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Hughes 500 - you are absolutely right and most of us fully understand the energy management concept which is why there is no 'one size fits all' solution to entering auto because there are so many different combinations of entry parameters.

Consider PG's (and others) assertion that cyclic flare is the best form of energy management - why waste 100hp/sec (Dick Sandford's figures from the Robinson Safety Course) just keeping the Nr up by flaring when you could stop the Nr decaying by lowering the lever fully AND still have that extra energy available to you.

If you have lots of speed then fine, use flare to minimise the height loss (or even gain some if you are quick and have a responsive aircraft) but for goodness sake lower the lever (maybe not fully in the high speed cruise) - why would you use your precious energy up overcoming rotor drag (from having pitch applied) when you don't need to.

Try the aft cyclic first technique in a max power climb at 60 kts and you will probably die because in this case and many, many others, the lever down MUST be a priority.

Remember - engine fails.......pause, 1 second, two seconds for real world reaction times - where is your Nr now in the fast cruise at 14 degrees of pitch???? decaying very, very rapidly! Lower the lever AND flare might just save your bacon - flaring without lowering the lever might slow down the rate of Nr decay but it won't stop it, not at such high collective pitch angles.

None of the anti-aft cyclic heretics such as myself have ever said lower the lever without moving the cyclic, we have all said use both controls but lead with lever always.

muffin

In the past couple of years a new clause has crept into my insurance policy that specifically bans deliberate autos (except in emergencies) unless an instructor is on board. So consequently I am unable to practice them. Very interesting thread nevertheless.

nigelh

Not surprising really !!! I have always had a deal with my insurance that I never do full touchdown autos in my helicopters . We have covered this before but if you can keep rrpm in the green , flare and then level off at say 5-6ft , at a slow forward speed , then what happens next is pretty un important . As an owner I would be very happy for full down autos to have to be separately covered by insurance companies as that would save countless utterly pointless write offs which then push my premium up !!!!!

chopjock

IIRC that only applied to turbines or was it pistons? But not both.

Ornis

Energy (or work) is power times time: kilowatt-hours for electricity or horsepower-seconds in Robbie-speak, hp-s (not hp/s).

After engine(s) failure, the energy in the rotor will keep the aircraft flying for a second or so. Then it's gone and the rotor will stop, forever. It's how one adds energy to the rotor immediately that is the crux. In the cruise one can easily get some from the kinetic energy - motion - using the cyclic. In the hover one can't, one can get it only from descending - potential energy - and then only while the rotary wing is flying. Otherwise the aircraft is falling, not flying. Which introduces the other important factor - the angle of attack. Once the AoA has passed the stall angle the rotor stops, forever. So, lowering the collective lowers the AoA which helps keep the wing flying and slows the rate the energy is used by decreasing the lift and drag. Lowering the collective does not primarily add energy to the rotor.

Clearly entering autorotation in a genuine emergency is not as easy as falling off a log, or this thread wouldn't have started. More importantly, a helicopter wouldn't have punched through the solid roof of a pub, it would have landed. Somewhere else, probably.

henra

I think this is an important aspect in this discussion.
While theoretically cyclic back will be an effective means of keeping and restoring RRPM this will only work in a certain AoA range. With the collective in your arm pits you will likely see a significant decay in numbers even while in cruise flight.
That said the same will happen if you leave the cyclic fully forward and just drop collective. You will unload the disc and not get the necessary upward flow through the disc. At least not until a very high RoD builds up at which point the NR may already have dropped below PONR.
In Autogyros this is a frequent cause of fatal accidents:
Push the cyclic forward a bit too agreesively, watch the rotor NR decay in 1 or 2 seconds beyond PONR and drop with a stopped rotor from altitude, even with your engine in the back still pushing.
So both actions have to happen. Relax cyclic, lower collective.
You can discuss to which extent you do what but failing to do one of the two will likely end in tears.

Hughes500

Nigel
Sorry cant go with you on the power recovery. Think of it as in the mind. If you regularly do them to the ground then the brain does not have that added problem to consider if the donkey goes on holiday !
Examined someone the other day who did a perfect power recovery but a bit high. Then asked him to do it to the ground which he had never done, all I can say is if I wasn't on the controls the insurance company would have had a big bill and I quite like being over 6 ft tall not under !!!!!!!

Muffin

Are you sure your insurance says instructor as opposed to competent person ? Personally I would have that removed or changed to EOL's which I could understand. I think you will find it is a terminology problem
Just remember keep it in the green

awblain

Horsepower-seconds is indeed a unit of energy.

The kinetic energy of the rotor depends on the moment of inertia of the rotor, and the rotational speed squared. Note that it doesn't depend on the blade pitch, although the deceleration rate does.

The rotor typically stores ~2-5s of FULL engine power, so you typically have rather longer than 1s stored up, but you're right - there's not enough: you need to transition quickly to converting potential energy or kinetic energy from forward speed in order to maintain that reserve of energy.

To stop the rotor dead would require some specific choices. If it slows markedly, the the lift will be inadequate, the aircraft will drop, and this would certainly increase the airspeed from a hover, which could provide suitable airflow to get it going again, depending on its configuration.

chopjock

Muffin,
I just checked with my insurance company. Believe it or not, that only applies if you have a piston engined machine. Perfectly insured to practice with out an instructor if you do it in a turbine.
Work that one out.

Attila

When I did my Bell 214B conversion in Oman, the QHI warned me about rotor decay in the event of engine failure, 33 inch chord blades, remember!!

At 120 knots in the cruise he told me to be ready to lower the collective and to watch the rotor RPM as he counted down to the "engine failure" and chopped the throttle. As I lowered the lever very quickly I watched the RRPM decay instantly to 85%.

Some time later whilst carrying out a topping check at Seeb International I had an engine surge whilst climbing at a great rate of knots and before I knew it, I had closed the throttle, lowered the lever, rolled on some bank and applied aft cyclic to load the head and put out a mayday call. The rpm were still in the green but the engine wasn't very happy as part of the bleed band had gone through the compressor, so I had to shut it down for an engine off landing back onto the main runway. I then discovered, at 5000 ft in the descent, that the auto rpm were outside limits, a little too low for the AUW, which did make an interesting approach, i.e. a little faster than normal with a slightly exaggerrated flare to recover them, followed by a run-on landing without any damage to the aircraft.

Incidentally, there was a rate of climb limit in the Flight Manual, not to exceed 2000 ft/min below 2000 ft agl and during the above, I was well over 7000 ft climbing at 4000 ft/min. Interesting day...........

Lonewolf_50

Huges500 (in cruise) I note that this example is someone talking about using both hands to solve the problem. The false dichotomy of "either or" being dispensed with would improve this conversation (should it continue) immensely.

AnFI

AWblain: Nice to see someone being accurate about the energy picture, there are a few incorrect posts and referenced posts above in which HP/Sec is used.

Horsepower seconds is Hp x Sec OR HP.Sec NOT Hp/sec

Hp is Power, ie energy per time - so Hp times time is an amount of energy.

EXAMPLE: 20000Hp.sec means you can consume energy at the rate of 200Hp for 100seconds.


It's a abstract but useful unit for energy since most people have a feel for how many horse power they need to acheive a mode of flight. If you know the Hp you need and you know how many Hp.sec there are in your HEIGHT, SPEED and RRPM then you know how many Seconds of that Hp delivery each of those 'stores of energy' can provide.

Interesting Factoid:
For every 330lbs you weigh the VSI is calibrated in units equivalent to Horsepower - so if you weigh 3300lbs then 1 unit on the VSI is 10Hp
(assumes VSI calibrated in sensible units ie 100fpm)

Tailspin Turtle

Attila: I don't know whether this is an example of it, but the avoid region is only applicable for unaccelerated level flight. If you're in autorotation or even a low-power rate of descent, the avoid region is very much smaller. In a high-power low-speed climb, the avoid region (if the manufacturer had to depict it), would be somewhat bigger than the one in the handbook. As Attila notes, the 214B was capable of very high rates of climb since it had a Chinook engine in what was basically a Huey.

paco

"but the avoid region is only applicable for unaccelerated level flight"

Beg to differ - the top half above the knee is for that regime, below the knee is for the climb, where your hand is assumed to be on the collective.

cheers & happy xmas

Phil

Tailspin Turtle

Phil: Thanks for the amplification. The test pilot is allowed to respond much quicker on the collective at the knee of the curve and below. However, as far as I know, the test condition is still with power (collective pitch) for level flight, not climb. Since the delay required is almost instantaneous (0.3 second?), having the collective pitch higher (which means the blade drag is higher) than the test condition for establishment of the avoid area results in a quicker decay in rotor rpm for a climbing helicopter, meaning the knee is not, strictly speaking, valid, nor is the rest of the avoid area.

The difference in avoid area for level flight versus climb may be a small and even negligible for most helicopters but for one like the 214B, a single-engine helicopter with enough power, rotor, and tail rotor to hover out of ground effect at something like 11,000 feet, my understanding is that it did make a difference.

RotorIP

Beg to differ the test pilot can't do anything for 2 secs. Average reaction time for a pilot . The HV curve is very restrictive and no it's not for unaccelerated flight, empire test pilots jump in here. The HV curve as has been described to me by test pilots is to ensure you can land OEI or no engine in a single without damaging anything doesn't matter if your accelling or decelling or flying at one speed. Think steady state is worst case. Yes incredibly long when your sitting there and yes NR bleeds fast, is there a test pilot out there that can clarify this?
Seriously discussion about hp per second? What are you talking about?? It's useless info....oh yeah the engine quit so if I pull out the calculator ...really??
Autos are simply lower collective don't bottom it as NR will climb incredibly fast at altitude, if you need to check aft to build NR but only if you need to.
I go back to talk to training staff.....

RotorIP

Sorry don't know what type of rotor your referring to, ancient metal weighted tip Blades maybe, composite blades bleed NR very fast, went from 100% to 90 % in about a second and that was waiting for it and knowing the throttle chop was coming.
It's exactly this misinformation that will cause havoc in the industry. A 214 and a 225 are completely different animals you can't generalize.
Reading thru this thread nobody seems to know what the starting parameters were of the helo in Glasgow? Ie what was the actual airspeed and altitude when the engine quit? Was it recoverable ? Nobody seems to know they are just speculating and offering advice on technique. it really doesn't matter why the engine quit. Likelihood of two engines quitting at the same time almost 0. That's what we should be discussing, before it starts most crashes involving multi engines occurred because they were operating in a performance area requiring two engines. Modern Helos have much better performance OEI and can be operated in a manner that does not require two engines.

AnFI

RotorIP: "Seriously discussion about hp per second?" NO !! - It's HP.Secs !! - its arbitrary and doesn't require a calculator - It helps people 'get a feel' for how much energy is where and how valuable it is - once you've done the exercise you don't need to calculate anything - you'll have a 'feel' for it.

Understanding ENERGY is fundamental to flight - that particular unit is irrelevant of course! - but the fact that you (obviously) don't understand it is alarming! ( Power x Time = Energy )

(and waiting 2 secs in the high speed cruise is questionable - given a pilot is intervening on the cyclic at what 1-3Hz perhaps? (Second opinion?))

"Autos are simply lower collective don't bottom it as NR will climb incredibly fast at altitude, if you need to check aft to build NR but only if you need to.
I go back to talk to training staff..... " Good idea!

RotorIP

No I understand energy and teach it but your reply is exactly my point. Nobody teaches HP.secs. Seriously I don't know of an instructor or ground school anywhere that taught something like that because it has no relevance to actually flying. Maybe at the test pilot school for calculations but that's it, too many variables.
Waiting 2 secs before reacting is a certification criteria. Look it up.

AnFI

"has no relevance to actually flying"
Pilots do need to understand the Fundamentals of Energy:
Where it is
What it's worth
How fast you need it
How you can access/exchange it
- it's not rocket science, shirley?


HP.Sec is just a vehicle for doing that - some people use the unit the BANANA - you may have heard of those?
(Talking BANANAS: You may say u need 4 Bananas to fly at 60kts, 8 to hover, 9 at 140kts. 100ft is 1 Banana, 30kts is worth 1Banana 60kts is worth 3 bananas, 120kts is worth 7 bananas.) I don't like bananas!

but the Hp.Sec is pretty simple and easy and gives a clear representative understanding of the principles - without getting out a calculator.