PPRuNe Forums - Increasing engine power in a helicopter

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- - Increasing engine power in a helicopter (https://www.pprune.org/rotorheads/641267-increasing-engine-power-helicopter.html)

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the Scout and its naval twin....should have been rocket ships!

The most fun flying I ever had, care free fighter like handling, wonderful governor, Nr never moved an iota irrespective of what you did with the lever, pity about the fuel consumption and payload/range though. Was replaced with a Kiowa on low skids, much more productive, though deck handling was a little more sporty.

SAS - but it lacked the modern, powerful engines and anything remotely approaching advanced blade design:)

I gather it was a rocket ship in autorotation but only vertically:ok:

Off topic ..... I have enjoyed reading helicopter history and nearly every machine was under-powered in the original versions thus the need for increases during the following years.

First Bell 47's were 178 hp and ended up 250-300 hp to make them practical.
Even the modern R22 bumped up the power shortly after manufacturing began.

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Originally Posted by [email protected] (Post 11069091)

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.

Interesting, thanks for that

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Originally Posted by Hot and Hi (Post 11069112)

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: https://www.pprune.org/rotorheads/64...l)

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

Sounds a bit like flat rating

And another variation of how increased engine power can be applied usefully in helicopters

Seems like there’s several ways in which this can be done

There is a practical limit to forward speed for single rotor helicopters which is the ratio of tip speed to forward speed called the advance ratio.

There are limiting factors on both the advancing side (sonic issues) and the retreating side (low V squared, high AoA and stall).

Advanced blade designs can operate at higher AoA on the retreating side and higher speeds on the advancing side.

Having a lower tip speed due to a shorter rotor helps on the advancing side but you would have to increase the rotation speed to keep the mass flow the same so it's not a free lunch.

Ray Prouty used to say about helicopter design 'What you gain in the hover you lose in forward flight and vice versa'.

One other aspect of increasing engine power in a twin is that it allows PC 1 profiles at higher AUM so more bums on seats in the cabin.

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One other aspect of increasing engine power in a twin is that it allows PC 1 profiles at higher AUM so more bums on seats in the cabin.

I think in the Twin engine world, this is the primary reason for increasing power - as has been noted you typically can't use the maximum power from both engines AEO due to transmission limits. The confliction that arises is that turbine engines operate most efficiently close to their design point, at high Ng, so for best AEO fuel efficiency you want the engines to be running 'flat out'. But, that means if you lose one you take a big hit on power remaining. Conversely, you can have powerful engines that are 'barely working' when AEO, but are able to pick up most of the load OEI, which results in good payloads but poor AEO fuel efficiency. Probably the best example is the S76B vs the S76A+, although modern design of engines and transmissions is closing the gap.

The SK76 B model was great for short trips. Unfortunately it burned notably more fuel, but it was also heavier than the A models and with a full passenger load it could often carry less fuel. Candle burned at both ends.

Another way of looking at this is by comparing the more powerful variant of the AS350 (the B3e) with its previous variants.

The As350B3e with an 847 shp engine can do full fuel and full pax (and even rumoured to have "landed" on Mt Everest) whereas the previous variants couldn't. All this capability of course comes at a higher fuel consumption and therefore higher cost passed onto the paying customer.

Is it needed? Well that all depends on the mission profile. Unlike aeroplanes, due to their utilitarian nature most helicopters begin their mission when they arrive at their destination. Often, to turn a profit for the task at hand, a powerful and high fuel consumption engine is not required.