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Increasing engine power in a helicopter

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Increasing engine power in a helicopter

Old 26th Jun 2021, 03:55
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Increasing engine power in a helicopter

Following on from the interesting and educational post on rolling take offs I have another question


When helicopter manufacturers install more powerful engines on the same model of helicopter for example how does this additional power increase performance ?


To clarify, is the additional power used to turn the main rotor faster and / or install a larger main rotor or is it just the ability to ‘pull more pitch’ a larger ‘bite of the air’ with the collective inputting a higher angle of attack and the additional power being used to maintain that angle without the rotor speed decaying ?


Perhaps a combination of the above ?



Getting more thrust out of a basic jet engine and how it improves performance on a fixed wing aircraft is simple and straightforward to understand


Trying to understand how it works with helicopters
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Old 26th Jun 2021, 04:26
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They just change the charts...... Sorry could not resist.....

More learned pepes will come on and explain it better than me.... I yield and defer my time.....
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Old 26th Jun 2021, 06:36
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Having been awkeneded from a sound sleep by a Labrador Retriever who either has a smaller bladder than mine or knows when I am sleeping the sleep of the dead....I feel compelled to give my offering while I await the Princess to come back so we can all return to our slumbers.

The answer should be simple but it as in most things helicopter a bit. harder to lay out.

Assuming it is a straight forward increase in engine horsepower uptick such as when the Huey went from its first engine to later models....usually the aircraft was limited to the same certified weights, and torque limits of the original aircraft or as in the case of the Chinook in the A Model that saw stronger engines. installed.....the only improvement was in the retention of performance at the same structural limits.

In the case of later improvements of the basic aircraft that came along with upgraded engines such as the latest models of the Huey being flown by the USMC the new aircraft is a completely different airframe as compared to the older versions.

As it is 0230 in the middle of my night....this input was provided you due to Miss Reagan's wet nose summoning me to temporary duty without pay as the Door Man of the Goose Bay Tea House.

Now back to bed it is for me and that damn dog and whichever Cat it was that was snoring away at the foot of the bed.....probably the three legged J.R. Mewing.


Now that I am awake....with coffee number one in hand.....I might clarify what I posted earlier.

In the case of the CH-47A....we went from Lycoming T-55 L5 engines to L7's, and finally L7-C engines with no other changes to the aircraft.

With 7C engines the old girls would do everything we asked of them.

The 7C had been designed for the CH-47B and early model CH-47C's and later we went to L-11 engines for the C Model.

The B model had some minor changes...square tail being the most important as it greatly improved yaw stability and different rotor blades than the A model.

The C Model with L11 engines was a real horse....full of fuel at a hover....shutting down one engine could be done without any real difference in the handling of the aircraft....other than a split in Torque needles and the engine gages showing the shutdown and at most just one or two Rotor RPM decrease.....no other signs of just being on one engine.



Last edited by SASless; 26th Jun 2021 at 14:02.
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Old 26th Jun 2021, 06:37
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Put simply, the additional engine power is used to maintain the rotor RPM at higher pitch settings than previously would have been allowed, so more rotor thrust can be maintained and thus higher MTOW or hover ceilings are created. There have also been revisions to the main rotor blade profiles over time (the Squirrel AS350 / AS355 being an example), but rarely is the rotor speed is increased to generate more thrust (to the best of my limited knowledge).

Last edited by ApolloHeli; 26th Jun 2021 at 19:36.
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Old 26th Jun 2021, 07:44
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Updated engines are often installed to allow operations at higher Max All Ap Mass and higher density altitude.
The older engines may have been limiting the operational envelope. Rotor RPM usually unaffected apart from very minor droop characteristics, essentially rpm remaining at the old setting.
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Old 26th Jun 2021, 08:12
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Originally Posted by ApolloHeli View Post
...or that a larger main rotor has been installed (to the best of my limited knowledge).
Check the H135 P/T3. Longer rotorblades than the former versions.

skadi

Last edited by skadi; 26th Jun 2021 at 08:22.
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Old 26th Jun 2021, 09:02
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As many wise folk have said, to my mind there are essentially 3 options when installing an uprated engine:

A) uprated engine, but keep the same drivetrain/rotors, but be able to apply more torque (turning force) to keep them turning at steeper pitch angle (more drag) at the same RPM. This enables an increase in overall lifting capacity but assumes that the drivetrain is capable of handling an increase in torque, the aerodynamics of the rotor blade work sensibly at an increased pitch and the fuselage is capable of coping with increased mass.

B) Uprated engine used to overcome a limiting factor, but not used to increase normal torque. For example, your engine might be limited at high temps/density altitudes by outlet/turbine temperature, so changing the hot components of the engine for improved parts might allow a higher rated temperature, meaning you are able to achieve the same torque in worse environments, thereby improving lift in those without changing any other components (because you're not really changing the max rated torque of the drivetrain or pitch angle of the blades).

C) change everything as part of a redesign - obviously gives improved lift. New blade profiles, transmissions and engines = step change in performance but significant cost.

Case B) often also applied to twin engine helicopters where improved engine performance doesn't increase twin engine lift but does increase single engine failure (OEI) safety margins, which can in turn lead to an apparent increase in overall performance due to regulations allowing it, rather than the aircraft being physically capable of it.

Case A) has been known to apply where the original aircraft was "over-designed" for the original engines (e.g. engine manufacturer failed to deliver promised performance from shiny new wonder-engine, but the airframe was designed for it or where it was known that a new engine was likely early in the airframe life.)
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Old 26th Jun 2021, 10:04
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[QUOTEnever a case that the rotor speed is increased to generate more thrust][/QUOTE]

My first rotary steed was the mighty Gnome-engined Whirlwind (S-55), and delving back into my tech I vaguely remember being told that when they re-engined it from the original piston the RRPM was increased from approx 170 (?) to 220 with no other changes. However, it is back in the mists of time, and it could just as easily have been the Wessex. Whatever, I learned a lot about limited power operations during my 3 years on the Whirlwind, so I hate to imagine how limited it must have been with the piston engine.
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Old 26th Jun 2021, 17:33
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On some helicopters the engines are capable of exceeding the transmission limits in certain conditions. For example, on the SK-76 A models I used to fly you had to be aware of both sets of limits; obviously the first limits you meet have to be respected.

In a case like this, the main gearbox and other components need to be upgraded if more powerful engines are installed.

As per the "B" model, which had more powerful Pratt and Whitneys than the "A" and had a more robust gearbox (a thirsty machine). There was then the C model, with smaller engines for economy, coupled with the B gearbox. And commonly known as the "B minus".... at least you only had the engine limits to worry about in those.
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Old 26th Jun 2021, 19:02
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Originally Posted by skadi View Post
Check the H135 P/T3. Longer rotorblades than the former versions.

skadi
Clearly I wrote that before my morning coffee - I also believe one of the MD500 models includes lengthened blades for hot & high performance as well as an uprated engine.
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Old 26th Jun 2021, 20:11
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Crew: “ Yahoof we’re getting a bigger/better engine.” Happy Cheers! Visions of better hot/high performance.
“Oops my bad they have also increased the max gross” Moans and groans from crew.

Passengers are like gas they expand to fill whatever container they are put in. Once started a contract with a 206B …passengers brought a lot of stuff. Changed to a 206L-1 passengers brought more stuff and an extra guy.
Changed to a 205A-1 …passengers met the challenge…more guys, more extra stuff.
Too bad we didn’t have a Chinook they would have had us back up max gross in a week.

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Old 26th Jun 2021, 20:34
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A Chinook....they would have had a passenger sitting in the companionway to the cockpit and stowaways on the Ramp.

We loaded rice pickers by folding up the seats...cramming as many as we could into the cabin....filling the ramp up...and when ready....rolled forward and jammed the brakes on and as everyone shuffled forward...raised the ramp....then off we went.....135 pax at times.
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Old 26th Jun 2021, 21:31
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Originally Posted by ApolloHeli View Post
Put simply, the additional engine power is used to maintain the rotor RPM at higher pitch settings than previously would have been allowed, so more rotor thrust can be maintained and thus higher MTOW or hover ceilings are created. There have also been revisions to the main rotor blade profiles over time (the Squirrel AS350 / AS355 being an example), but rarely is the rotor speed is increased to generate more thrust (to the best of my limited knowledge).

That answers my question in the clearest most understandable manner, now it all makes sense !

Thanks for that
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Old 26th Jun 2021, 22:10
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No need for all that. You want more power? Just kick out the 180 lbs sitting next to you!
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Old 26th Jun 2021, 23:36
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[QUOTE=


As per the "B" model, which had more powerful Pratt and Whitneys than the "A" and had a more robust gearbox (a thirsty machine). There was then the C model, with smaller engines for economy, coupled with the B gearbox. And commonly known as the "B minus".... at least you only had the engine limits to worry about in those.[/QUOTE]

And of course Nr for the B and C models was increased from 100% to 107%

It's not unusual to bump up the Nr during original development and/or for followon models, with or without a change in engines.
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Old 27th Jun 2021, 07:16
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Increasing the Nr has two advantages for an increase in max gross weight. Firstly, increasing the speed of a rotating shaft decreases the torque, for the same power transmission.

Secondly, because the tail rotor is directly geared to the main rotor system, increasing its speed will also increase in proportion and reduce the blades angle of attack for the same thrust, making them less prone to stall at high pitch angles.

Obviously, increasing the Nr will result in a compromise elsewhere, such as bringing the aircraft closer to advancing blade speed problems in the high speed cruise.

The A109 series of helicopters are designed with a two position Nr switch. The Nr is set to 102% for flight below Vy and 100% above it. If the switch fails at 102%, the cruise speed has to be kept to 145kts or below.
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Old 27th Jun 2021, 08:58
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Stilton - one trend with more powerful engines has been to reduce the size of the rotor because you can work it harder with more power.

Shorter rotors mean a lower tip speed which allows faster forward flight.

You can see this from the increase in speed of modern generation helicopters when coupled with advanced blade design.
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Old 27th Jun 2021, 09:35
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Originally Posted by stilton View Post
That answers my question in the clearest most understandable manner, now it all makes sense !

Thanks for that
OK, that's the answer for comparing old and new engine at same density altitude. That assumes that the performance was engine limited. Often it is not, and the maximum power allowed is limited by gearbox, rotor head of other airframe limitations. In that case, the increased power cannot be used. (Well, you practically can use the additional powers, but you are discouraged by one of the other means from making use of the additional available power. And sometimes you should have regardless, but that belongs to another thread.)

Consider however the following: An engine delivers its nominal power only at sea lever and standard atmospheric conditions. Take the engine up to some altitude (density altitude, to be more specific) it might now only produce half of its nominal horses. (See also here: Reduction in Max Continuous Power in turbine helicopter with increase in altitude?)

Solution:
  • Double nominal engine horse power
  • Tell pilots (see above) to please not use the now available additional horsepower at low DA
  • When at that altitude contemplated above, the new engine - at half of its nominal power - still delivers what the old engine did at sea level
No changes to speed or MTOW, just the same documented performance available at higher DA
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Old 27th Jun 2021, 12:37
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Crab,

For that to hold....the Scout and its naval twin....should have been rocket ships!
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Old 27th Jun 2021, 13:33
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SAS, Yes but they managed to kept the speed down by building in loads of form drag. Or, depending on which way you look at it, they never finished the design and just left half of it hanging out in the breeze.
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