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Restricted takeoffs, VRS, and ground effect

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Restricted takeoffs, VRS, and ground effect

Old 28th Jul 2015, 08:40
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Lots of replies! For the legal matters, we are not required to follow a PC1 profile. To be honest I'm not 100% sure what that would entail, I assume it's a specific profile that gives a certain safety margin? For weight/power considerations, we maintain at least a 5% margin over OGE power during our ops. I'm with Gomer Pylot in that I don't know the details beyond that.

whoknows, I've been meaning to do that for quite a while now! I'll try to get a video and take some snips from it or something.

Crab, John is right in that our "normal" range is 97-100% Nr. It's definitely an interesting idea to have in the back pocket though, since the beep range does go up to 102.
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Old 28th Jul 2015, 09:41
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I fly two different types of twin, both with PC1 performance, and very different profiles, two areas I regularly see ignored are the max ROC on the vertical section of the take-off and also the torque limits. Both are designed to contain the Nr in the event of an engine failure.

The simulator provides fascinating lessons in this regard, when the book says "apply 10% torque above hover requirement" a lot of pilots just give it the full beans. When an engine stops the full beans guys find themselves with a sudden (and often catastrophic Nr droop) while the guys who fly the accurate proflie find the NR drooping to exactly the right spot for either a reject or fly away.

People with very clever science/maths qualifications who have graduated from complex test pilot courses work this stuff out, I wonder why when they've gone to all that trouble an average line driver ignores their work and puts themselves in a very difficult place to explain away if they have a problem.

In the last 28 years I have only once rejected for real off a PC1 take-off and that was actually when a front seat passenger dropped something in attempting to catch it turned off the hydraulics, so it isn't always an engine failure you're waiting for.

SND
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Old 28th Jul 2015, 10:54
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Chucklehead - I have just been perusing my colleague's 212 RFM and it shows a vertical take off profile as follows:

Vertical climb to 40-60ft using a maximum of hover Tq plus 15%,

Continued climb with rearward movement to keep the LS in sight up to CDP of 160 ft with the same Tq limits,

Then rotate to achieve VTOSS (30kts plus windspeed) using not less than 72% Tq,

Climb to 200 ft at VTOSS.

You are right that the beeping up only comes following an engine failure.

I don't know what the certifcation of a 212 entailed but I am guessing with the age of the design it was well before the advent of Cat A and B. Hence there don't seem to be any graphs in the RFM which state Cat A or PC1.

Not sure where that places you in the modern era!
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Old 28th Jul 2015, 13:15
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I uploaded this CAA Bell 212 Cat A supplement for a previous discussion some years ago; it is not an approved current copy, but may give some idea of the profiles available.

It is also based on very early PT6-3 engines:

Part 1

Part 2

Part 3
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Old 28th Jul 2015, 16:45
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It makes me wonder why backing up downwind to keep the LZ in sight and so enable a forward descent / run on in event of an engine failure has any real advantage when in so doing puts more stress on everything and thus more likely to have an engine / tail rotor failure in the first place!
I cringe when ever I see this, using all that power whilst backing up, when you could be doing a towering take off into wind and on your way in much less time.
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Old 28th Jul 2015, 17:05
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Some discussion of why the twin behaves the way Crab and John E have so properly described:


In the vertical takeoff, the loss of an engine creates a power deficit that produces the need to descend, of course. The deficit is measured by comparing the power needed to HOGE as compared to the OEI power remaining. If the OEI power is very close to the OGE power, as is the case when very lightly loaded, the ride down is fairly slow and the landing easily cushioned. Since the speed of that descent is driven by the size of the power deficit, as the takeoff mass is made greater, the OEI power deficit is greater, and the rate of descent becomes greater. If the power deficit is appreciable, the aircraft doesn't stabilize in a fixed rate of descent, it actually accelerates downward, so that the stop at the bottom becomes pretty harsh at higher reject altitudes (higher barriers) and at higher weights. The flight manual WAT curve (Weight, Altitude, Temperature) shows the weights where the power ratio is acceptable to perform the published procedure. For vertical procedures, the WAT curve calls for much less all up mass than a ground level procedure where you can accelerate to somewhere close to VY before the engine failure. Of course, the ground level procedure rejects into a much longer heliport, because you have to bring that fast helicopter to a stop while in the protected runway length. In other words, the various procedures for Cat A swap all up mass for speed/runway length.


VRS is somewhat involved in a vertical reject, but the descent is more governed by the simple application of too little power and too much mass, so rotor thrust does not equal weight, and the helo falls, and accelerates as it falls.


Generally, for a vertical reject from over 100 feet to be successful, the OEI power must be an appreciable fraction of the OGE power required, perhaps 75 to 80%. That is almost exactly what HIGE power is as compared to HOGE, so a rule of thumb could be that if you can barely HOGE OEI, you probably have a darn good vertical reject capability.
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Old 28th Jul 2015, 17:51
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Chopjock - you must understand that these profiles are, if flown exactly, guaranteed to permit a safe reject back to the helipad or a safe transition into forward flight and climb to 1000 ft providing the weight and temperature limits are adhered to.

They have been proven in flight testing whereas your pull pitch and pop out vertically has not.

The argument about exposure time is valid to a degree but with higher collective pitch comes more rapid and greater Nr decay in the event of an engine failure.

The profiles are actually quite gentle due, as Nick has explained, to the need to keep the power available and the power required quite close; your vertical departure will use more torque and therefore put more stress on the airframe and TR than the up and back version.
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Old 28th Jul 2015, 18:43
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C'mon chopjock......stop it. How long have you been on this forum, now? Stop being a dickhe*d now.............................................
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Old 28th Jul 2015, 22:59
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Without the need for name calling, I believe that CJ flies single engined machines where the performance considerations are slightly different.
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Old 28th Jul 2015, 23:23
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Originally Posted by chopjock
It makes me wonder why backing up downwind to keep the LZ in sight and so enable a forward descent / run on in event of an engine failure has any real advantage when in so doing puts more stress on everything and thus more likely to have an engine / tail rotor failure in the first place!
What 'stress', chopjock? And how does it create a greater likelihood of engine/tail rotor failure?

Originally Posted by chopjock
I cringe when ever I see this, using all that power whilst backing up, when you could be doing a towering take off into wind and on your way in much less time.
Using 'all that power'? SND and crab@ have both explained that it is a maximum of hover Tq plus10/15%. It is actually a relatively gentle application and very controlled, with little stress compared to many SE departures of 'pull max take off torque, nose forward and rotate'.

I've done a reasonable amount of both. I'm the guy that crab@ refers to as an ex-mil driver who would reef in power to get up quickly, and had to re-learn to be a bit more considerate to follow the correct profile.
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Old 29th Jul 2015, 06:36
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What 'stress', chopjock? And how does it create a greater likelihood of engine/tail rotor failure?
Ok is an engine or anti torque system more likely to fail / stall (LTE) at high loading or not so high loading.?
Yes I only fly singles and I know on a hot day with a light wind and quite heavy with fuel / pax etc it would require more power to climb out backwards down wind than to lift straight up off a cushion into wind. (I'm not talking about yanking up on the stick here).
I know the twins are doing it at a lighter than max weight to allow for this (helipad style), it just seems if the same power used for backing up was used to get going into wind, the exposure time to the risk would be less, or even be accomplished using less power (or an increased take off mass) and in my mind less risk of something breaking.
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Old 29th Jul 2015, 06:54
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Chopjock - the profiles are designed so there is no exposure - at any stage of the process, an engine failure will result in either a safe landing or a successful flyaway.

Single engine is different and I would tend to agree (after 32 years of mil flying) that reducing exposure time is a good idea.
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Old 29th Jul 2015, 09:20
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John E, thanks for the references! With those numbers we may be close to cat A in the winter but pretty far out in the summer. That's more or less what I suspected, I've always expected to best case spread the skids if we had a failure up past 100'.

Crab, that RFM profile sounds pretty similar to what we do during normal ops. The only exception is that after 30 KIAS or so I normally climb while accelerating to about 60 KIAS for max rate of climb. Do you know why they would recommend climbing out at VTOSS instead? Does it give a better ROC when you take the acceleration into account? I would assume that this would only be valid up to a certain distance..


My reasons for backing up during takeoff would be a) keeping the landing area in sight, b) increasing your forward run available and therefore your obstacle clearance, c) to allow a bit of forward drift from the nosedown tendency when you reduce collective, and d) to allow for landing with a "skids level" attitude as opposed to nose up due to the mast tilt (at least for Hueys). Am I off base, or are there others that I'm missing?

Thanks for all the replies!
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Old 29th Jul 2015, 10:10
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Rejected Helipad Take-off

The other advantage of the rearwards departure is that the rejection profile (forward and down) allows a little additional airspeed (airflow over the disc) which improves performance, while the slight flare used to terminate helps with NR maintenance/recovery prior to landing. At higher weights the difference (according to the A109 simulator, at least) is significant.
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Old 29th Jul 2015, 15:31
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I note that all published vertical takeoff procedures are not particularly gentle. At least one Category A vertical helipad procedure (S-76) uses rapid collective application from wheels light to full takeoff power for vertical climb to TDP’s up to 110 ft.
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Old 29th Jul 2015, 15:52
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Chucklehead - I think the VTOSS climb is more to do with angle of climb than rate of climb which, as you say, will be better at 60 kts (I think it says 56 kts in the RFM).

Certainly following an engine failure at or after TDP, the attitude change to capture VTOSS for the climb ensures you get a healthy vector away from the ground.

i think all your reasons for backing up are extremely valid - that's probably why the profile is designed that way.

Helitester - perhaps Nick Lappos has an answer to that apparent anomaly.
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Old 29th Jul 2015, 20:21
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Well chopjock another sterling job - reeling those innocent ones in....again tut tut.
Helitester, S76 do this because they have to break earth orbit @ 11k/mts/sec. Simples.

Next Q?
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Old 29th Jul 2015, 20:30
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Do you know why they would recommend climbing out at VTOSS instead? Does it give a better ROC when you take the acceleration into account? I would assume that this would only be valid up to a certain distance..
Yes, it's for obstacle clearance, rather than obtaining best rate of climb. For example, the profile for the type I fly requires the aircraft to be flown at Vtoss (30 kts) up to 200' above the takeoff surface, then an acceleration to Vy up to 1,000'
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Old 30th Jul 2015, 07:47
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Helitester;

Its' actually quite a smooth upward acceleration, the surprise comes on the reject, where in the words of the RFM you should "go light in your seat" Training for that profile is now done in the sim with occasional demos with both engines running in the aircraft. The problem is the propensity for pilots to hit the tail in the touchdown (RTO managed that a couple of times I believe) Certainly the S76 vertical, which is a true vertical with no rearward part is the most aggresive PC1 take-off I have ever flown (and I do it most days), but the power application should be smooth, yank it in and there are good opportunities for an over torque. Every other PC1 takeprofile I have ever tried is a very gentle and controlled experience,

SND
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Old 30th Jul 2015, 08:32
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On the subject of that S-76 takeoff, I'd like to poll the crowd on another "max performance" takeoff issue. I've met some proponents of a rather rapid power pull, vs a smoother, continuous pull into takeoff power. The argument being that the rapid pull can give you a higher vertical velocity vector and get you into a safe regime of flight faster.

I prefer the smoother pull because, in my experience anyway, the rapid pull tends to lead to a more unstable takeoff in which you lose a significant amount of what you gain, with a greater tendency to overtorque (esp due to tail rotor inputs). Any preferences, and arguments for one or the other that I maybe haven't considered?

Thanks again everyone! Interesting stuff all around.
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