Forum: Technology

I've read this article on power and thrust of jet engines, but I still have some questions. The essential equation Power = Force * Velocity puzzles me.

Let us assume we have two aircraft in the air. One 10 000 lbs helicopter with two 1500 shp turboshaft engines. The helicopter is barely able to hover, using maximum rpm/torque combination. The engines is obviously providing a power close to 3000.

The article states

Quote:

Power of a jet engine is force times velocity.

As we discussed earlier, a static engine does no work no matter how much thrust it produces because it results in no motion. We must instead focus our attention on a plane that is in motion.

Now imagine we have a 10 000 lbs VTOL (vertical take-off and landing) airplane hovering. The turbofan engine is obviously providing 10 000 lbs of downward thrust. What is the power output by this engine? An amount equal to the helicopter's engine? According to the statement above, the engine is providing zero power.

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Your examples are like comparing apples to oranges.

Turboshaft/turboprop engines have free-turbines that extract 'horsepower' from the gas-stream (accelerated gasses) and convert it into mechanical (rotational) energy. This rotational energy, similar to the output shaft of a steam, gasoline, or deisel engine, can be used to do 'work' via a gearbox or transmission.

Turbofan/turbojet engines do not have such a free turbine and do not make 'horsepower' in the conventional definition of the word since they are not actually capable of doing any work. (Unlike a spinning shaft) So a conventional jet engine only produces an accelerated mass.

With this the 'technical' formula for converting thrust to horsepower is; speed in ft/sec X lbs thrust then divide the product by 550.

In this formula you will see that a jet engine standing still, even though at full power, makes 0 Horsepower since it is not moving anything. (Only accelerating a mass) Industry standard is 2.5lbs of thrust = 1 horsepower when an engine is 'static'.

IE the PW-229 of a Viper makes 11,664 HP if it makes 29,160lbs of thrust static.

So if that Viper is moving at MACH 1 which is about 750MPH or 1,935.6ft/sec (Depending on altitude) and making 27,000lbs of thrust, (1935.6 x 27000) / 550 = 95020 HP!

You can see the faster a jet engine 'moves' an aircraft (doing work) the more horsepower it makes. So if thrust is constant with increasing airspeed, the HP goes up.

This is why Turboshaft and Turboprop engines are rated by 'power' (shp or kW) while Turbofan and Turbojet engines are rated by 'thrust' (kN or lbs)

In a turboshaft engine installation in a helicopter (your example) one has to account for rotor efficiency, friction of clutches, gearboxes, transfer shafts, tail rotor HP consumption etc... The FULL 3000Hp never reaches the air to be used for hover.

So for the second part of your question; yes a hovering F-35 at full power makes 0 (zero) horsepower because it is not 'moving' anything or doing any 'work.' It is simply accelerating air downward with 40K lbs of force, allowing an 'equal and opposite reaction' of keeping 40K lbs of weight motionless in mid-air.

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TEG

Filling in a gap in TEG's excellent explanation, an engine with shaft output (turbo-shaft, diesel, gasoline, etc) has its power output computed by torque x angular velocity. In our system, that works out as torque in ft-lb x rpm / 5252.

In this sense, the F-35 lift fan power could be computed from its shaft torque and rpm.

The helicopter's rotor converts the engine power to thrust, so looking at the entire helicopter engine plus rotor, there is no power output since the helicopter is hovering at zero velocity, similar to the F-35 hovering..

TEG, slightly incorrect on the velocity in that equation of the Viper. 750MPH is actually 1,100 FPS. So the viper at Mach 1 produces about (1100X27000)/550= 54,000 horsepower! Still an incredible amount of power!

Sorry...

TEG = That Engine Guy

TEG ≠ math major or calculus degree...

I'll double check my slide-rule to be sure it's calibrated and operating properly... LMAO

Guess you'd need to bump up to MACH 2 to reach 100K HP.

TEG

No worries TEG, Simple mistakes! Im very far from being a math major as well, just happened to notice something seemed a little odd. Shocked

Last edited by labrador on Jan 03, 2010 - 03:09 AM; edited 1 time in total

Thanks. I have another question regarding the same subject.

A Sea King helicopter uses approximately 1,200 lbs of fuel pr hour according to this web page. It weighs fully loaded appr. 20,000 lbs and thus the engine (rotor) provides 20 000 lbs of thrust if the aircraft hovers.

A F-16 running at military thrust uses appr. 8,000 lbs of fuel pr hour. However, it only provides 18,000 lbs of thrust (F100-220E).

Is this fuel efficiency typical of turboshaft versus turbofan engines?

Double post.

If I haven't said it anywhere before, TEG is one of the most important assets this forum has. I haven't read a bad post from him yet and I don't think I ever will.

Best Regards,

JoeSambor wrote:

If I haven't said it anywhere before, TEG is one of the most important assets this forum has. I haven't read a bad post from him yet and I don't think I ever will.

Thanks Joe! Cheers

Lab,

I'm going to use some standard MIL settings listed at J@Ne'$ site to keep things even, but they are in line with the ones posted from the given reference.

The T58-GE-402 only weighs 156.5 kg (345 lb), it is only 525.8 mm (20.7 in) in diameter and 1,397 mm (55.0 in) long.
The F100-PW-220E weighs about 1,472 kg (3,245 lb), it is 1,181 mm (46.5 in) in diameter and 4,855 mm (191.2 in) long.

T58s make horsepower, and F100s make thrust.

The T58 has an SFC of 0.60 lb/h/shp and makes 1,500 shp at MIL.
The F100 has an SFC of 0.73 lb/hr/lbs st and makes 14,500lbs st thrust at MIL.

The Sea King uses two T58 engines, burning 1800 lbs of fuel in an hour at MIL power to produce 3,000 shp.
The Viper uses one F100 engine, burning 10585 lbs of fuel in an hour at MIL power to produce 14,500lbs thrust. (Static)

The 3000 shp of the Sea King can 'hover' or move at a maximum of about 166 mph.
The 14,500 lbs of the Viper can push it to about 150 mph (just for take-off) in a couple thousand feet, and much faster when airborne! (Maybe MACH .9?)

You can't simply say the T58's are making '20,000lbs of thrust', they're not. They're providing 3,000 shp to the rotor head/blades that produces the 20,000lbs of "lift" to hover/move the chopper. The shafts are driving the rotor system. (Like a turboprop uses shaft power to move a propeller) The magic is in the rotor's efficiency, the rotor-head's gearing, number of rotor blades, etc.

The V-22 Osprey can hover at about 50,000lbs, The two Rolls-Royce Allison T406 turboshafts, make 6,150 hp (4,590 kW) each, which (by dead guessing) make 25,000lbs of lift (thrust) each at the rotor-disk. To do the same with a non-afterburning turbofan engine, you'd need something like a CFM56-5A1 to do the same job. (The T406 weighs about 1000lbs + rotor assembly, where the CFM56 is 5000lbs + nacelle)

So yes, a turboshaft engine, coupled to a rotor (Or turboprop engine) is MUCH more efficient at propulsion than a turbojet or turbofan at helicopter or civil aircraft speeds. (Even large high-bypass turbofans) This is why you don't see "Osprey type" or Helicopters flying around with turbofans for propulsion. Turboshafts on helicopters move massive amounts of air with the rotor-disk to provide lift, but that lift (thrust) is limited in the amount of speed it is given.

So while the F100 at MIL moves air at 224 lbs/sec, the Sea King's rotor-disk would be moving all the air above it's 62 ft diameter. See the relationship? A 3 foot hole moving air SUPER fast, or a 62 foot disk moving LOTS of air at a much slower speed. How fast can a Sea King accelerate straight up in a hover? Not much. I'm no helicopter guy, but I doubt the air under a Sea King, would be going as fast as an F-16 at take-off.

I hope you see what I'm getting at; I'm not into helicopters that much, but I'm trying my best.

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TEG

Forum: Technology

Power and torque of a jet engine