MaxP or Towering

[advancing_blade]

Evening all

I am in the late stage of my FAA instructors course although I learnt to fly in Canada and the UK I am used to performing what I have always herd referred to as a "towering take off." From light on the skids / hover > vertical ascent with minimal cyclic movement and prior to loosing vertical speed, transitioning into a forward attitude (very gradual) keeping the tip path plane just above the trees etc.

However I am now learning to teach an FAA Max performance take off, which champions the use of maintaining the same attitude as that of light on the skids, which gives forward movement all the way. This is to give the rotor disk cleaner air and less induced flow, but uses up limited available space long before ETL.

Which method do you savvy folk reckon is the better method? I can see plus points to both, but lean toward the towering method. Don't want to start an FAA / CAA / MOT debate, but interested in opinions. Thanks :hmm:

[GLSNightPilot]

I don't see the value of the 'towering' takeoff. This seems to be a Brit thing, & I've never seen it used in the U.S. It takes less power, IMO, to do a moving takeoff, keeping the pitch attitude constant throughout, than it does to try to go straight up, to a zero rate of climb, or close, then try to transition to forward flight. You'll likely lose some altitude, & end up closer to the obstacles. The only time either of these is of any use is when there are obstacles to clear, anyway.

When you fly UH1's when it's very hot, high, & humid, you soon develop a technique that works for getting out of an LZ, & I've never seen a huey driver, current or former, use the 'towering' technique, & I've been around a lot of huey drivers over the decades. We just put the nose on the top of the trees and keep it there until the trees go underneath it.

[SASless]

Being Bi-lingual....have used both techiques....and have had numerous intellectual agruments over a pint or two dozen on the subject. We even did a comparison....a dinosaur from Oz and myself in one of Big Al's lovely Bell 47G2's at the Redhill smashplatz. It would seem the power demand is the same....assuming not reaching ETL prior to crossing the barriers....the arguing point seemed to center over the issue of giving up ground as you ascend using the US Army (FAA) method or being well up in the air should you lose a Donk while using the UK/European method. For twins it is a no brainer.....ascend to a height clear of the barriers then go.....but for singles...it really is a toss up. :confused:

GLS....please remember there are two kinds of helicopters.....Chinooks and slingloads. Have never had a problem taking off with a Huey....except when the main blade comes untied and the airframe wants to spin like a top......but then they always did make for an unstable slingload !

[[email protected]]

The British Military teach the towering takeoff for singles and twins as it is easier to carry out a vertical abort following a power failure or just running out of power.

The US Army/Ausralian Army/FAA max power technique is a cross between a cushion creep and a normal transition and, while it gets you accelerating towards ETL quickly, requires you to nominate an abort point where you must have enough room to get back to the hover/land before you hit the obstructions ahead.

I have tried both in singles and twins and personally prefer the towering method in a twin- I would rather be sure I was going to clear the obstacles before I transition. In a single its relatively academic - if the donk quits you're going down regardless

[headsethair]

GLSNightpilot: "We just put the nose on the top of the trees and keep it there until the trees go underneath it."

So - how do you get the nose on top of the trees ? If you're in a tight LZ, surely you have to use pure towering (with a little bounce off ground cushion if you're near the red on power) to get the machine anywhere near tree top height ?

If you've got the space, then a creep is best - anything to get some airspeed/translational.

And - yeah I know this sounds obvious - get a really good check on the wind before towering. The wind direction coming over the trees - not through them. I once saw a beautiful tower turn into a nightmare - the pilot arrived at tree top height to discover he was downwind.....with little power in hand. Sweaty translational search.

[ShyTorque]

Sometimes the ONLY option is a towering takeoff, that is why the British military teach it. In such cases there is a risk of a heavy landing in the event of an engine failure, even in a twin, but at least it would be vertical on the u/c and not involving trees or other obstructions. The correct way to fly the towering T/O is to move forward to gain airspeed as soon as the aircraft will clear the obstruction. Obviously we are NOT talking Class A performance stuff here!

The so called "Max performance" takeoff is good IF there is sufficient space to get translational lift and maintain a sufficiently steep climb angle (hence the aircraft manufacturer being required to prove the performance figures for certification and supply such information in the Flight Manual).

I have been in the "other seat" in a twin quite a few times when I have felt that if an engine quit we would have been through the trees and not over them and a better option would have been a towering takeoff, possibly in another direction. The risk involved in a vertical landing can be quantified (and practiced in a simulator) but going through trees, albeit at a lower ROD, or even with a ROC, carries an unquantifiable one.

Common sense and training should give the pilot a clue on which takeoff profile to fly!

[SASless]

Shy....

"Commonsense and Training".....are you suggesting that pilots think ....now be careful here.....management will take exception to that.....why who knows where that will lead!

[ShyTorque]

SASless,

:D :ok:

Management do like to think we are stupid as they are, perhaps we are for putting up with THEM!

:E

[pa42]

Aha, great exchange, this subject one of my pet peeves with US flight instruction.

In my checkered past as trainee/flight review victim (17 yrs) I've had the joy of flying with about 20 CFI's--each of whom seemed to subscribe to a different but absolutely required inalterable Max Perf TO profile. Towering, or full-power-near-horizontal-departure-from-hover, or lightonskids & full-power to ETL then pitch up (different pitch different instructors), or circle very tightly (35 deg bank) within the clearing, or . . . or else some perversion of those which I ended up mimicing without understanding, since the presiding instructor did not speak English well enough to understand my efforts to clarify.

I thought to find revelation of truth in published instructional manuals. No, chaos again.

So the opinions appearing on this thread have been valuable! Thanks.

My solution has been to make a thoughtful effort to analyze each technique for the time-duration of exposure to death by engine failure (of course, missing the obstacle also). What we need for teaching safety is MUCH more performance data in the AFM--for instance, whereas best angle of climb speed in f/w is always called out, in helis that I've seen there's no mention. (Notably R22 and training texts in general.)

Absent factory data, would it not be appropriate, folks, to have the student learn and perform two or three of the profiles described above, with the CFI OPERATING THE STOPWATCH from when student calls Fatal Exposure (20' & 20 k?) to when he calls Autorotation Possible (60k clearing obstacle?)?

And then same exercise when you're at maximum normal training altitude doing pinnacle/confined training?

The student (and the CFI!) then have some absolute (perhaps inaccurate, but better than nothing) appreciation of comparative MaxPerf methodologies. AND useful training in test pilot methods for learning the limitations of a new-to-the-pilot ship.

Taken another way, is it productive to go beyond philosophical arguments (example: this post here) about angels on the head of a pin, and instead gather some Real Numerical Facts about comparative performance of one helicopter, one crew flying different profiles? (One pin w/two angels . . .)

Forgive me, I keep forgetting that reality is sometimes unfashionable (viz. Matrix Reloaded).

Dave

ps: And in case you don't survive, leave us a note specifying which profile you plan to practice . . . if you DO survive, post the useful numeric results here.

[[email protected]]

Pa42, I have tried the different techniques with the same simulated power limits in the same scenarios and tried to come to a conclusion about which is the best technique.

If you have insufficient power to achieve a vertical climb to above the height of the obstacles then you need enough room to accelerate to above ETL and achieve best angle of climb speed (straight up if you are not limited on power) which is usually between 30 and 45 knots.

Then you have to decide how to get there - do you a. from the ground, pull to max power and climb and accelerate together until you can rotate at best angle of climb speed (FAA and Australian AAC) or b. use a cushion creep technique pulling to max power but accelerating close to the ground until you get your climb speed and rotate (British Army and others).

As I say I have tried both and in my opinion method b gets you climbing quicker and the abort -if you need it-is easier to fly.

After much nagging from an Aussie transferee I experimented with the 'put the nose on the top of the trees and keep it there' technique and have to say it made me feel distinctly uncomfortable as an engine failure in the latter stages meant an inevitable visit to the scenery.

As with all these things, what you feel most comfortable with is usually best because you will fly it better but take the opportunity to go and practise the various techniques to at least learn to recognise the attitude your helo requires for best angle of climb - that makes the judgement a lot easier.

[GLSNightPilot]

Headsethair, I didn't explain that very well. From a low hover, start climbing at an angle that appears to put the nose just over the top of the obstacle, & then maintain that angle. I agree with you, forward movement requires less power than a completely vertical climb, since you get some wind through the rotor system, & might get translational lift by the time you clear the obstacles. Obviously, as others said, I'd prefer to reach translational lift, and higher airspeed while in ground effect, then climb over the obstacles, but I thought the point of the thread was clearing obstacles when this isn't possible. There may be times when only a completely vertical climb is possible, but this is rare, or at least should be.

Some posters seem to be worried about engine failure to the exclusion of everything else. I tend to consider all the things that can go wrong, & try hardest to deal with the most likely thing. While trying to clear obstacles, ISTM that hitting the obstacles is more likely, & more dangerous, than an engine failure. I could also worry about losing a rotor blade, or any number of other failures, but I try to deal with the most likely things. Maybe it's a matter of personal history, but in over 30 years, I've never had an engine failure, but other things have failed.

[B Sousa]

Lots of very good Opinions above. For me, what your asking is simple. Your Learning the FAA course to pass an FAA checkride. Do as the FAA standards require. Anything else you have been taught might get you an unsat.
In the real world once you have the ticket, then you can use what you feel is best.

[Crashondeck]

In the UK PPL of old exercise 25 Limited power we were taught Running T/O, Cushion Creep, Towering T/O and vertical climb. It seems that the UK consider that Towering and max Perfomance T/Os are different techniques and each have their uses.

I remember being taught that a towering T/O starts with a low hover or even a small ground cushion bounce followed by a vertical climb. Decision point was when the a/c stopped accelerating upwards. If you were clear of obstructions, you continued by starting a gentle transition that took you clear of the scenery. If you weren't you landed on and kicked out ballast. In effect it was only used for clearing relatively low obstacals such as hedges and bushes although it depended on how much power you had in hand. Of course you always did a power check before you started to ensure that you werent wasting you time.

The max performance T/O or running T/O as it seems to be called over here is on much use on a very flat, smooth area with plenty of room. It can get a little exciting in a Robbie if the ground is not smooth.

[straitman]

Lots of very good Opinions above. For me, what your asking is simple. Your Learning the FAA course to pass an FAA checkride. Do as the FAA standards require. Anything else you have been taught might get you an unsat.

Never a truer word was spoken! Beyond that you will gradually learn (as was previously mentioned) that it's horses for courses. It really depends what you are doing.

i.e. In ground effect transitions work really well on airfields or large open areas with a smooth surface. They don't work particularly well from an offshore platform.
Conversely why go 20 - 30 ft vertically prior to rotation when you are on an airfield v's it's a necessity in a confined area or from a platform.

Good luck:ok:

[advancing_blade]

Thank you for the input people.

Quality as always, just as expected from Ppruners. The web site for the thinking pilot

:ok:

A-B

[NickLappos]

The terminology is always a bear. Here are the three basic places that take-offs settle, regardless of their names:

1) The maximum weight you can lift with a helicopter and get into forward flight uses a slow acceleration, at low altitude, gently accelerating in ground effect until nearly up to Vy. I made 1000 of these in Vietnam, where it took 200 yards to rise above the concertina wire at the perimeter.

2) The maximum performance you can use is a vertical climb, which requires about 1.5% more power than a steady hover OGE. This is the reason why you make a towering takeoff - to rise vertically above close obstructions and then transition into forward flight. During the vertical portion, you might be in the avoid area, of course.

3) The best angle to a barrier with the most weight is made with a ground cushion procedure, where you rapidly accelerate forward while in ground effect, then rise by trading acceleration for climb rate somewhere at or above translational lift. This is very similar to the airfield Cat A procedure most twins have published, since that takeoff also maximizes the climbout angle and minimizes the land back distance. The US Army used to teach an oblique takeoff, where you kept a constant angle from the hover to the obstruction, and where you left the ground cushion almost immediately. This has been proven to be significantly worse than the ground cushion procedure, since all the excess power you have IGE is frittered away as you rise.

[NickLappos]

Mr. Selfish,
The constant angle technique was what I was taught, as well. A NASA program about 20 years ago showed that the best way to spend the power to takeoff was to accelerate IGE, so the excess power was available to get you to some forward speed in less distance. Even the first few knots is worth a bunch, in an S-76, 10 knots of wind or forward speed will allow 1000 pounds more performance from the rotor, making the climb angle at 10 knots much steeper, so the distance you spent getting the speed is recovered in the climb phase.

[[email protected]]

Ah well, it's nice to know that NASA comissioned a programme to discover what I was able to conclude messing about in a Lynx on Salisbury Plain - I suspect that their research cost a few hundred times what mine did!!!

Use the force Luke - and if that doesn't work the ground cushion will do nicely!

[vorticey]

using the extra power at hover to scoot foward has always seemed a good idea, (i was tought constant angle aswell)
:ok:

[Shawn Coyle]

There are several problems here -
1) a distinct lack of agreed terms (or perhaps appropriate terms?)
i.e. what is a 'maximum performance takeoff?'
2) a lack of good performance information. The civil manuals do not tell you power required to hover - only that you can hover in ground effect (for Part 27 helicopters and most older helicopters). And at one height only.
With no account for wind.
And no way to know what your maximum angle of climb airspeed should be (assuming you can't hover Out of Ground Effect).
Why are we sitting still for this? The Fixed wing world that is involved in commercial operations is far better off.

[Steve76]

The MP "constant angle" departure from a hole gives me the creeps. It really does. Lifting into a hover and then pulling all she has while launching off your comfortable energy conserving ground cushion is scary.

My greatest desire is to get the rotor above the trees. Once in the clean air the aircraft will either want to fly or it will want to settle.

If it settles....then it is an easy lowering into a nice known quantity.

If it settles during the constant angle departure then you have issues with the planet.

I have seen plenty of guys launch off helipads around here with the intent of doing the constant angle departure and EVERY time I can honestly attest that I had a moment of asking myself if we were going to make it (I usually do the "jump on the imaginary brake pedal thing that guys do when their wife drives them somewhere......:) )

Just the slightest drop in the wind or a swirl to the rear and the landing gear was in the trees with no room or power to solve the problem.

A vertical climb to 30-40 ft, FEELING what the machine wants to do up there and then a transition into fwd flight with an emphasis on airspeed over altitude is much more comfortable.

Five cents worth from a NZ/OZ/CA perspective???

[Winnie]

From behind the curtain in Canada, we have to teach what is called the "vertical take-off" for situations with no space for acceleration, and the "Towering" when you have some space. That is put forth in the Flight test standards, so that is it. However, I agree that the Vertical route is more comfy, since you can always go back into the hole you left, even if it may be a somewhat hard landing...

In the states we thaught the Max performance route, which I found slightly different than the towering. Here you don't have to use all power to get out, sufficient to get you going is enough, as long as you got enough. I personally like that better, but still better is the vertical departure, since you don't have to do a quick stop or rapid deceleration in case of those pesky warning lights.

Don't know if that is of any help but...:ok:

[heedm]

Very interesting post. I just spent the last week doing multiple vertical departures (straight up, clear the barrier, move forward). Experienced loss of wind a couple times and decided I'd prefer to get bigger engines or less cargo.

A couple points to add:

1. Climbing vertically can actually use less power than hovering OGE. Induced flow changing blade angle of attack is the biggest culprit, if I recall correctly. Rather than thinking about it, try it. Hover OGE, note power. Increase collective slightly to initiate a climb. Once climb established, set power to same as recorded hover OGE power. You should still notice a climb. Lower power slightly to see if you can maintain a climb. I tried it in a B412 and it worked.

2. Max angle of climb cannot be computed for helicopters as easily as it can for f/w since our thrust vector changes. Too much to go through here, but just get a basic f/w aerodynamics book for the analysis. Something could be done for helicopters, but many more factors would have to come into play (especially AUW and DA). Point is, it's not as easy to obtain.

Matthew.

[vorticey]

isn't the whole idea of a constant angle departure, to get out when you CAN'T tower out (go strait up and clear every thing by 10 feet or so)?
strait up has to be the safest way, the chances of losing lift at a critical moment or just mis judging the power required for a max p T.O, are much greater. if there's not enough power to go strait up then looking at another way is the go. i shall experiment!

STEVE, if just the landing gear was in the trees, you've done well!:ok:

[SASless]

Much chatter about profiles....now how about some discussion on power checks...power available checks...wind checks....determining available takeoff distance to barriers....determining height of barriers....determining direction, velocity of wind, and gust strength and duration....consideration of mechanical turbulence effects....sun direction...??? All things that go into making an informed decision as to when, how, and what way to takeoff.

Let's define a confined area.....not all confined areas are the same....would you always take off downhill for instance? Have you ever operated from an area that a slight uphill departure could lead to 4,000' altitude gain after 100 meters whereas downhill put you into a narrow steeply descending rocky floored canyon?

This is not a black and white issue....nor does one set of rules always fit the circumstances. Those of our numbers that have done some serious bush flying understand that concept. It all comes down to Risk Management.:O

[NickLappos]

heedm,
You are correct that once stabilized in an appreciable vertical climb, the power is less than that you would need for the same climb done at Vy, but all climbs need more power than steady level flight. I have done some careful work to see what it takes to make a vertical take off as compared to a steady HOGE, and I have never successfully risen above about 2 rotor diameters without using 1.5% more power than the power needed to HOGE. Even with that 1.5% more, the climb rate was a paltry 50 feet per minute.

PPRUNERS, Give it a try. Take a very still day, and just HOGE at 1.5 times the rotor diameter (65 feet for an S-76, for example) and carefully note the power. Try to have no climb or descent, it might take a little averaging of the torque swings. A good way to do it is to release the collective, and just see if you average a nice HOGE, rather than get involved in a nice pilot-in-the-loop oscillation. Radar altitude really helps here. Set back down to a low hover and then pull exactly that power. In my attempts, I could only climb slowly to a few feet above the 1.5 diameter, then watch as the aircraft bobs up and down in neat altitude oscillations around the hover height.

I could only avoid this oscillation by starting all over again and using about 1.5% more indicated torque (actually 2% of the power I was pulling, ie 2% of 75% total torque). If I pulled the extra power, I slowly and steadily climbed right up to several hundred feet.

[heedm]

I'll give that a try, Nick. Sounds much more believable than what I posted. I guess in the steady state we're able to use less power than required for hover OGE, but overall we need to use excess power to initiate that vertical climb.

This brings me closer to a theory I have on vertical takeoffs. We check power at 4' and then lower to 1', smoothly but rapidly increase to a high power setting. I believe that at 1' we get the maximum benefit from ground effect, obtaining the most excess power to initiate a climb, and then maintaining a high power setting we obtain the maximum vertical rate of climb possible. It may be that this technique allows you to climb to an OGE hover even when you don't have enough power to maintain the hover (although the rates of climb would be incredibly slow and it would require exceptional skill and luck to see it).

I think the 1' makes more sense than starting from a 0' hover because there would be fewer control inputs and more efficient use of available power.

Matthew.

[NickLappos]

Matthew,
You are right on both counts.

The power to hover OGE is less than the power needed to climb vertically, but you get a speedier climb rate by starting the takeoff at a low hover. or gear ightly on the deck, using the extra power from the IGE hover (about 12% power available) to make the climb happen.

In a narrow sense, a vertical climb is actually more efficient than a Vy climb, due to rotor inflow effects - almost twice as much climb rate for the change in torque. In other words, if we add 100 horsepower to a vertical climb, we get almost twice the climb rate than if we add 100 horsepower to a helo in forward flight. But since the HOGE takes so very much more power than forward flight, Vy is always the best climb speed in terms of peak climb rate for the total power consumed.

As a matter of interest, the "elevator factor" is perhaps worth discussing. Basically, if you start in level flight and add some power, the exact climb rate can be calculated. A horsepower is 550 foot-pounds per second, the power needed to continuously lift 550 lbs at a vertical speed of foot per one second. Take the weight of your aircraft, and the horsepower that you add to the level flight, and the exact climb rate can be calculated.

For an S-76, at 10,000 lbs GW, if we add 10% torque (which is 160 Horsepower in a B or C), we add 550x160=88,000 fl-lbs. This is enough to cause a climb rate of 8.8 feet per second, or 528 feet per minute. Actually, the rotor is about 86% efficient, so we lose 14% of that climb, and end up with 454 feet per second.
The climb-descent chart in the US Army flight manuals is based on this effect. For the Black Hawk, 100% torque is 2828 horsepower, so use that calibration to check the chart.

http://www.s-92heliport.com/climb chart.jpg