handle on this, but could be said to be guilty of only considering military scenarios and types - ones that currently have a lot of power in the training environment. (how many training trips get done at maximum weight and high pressure altitudes and temperatures with minimum or no power margin?)
It would be interesing to see what the feedback from the folks flying in Afghanistan has been, and even more interesting to have someone put together a matrix of helicopter maneuvers that a student pilot should be able to demonstrate in a private and then commercial flight test. Some countries require sling load training as part of the commercial syllabus, where the FAA has a separate 'rating' for slung loads, as just one example.
One of the issues is that there is little (no?) standardization in the civil training world either for flight test requirements or for instructor training. Not helping this is the ludicrously low hours and relative lack of instructor training needed to be a helicopter instructor in the USA.
The military has perhaps a better handle on this, but could be said to be guilty of only considering military scenarios and types - ones that currently have a lot of power in the training environment. (how many training trips get done at maximum weight and high pressure altitudes and temperatures with minimum or no power margin?)
It would be interesing to see what the feedback from the folks flying in Afghanistan has been, and even more interesting to have someone put together a matrix of helicopter maneuvers that a student pilot should be able to demonstrate in a private and then commercial flight test. Some countries require sling load training as part of the commercial syllabus, where the FAA has a separate 'rating' for slung loads, as just one example.
Joined: Jun 1999
Posts: 362
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From: Australasia
pa42,
The 10% rule of thumb is certainly fairly widespread in countries influenced by the Royal Air Force Central Flying School (their instructors' school).
It is very simply the result of flight path control along an upwardly inclined take-off gradient that does not trade-off hover height for forward speed. When you work out the desirable speed/altitude/attitude/pitch parameters that ensure a sensible and consistent approach to a safe take-off, you come up with a positive power increment over hover power that is not so large as to result in significant nose-down attitudes and unnecessarily high blade angles.
In very high powered machines, it is relatively easy to get to such a low nose attitude that an early engine failure will have you in the dirt before you can get back to a level flight attitude for the touchdown or save much in the way of rotor RPM. That is usually why the HV diagram has a low speed/low altitude lump in it.
In very low powered machines, any power margin may well be a luxury but the risk management principles embodied in the take-off technique remain essentially unchanged.
As for your question about using the available power (notwithstanding the obvious complications if there is lots of it), the engineering concept is to use as much as you need but avoid max'ing things out because the internal stresses are what limit the engine life. That is why big jets use derated or "flex" power at every opportunity.
Stay Alive
The 10% rule of thumb is certainly fairly widespread in countries influenced by the Royal Air Force Central Flying School (their instructors' school).
It is very simply the result of flight path control along an upwardly inclined take-off gradient that does not trade-off hover height for forward speed. When you work out the desirable speed/altitude/attitude/pitch parameters that ensure a sensible and consistent approach to a safe take-off, you come up with a positive power increment over hover power that is not so large as to result in significant nose-down attitudes and unnecessarily high blade angles.
In very high powered machines, it is relatively easy to get to such a low nose attitude that an early engine failure will have you in the dirt before you can get back to a level flight attitude for the touchdown or save much in the way of rotor RPM. That is usually why the HV diagram has a low speed/low altitude lump in it.
In very low powered machines, any power margin may well be a luxury but the risk management principles embodied in the take-off technique remain essentially unchanged.
As for your question about using the available power (notwithstanding the obvious complications if there is lots of it), the engineering concept is to use as much as you need but avoid max'ing things out because the internal stresses are what limit the engine life. That is why big jets use derated or "flex" power at every opportunity.
Stay Alive
Joined: Oct 2003
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From: Over here
Long ago and far away, when the UH1 was cutting-edge helicopter technology, we used to bet a beer on who could take off with the least power. Like lying, you didn't want to be the first to go, since burning off some fuel in the (often very close) pattern made it easier. We never hovered, we just got light on the skids and started dragging forward until we lifted off. Not a recommended technique in most cases, but sometimes military necessity demanded getting airborne with the maximum payload, book gross weight limits be damned. Hover power was not required, much less 10% above that. I still benefit from that practice, because it demanded a smooth control touch and patience. I don't remember any formal training that called for a specific amount of torque, at least in U.S. Army schools. After flight school, we started learning to fly for real. In the GOM, we regularly take off with barely hover power, both from the beach and from platforms/rigs. The same techniques are called for, but we do hover before takeoff.
As for max power on takeoff, I think this illustrates yet another difference between planks and real aircraft. Planks depend entirely on airpseed and altitude. Helicopters don't. As long as I have sufficient rotor RPM, I have a good chance of a survivable landing anywhere. The higher the pitch, the lower my chances of maintaining RPM, or conversely, the lower the pitch, the easier it usually is to keep the RPM in an emergency. My technique is to pull whatever power I feel necessary at the time, and no more, thus keeping stress on the engines, transmission, tail rotor, and everything else at a minimum. I want everything to keep working all the way. You are certainly free to use whatever technique you prefer in your cockpit.
As for max power on takeoff, I think this illustrates yet another difference between planks and real aircraft. Planks depend entirely on airpseed and altitude. Helicopters don't. As long as I have sufficient rotor RPM, I have a good chance of a survivable landing anywhere. The higher the pitch, the lower my chances of maintaining RPM, or conversely, the lower the pitch, the easier it usually is to keep the RPM in an emergency. My technique is to pull whatever power I feel necessary at the time, and no more, thus keeping stress on the engines, transmission, tail rotor, and everything else at a minimum. I want everything to keep working all the way. You are certainly free to use whatever technique you prefer in your cockpit.
Joined: Dec 2001
Posts: 1,835
Likes: 3
From: Philadelphia PA
Gomer Pylot:
Nicely summed up a lot of things.
Bu there is something missing between the FAR Part 27 certification and the operations rules / practices from what I've seen.
Part 27 (and 29) require that the takeoff profiles and distances and airspeeds recommended for same keep the helicopter free of the HV diagram. The bottom of the HV diagram for civil helicopters requires that the power used is takeoff power (which may be all you have at the conditions used for the HV curve of minimum 7,000' DA and maximum weight to hover OGE). Yet we don't use that sort of power normally in our day to day work.
If we're worried about takeoff distance or getting some air between sphincter and ground, then climbing out at the airspeeds recommended may take too much distance. It would be safer for an engine failure than climbing out sooner, but that may not be much use to you when faced with an obstacle.
Sorry to ramble on like this, but we do need some more guidance on the best way to do things.
Nicely summed up a lot of things.
Bu there is something missing between the FAR Part 27 certification and the operations rules / practices from what I've seen.
Part 27 (and 29) require that the takeoff profiles and distances and airspeeds recommended for same keep the helicopter free of the HV diagram. The bottom of the HV diagram for civil helicopters requires that the power used is takeoff power (which may be all you have at the conditions used for the HV curve of minimum 7,000' DA and maximum weight to hover OGE). Yet we don't use that sort of power normally in our day to day work.
If we're worried about takeoff distance or getting some air between sphincter and ground, then climbing out at the airspeeds recommended may take too much distance. It would be safer for an engine failure than climbing out sooner, but that may not be much use to you when faced with an obstacle.
Sorry to ramble on like this, but we do need some more guidance on the best way to do things.
Joined: Oct 2003
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From: Over here
Shawn, max power is required? Are you saying that while supposedly under the HV curve, with less than maximum power, we may actually be inside the curve? Or am I misunderstanding you? It seems rather counterintuitive. I've always assumed that if I were using less power, then I was further outside the HV curve. Have I been mistaken all these years?
Joined: Jun 1999
Posts: 362
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From: Australasia
Gomer,
Testing at the maximum (power=blade angle) means that the transition to autorotation is worst case - using less power may mean a slower transit through the speed/altitude paramenters but it should mean more RRPM in the event of a nasty than otherwise. It is also much easier to define as a certification standard, since there is little practical doubt about take-off power for flight test purposes.
Stay Alive
Testing at the maximum (power=blade angle) means that the transition to autorotation is worst case - using less power may mean a slower transit through the speed/altitude paramenters but it should mean more RRPM in the event of a nasty than otherwise. It is also much easier to define as a certification standard, since there is little practical doubt about take-off power for flight test purposes.
Stay Alive
Joined: Oct 2002
Posts: 396
Likes: 1
From: US...for now.
Parallel universes! Why, the helicopter industry is so full of them that I sometimes feel as though I'm stuck inside a tic-tac-toe diagram.
I am not a private pilot. I fly commercially and have a buttload of flight time. Hence, I am confident that if my one and only engine should fail I will be able to get the pitch down before the RRPM needle falls off the scale. I am also a plank driver. Having said that...
If I were always taking off from nice, flat, unobstructed heliports at which my direction of flight coincided with the takeoff direction, then I might use some sort of "minimum power" technique. But as a working helo pilot, I often find myself operating from heliports that don't meet those requirements.
My personal parallel universe philosophy in both fixed-wing and helicopters is that I want to have airspeed and altitude quickly- the sooner the better. I don't like making low-power/low-rate climbs to whatever altitude will allow a downwind turn (we all know that it varies in the real world) just because someone else's philosophy is to use minimum power on takeoff. When I do turn downwind (on course) I want to not only have sufficient altitude, but some excess airspeed just in case I have to turn back into the wind should the engine stop.
Here's a clue for the clueless: You don't get any TBO extensions because you use 90%Q on takeoff v. 100%. The components are designed to go the distance as long as you don't exceed the limits. So-called "babying" the aircraft has no quantifiable benefit other than giving pilots the illusion that they're doing it a favor. But if it makes you feel better, hey...more power to you (pun intended).
People are correct about not yanking in full power from the hover. In many cases, the resulting nose-down angle would be dangerous at such a low altitude. However, there is absolutely nothing wrong with smoothly increasing the power to max once through ETL and established in the climb.
So we may be talking apples and oranges. "Hover power + 10%" is perfectly fine as a guide to ease you through ETL, and I have no problem with that. But sometimes my 206 hovers at around 70%Q. Just don't expect me to leave the lever there at 80% when the terrain in front of me is not great and I want to be going "back thataway" anyway.
I am not a private pilot. I fly commercially and have a buttload of flight time. Hence, I am confident that if my one and only engine should fail I will be able to get the pitch down before the RRPM needle falls off the scale. I am also a plank driver. Having said that...
If I were always taking off from nice, flat, unobstructed heliports at which my direction of flight coincided with the takeoff direction, then I might use some sort of "minimum power" technique. But as a working helo pilot, I often find myself operating from heliports that don't meet those requirements.
My personal parallel universe philosophy in both fixed-wing and helicopters is that I want to have airspeed and altitude quickly- the sooner the better. I don't like making low-power/low-rate climbs to whatever altitude will allow a downwind turn (we all know that it varies in the real world) just because someone else's philosophy is to use minimum power on takeoff. When I do turn downwind (on course) I want to not only have sufficient altitude, but some excess airspeed just in case I have to turn back into the wind should the engine stop.
Here's a clue for the clueless: You don't get any TBO extensions because you use 90%Q on takeoff v. 100%. The components are designed to go the distance as long as you don't exceed the limits. So-called "babying" the aircraft has no quantifiable benefit other than giving pilots the illusion that they're doing it a favor. But if it makes you feel better, hey...more power to you (pun intended).
People are correct about not yanking in full power from the hover. In many cases, the resulting nose-down angle would be dangerous at such a low altitude. However, there is absolutely nothing wrong with smoothly increasing the power to max once through ETL and established in the climb.
So we may be talking apples and oranges. "Hover power + 10%" is perfectly fine as a guide to ease you through ETL, and I have no problem with that. But sometimes my 206 hovers at around 70%Q. Just don't expect me to leave the lever there at 80% when the terrain in front of me is not great and I want to be going "back thataway" anyway.
Joined: Jul 2002
Posts: 515
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From: Denver, CO and the GOM
Hi P-Fan1
You state your ideas very clearly - here's a rough analogy. Say you're flying single-engine fixed-wing and taking off from a 10-mile long salt flat. Would you do a maximum-angle-of-climb departure at a minimum airspeed, or would you let the airspeed come up strongly before you do a more "normal" takeoff.
Would you want to be 50' over the ground at 65kt hanging off the engine in a plank at almost-stall, when instead you can be in a nice 80 kt climb where if the prop stops you can just fly down instead of falling? Of course not. Now if there were 100' trees at the end of a 2,000' runway, different story.
In the helo, it's not about the strain on the engine (directly), it's about the pitch and resulting induced drag. If your engine stops while you are pulling a lot of pitch, you will lose a lot of RPM right now. Sure you can get the collective down quick, but meanwhile, even Johnny Lightning's RRPM will be in the bottom of the green. So there you are, waiting for the autorotative airflow to start so you can regain that RPM you so desperately need because it's all you're gonna have to cushion your landing, 'cause without solid forward airspeed, your flare won't do a thing.
Yep, they're helicopters, and in them we do helicoptery things. But the less power and more airspeed you are using in a departure (or approach), within the bounds of terrain, the mission you are accomplishing, and the H/V curve, the more options you will have if the fire goes out.
I agree with you though - it isn't going to be a TBO issue either way!
You state your ideas very clearly - here's a rough analogy. Say you're flying single-engine fixed-wing and taking off from a 10-mile long salt flat. Would you do a maximum-angle-of-climb departure at a minimum airspeed, or would you let the airspeed come up strongly before you do a more "normal" takeoff.
Would you want to be 50' over the ground at 65kt hanging off the engine in a plank at almost-stall, when instead you can be in a nice 80 kt climb where if the prop stops you can just fly down instead of falling? Of course not. Now if there were 100' trees at the end of a 2,000' runway, different story.
In the helo, it's not about the strain on the engine (directly), it's about the pitch and resulting induced drag. If your engine stops while you are pulling a lot of pitch, you will lose a lot of RPM right now. Sure you can get the collective down quick, but meanwhile, even Johnny Lightning's RRPM will be in the bottom of the green. So there you are, waiting for the autorotative airflow to start so you can regain that RPM you so desperately need because it's all you're gonna have to cushion your landing, 'cause without solid forward airspeed, your flare won't do a thing.
Yep, they're helicopters, and in them we do helicoptery things. But the less power and more airspeed you are using in a departure (or approach), within the bounds of terrain, the mission you are accomplishing, and the H/V curve, the more options you will have if the fire goes out.
I agree with you though - it isn't going to be a TBO issue either way!
Joined: Dec 2001
Posts: 1,835
Likes: 3
From: Philadelphia PA
A lot of good discussion, as always on pprune.
One of the issues is lack of knowledge of why we do things the way we do them.
I think that hover power plus 10% is a good way to do things, but it's not always a practical thing to do. And we don't have much guidance for the 'non-standard' situations.
At the risk of changing the direction of this thread - might I suggest that part of the problem lies in the lack of a low airspeed indicating system couple with the lack of knowledge of the performance characteristics in that region.
For example, no helicopter I've ever seen has a maximum angle of climb airspeed published (fixed wing do) - the reason is that it changes from zero (with lots of power to spare above hover OGE) to something else as the power required edges up to the power available. Not only is there no flight manual guidance for this, there is no academics for it, even at test pilot schools!!!
There are times when hover power plus 25% is the only way to go, and other times when hover power is all you have. And we don't teach people well about understanding the difference.
A window of opportunity for someone clever with helicopter performance and computers, methinks.
One of the issues is lack of knowledge of why we do things the way we do them.
I think that hover power plus 10% is a good way to do things, but it's not always a practical thing to do. And we don't have much guidance for the 'non-standard' situations.
At the risk of changing the direction of this thread - might I suggest that part of the problem lies in the lack of a low airspeed indicating system couple with the lack of knowledge of the performance characteristics in that region.
For example, no helicopter I've ever seen has a maximum angle of climb airspeed published (fixed wing do) - the reason is that it changes from zero (with lots of power to spare above hover OGE) to something else as the power required edges up to the power available. Not only is there no flight manual guidance for this, there is no academics for it, even at test pilot schools!!!
There are times when hover power plus 25% is the only way to go, and other times when hover power is all you have. And we don't teach people well about understanding the difference.
A window of opportunity for someone clever with helicopter performance and computers, methinks.
Joined: Oct 2003
Posts: 1,030
Likes: 0
From: Over here
Shawn said
and that's the truth. Sometimes I want to be up and away as soon as possible, and sometimes I'm in no hurry. Sometimes I want to be up and away quickly, and don't have the power to do it. Every takeoff is different, and each one demands a slightly different technique. You don't learn all you need to know in the first hundred hours, or the first thousand, or the first ten thousand, but the more you do it, the more you learn.
There are times when hover power plus 25% is the only way to go, and other times when hover power is all you have.
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
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From: EGDC
Shawn, in the Brit Mil we have long taught a best angle of climb speed when limited on power. As you say, with a large power margin, the best angle of climb is straight up - but on Gazelle, Lynx and Wessex about 35 to 40 kts is used. Typically this would be when a cushion creep transition is necessary due to power limitations and following this there are obstacles to clear requiring a steep angle of climb.
If you draw your power required curve and then put a limiting horizontal line representing the actual power available, then a line drawn from the vertical axis where the power limit line starts, down towards the descending part (low airspeed) part of the Power required curve (at a tangent to it) - it should indicate approximately the best airspeed for angle of climb.
I don't think it is very scientific but it gives a figure that seems to work well in practise.
If you draw your power required curve and then put a limiting horizontal line representing the actual power available, then a line drawn from the vertical axis where the power limit line starts, down towards the descending part (low airspeed) part of the Power required curve (at a tangent to it) - it should indicate approximately the best airspeed for angle of climb.
I don't think it is very scientific but it gives a figure that seems to work well in practise.
Joined: Dec 2001
Posts: 1,835
Likes: 3
From: Philadelphia PA
crab:
A useful technique, and the technical methodology is not bad, if you have the curves available. The problem is that you often don't know the power required (or available) and so, can't draw the curves and figure out the airspeed for absolute maximum climb angle at that moment. The 35-40 Knots is a compromise, and I'll be the compromise is based more on having a repeatable, reliable airspeed indication than on anything else.
The US Army did some tests years ago with a UH-1D, and they recommended 28-30 knots as the best angle of climb speed. they had used a special airspeed system to arrive at this number. The problem was that no-one could get a repeatable indication off the production system at those speeds.
I checked with my more learned FW brethern on the subject of Vx (max angle of climb airspeed), and rather than power required vs. Airspeed, it's actually based on rate of climb vs. airspeed, and you draw the line where the rate of climb starts to curl back (i.e. less rate of climb as the airspeed reduces). Haven't got that into the syllabus yet...
But, at the end of the day 35 to 40 KIAS works pretty well. It will probably put you into the HV curve, which is why it isn't an 'official' technique, as the FAA requires the climb profile in the FM to stay away from the HV curve. But if you know why you're doing it, in a Part 27 helicopter, fill your boots.
A useful technique, and the technical methodology is not bad, if you have the curves available. The problem is that you often don't know the power required (or available) and so, can't draw the curves and figure out the airspeed for absolute maximum climb angle at that moment. The 35-40 Knots is a compromise, and I'll be the compromise is based more on having a repeatable, reliable airspeed indication than on anything else.
The US Army did some tests years ago with a UH-1D, and they recommended 28-30 knots as the best angle of climb speed. they had used a special airspeed system to arrive at this number. The problem was that no-one could get a repeatable indication off the production system at those speeds.
I checked with my more learned FW brethern on the subject of Vx (max angle of climb airspeed), and rather than power required vs. Airspeed, it's actually based on rate of climb vs. airspeed, and you draw the line where the rate of climb starts to curl back (i.e. less rate of climb as the airspeed reduces). Haven't got that into the syllabus yet...
But, at the end of the day 35 to 40 KIAS works pretty well. It will probably put you into the HV curve, which is why it isn't an 'official' technique, as the FAA requires the climb profile in the FM to stay away from the HV curve. But if you know why you're doing it, in a Part 27 helicopter, fill your boots.
