Hover Power + 10%

[Letsby Avenue]

For pedants and sad instructor types only.....

A standard transition is hover power plus 10% - If the hover power is 50% for example, does one require 55% (hover + 10% of 50, ie 5) or 60% (hover + 10% of 100, ie 10)?

[NickLappos]

Being pedantic this evening, let me weigh in....

The exact quantity of the standard is subject to questions in its own right, since it is nowhere near 10% for careful procedures where flat approaches and departures can be made. In many types, basic HIGE power can get you off the ground successfully, if a slight dip at 12 knots on takeoff is acceptable.

That all being said, it is 10% of the hover power, so that your 50% power to hover would rule of thumb to 55% as the standard. Each percent equates to a bit of margin above that needed to hover, typically 5 to 10 pounds of extra lift for each extra horsepower. Knowing the calibration of the torque gage is nice, so that you can derive the exact contribution of each percent as equivilent weight or thrust margin.

[Steve76]

:rolleyes:

:p

[Letsby Avenue]

Thank You Nick - Guess this one will sink quickly out of sight now.....:ok:

[[email protected]]

Letsby, there is frequently confusion about power required for transitions and thrust margins expressed as a percentage.
A HOGE plus 10% thrust margin is basically derived from loading the aircraft up until it only just has enough power to hover OGE and then reducing the AUM by 10% - this gives you an AUM that will allow you to manoeuvre whilst in the OGE hover (some flight manuals specify actual rates of climb or the ability to cope with light turbulence).
The 10% figure that appears in many teaching guides is in fact 10% torque in hand - it is basically a safety measure to highlight that for a normal transition to forward flight (ignoring advanced cushion creep techniques and the like) you need to have available a margin of 10% torque over and above what you are actually pulling in the hover at normal hover height.
So if you are pulling 90% torque in the hover and your maximum available is 100% then you are OK. If your max available (due to other engine limitations like temperature) is reduced to less than 100% when you are using 90% in the hover, you will either need to use a more advanced transition technique or offload pax/freight etc.
Hope this is clearer.

[wde]

HMMM. Interesting.

I have never considered the 10% to be 10% of the Hover torque for that day...Hover at 60%, T/O Power 66%...rather

Hover at 60%, T/O using 70% (Hover Q + 10%)

As a former CF flight instructor, this is the only standard I and my colleagues have used. In any case, it really is just a value to set so that new pilots (primarily) don't reef in 100%Q for every take-off they do.

Cheers.

[SASless]

Well now....what about machines that use "pitch" for power issues? Or...what if you are TOT limited or N1/Ng limited? What numbers do we use now....percentages then get confusing too!:confused:

[Letsby Avenue]

Tis interesting indeed!! I have lost count of the amount of students I have apparently taught incorrectly as an ex AAC QHI :eek: Never mind - Crab has control now!:ok:

[[email protected]]

Letsby, I haven't taught at Wallop since 2001 but I didn't change my nom de prune when I left as I'd got quite attached to it.

The Lynx and Gazelle Flying Instructors Handbooks and Student study guides all refer to hover Tq plus 10% for a normal into wind transition so I'm sure you were teaching the correct technique.

SASless - I don't know what one would use on, say a Puma for example, when there is no tqmeter just a pitch indicator - there must be a Puma QHI who could tell you. If you do have a Tq meter in % then you could equate your N1 or TOT limit to a Tq figure and work backwards from there.

We all know that you can get into forward flight with not much more than hover power but the 10% figure has at least 5 uses:

1. It highlights that to make a standard transition into wind ie one in which you can climb and accelerate you will need a certain amount of power in hand above your hover Tq. (this is for Brit Mil types of transition not CAT A or any clever stuff like that).

2. As WDE says, it stops students trying to overtorque during a transition because they have a specific Tq setting to aim for.

3. It reminds you to use a different transition technique eg cushion creep if you don't have enough power in hand.

4. It is a good ballpark figure for vertical climbs out of confined areas where a towering take off is required.

5. It is also a good ballpark figure for downwind transitions.

[Letsby Avenue]

Crab - Did you ever fly to Kiel in a 350 with Stumpy leading the show? If so.. I was the good looking one next to Chuck!:cool:

I always remembered the instructors' guides saying 'Transition using hover power plus 10%' but I think that's what started the thread...

[SASless]

also....what happens if you only have hover power...just....and need to go? (...or less than hover power...) Ask any old time Gunship driver about limited power takeoffs...in "B" and "C" model Huey's!

:rolleyes:

[NickLappos]

Sasless,

I agree! That +10% rule of thumb would leave about 20% of the aircraft's payload back on the ground, a pretty poor showing!

The guys back in D Troop 1/1 Cav wouldn't have carried a single rocket with that rule.

I think its nice for beginners, but way too conservative for pros.

[helipedro]

Having a flat (nice)surface I to take off. I think it would be rather sad for the operator, customers and pilot to have to leave 1 passenger on the ground on a 206 so that the pilot could hover at 90%with a great smile in his face.All of us that have carried many 2 X45 gallon drums under a 206 we know thats perfectly possible to hover at 100%, and take off at 100 % probably losing a couple of feet between 10-12 knots. The 206 has a good rule of thumb...33lbsXeach 1% of torque.Obviously any tot or n1 limitations would have to be taken into account.

[[email protected]]

Sasless, Nick and Helipedro - the original question was aimed at pedants and instructors so, being both, I gave the standard QHI answer. I suspect that there isn't a training school in existence that doesn't stress that having power in hand to transition is a good thing - but we are talking about training not operations.

In recent years I have seen a definite lack of sensitivity to power limited ops in the military because many front line aircraft are so overpowered - whereas 10 or 20 years ago, many RAF pilots cut their teeth on the Wessex in NI, Hong Kong and Cyprus where being short on power was just a way of life and you learned all the advanced techniques very quickly.

However I have also seen PPL pilots completely ignore their power in the hover and exceed the MAP limit in the transition because they had the 'oh that was only needed during training' mentality.

[Flingwing207]

Throughout my training in the 300CB and the R.22 it was a point of pride to very rarely have to add power in the transition. The first time I flew an Enstrom 280C at 5400' pad elevation, I was glad to have developed that habit/skill, as was the aircraft's owner (sitting next to me).

[Head Turner]

Crab@SAA - Just one more item surely to add to your list is that by using minimum (10%) power this keeps the pitch at the lowest angle to provide a good take-off and if the engine stops then entry into auto is less stressful than if high pitch angles are used.

[Helibusdriver]

Flying the Norwegian oil patch it is required to have power to spare on all helideck take-offs. Maximum torque hovering over the helideck is 15% below the maximum allowed for takeoff.

This is to ensure that you during the takeoff procedure achieve sufficent vertical acceleration to clear the helideck edge in case of engine failure at or after reaching TDP (Takeoff Decision Point).

Examples:
S61N: max. torque for takeoff (5 minutes) is 2 x 103 %
Max. torque allowed while hovering for takeoff is 2 x 88 %.

B214ST: max. torque for takeoff (5 minutes) is 100%. Hovering in takeoff position max. torque 85%.

If torque is higher, weight reduction is required.

Having flown the S61 since -89, and the B214St since -92, this has always made sense. I guess the guys who created this procedure during the pioneering days in Norwgian offshore flying knew their trade.

By the way, these limits hardly ever creates any problems. The ST has acceptable performance for most missions. The S61 would most often be limited by other factors, such as wind, drop-down charts or 50' enroute climb perfomance etc.

[Shawn Coyle]

The IGE hover performance charts in all the civil manuals will have a hover height defined. That height is set so that with a fixed collective and a flat surface, you can transition to forward flight without touching the ground as you pass through 10-12 knots of airspeed.
This is to take care of the case where you can hover at that height, but no more than that height, using takeoff power (and not any transients). In that case, you should be able to safely transition to forward flight.
If this isn't one of the things you've been shown or used, it should be. Surprised it's not part of an FAA or CAA flight test.

[SASless]

Shawn,

It would be interesting to see a comparison of the different techniques required on FAA/CAA/MOT/JAR for the same maneuver, normal takeoff from a hover for example. Bet the matrix would be fun to look at and digest. About a year ago I started the thread about Bell 212/412 takeoff profiles from elevated (offshore) helidecks. That was an interesting debate on the "right" method to be used and demonstrated just how diverse the conventional wisdom can be. I suggest, the "official" requirements and philosophy of the licensing authority drives much of that diversity.

What say you?:*

[pa42]

Once again, after 17 years flying helicopters, I find that there's a parallel universe of training standards that nobody had told me about. 's OK; I've been sort of following the doctrine without formal introduction (using "some" excess rather than "10%").

I would like to track down the training environment (civil/military, North American/Euro, piston/turbine?) in which this 10% is taught as a standard procedure; I try to stay aware of those parallel universes. In particular, I would be eternally grateful for the titles and authors of training manuals which cite this as a standard procedure, for none of MINE do so. (Civilian R22 toy helicopter background, so sorry.)

AND does anybody know a good, solid, helicopter-specific REASON why so many takeoffs are done at hover power rather than at max power or close to it? The fixed-wing philosophy is FULL power, let's get up high ASAP because the crankshaft is sure to disintegrate within minutes. Helicopters, however, almost universally linger near the ground at minimal power settings, prolonging the exposure to engine failure while low and slow amongst trees and wires and other cuddlies. I can do it OK; I just wonder WHY I'm doing it!

[sarboy w****r]

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

[Letsby Avenue]

Of course if you're a pedant then 70% + 10% = 77%. Which, again, is where we started.:confused:

[Shawn Coyle]

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.

[4dogs]

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

[Gomer Pylot]

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.

[Shawn Coyle]

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.

[Gomer Pylot]

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?

[4dogs]

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

[PPRUNE FAN#1]

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.

[Flingwing207]

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!

[Shawn Coyle]

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.

[Gomer Pylot]

Shawn said 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 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.

[[email protected]]

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.

[Shawn Coyle]

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.