Propeller Thrust Figures
 

I asked this question before and was given permission to restart the thread sometime back. Basically the question revolves around how must thrust a propeller can produce.

Before we dive into propeller thrust calculation figures (which is of limited use as I don't know what figures to input for certain aircraft designs because I don't know enough about the geometry of the props), I could use some actual figures from real aircraft.

The first questions pertain to WW2 fighter planes, regarding basically how much thrust, and how many pounds of thrust were produced per horsepower at takeoff-power, at climb-speeds, at cruise-speeds/altitudes, at maximum speed and so forth. If that's undoable, I could just use some figures to look at.

Basically, the airplanes of major interest are the following

FIGHTERS

F4U Corsair
- F4U-1A:
- F4U-4:

P-51 Mustang
- P-51C
- P-51D:

F6F Hellcat
- F6F-3:

F8F Bearcat
- F8F-1:
- F8F-2:


ATTACK

A-20 Havoc
- A-20G:

A-26 Invader
- A-26C:


BOMBERS

B-17 Flying Fortress
- B-17B:
- B-17G:

B-36 Peacemaker
- B-36A:

B-50 Superfortress
- B-50B:

The universal answer is "it all depends"!

Static thrust is on the order of 2x or 3x the shaft horsepower of the engine.

In-flight thrust equals airframe drag (including cooling system, of course).

In a dive, thrust falls off substantially when approaching critical Mach, and the prop can even be a liability.

I remember being told that the thrust to weight ratio of the FR-1 Fireball with a light fuel load was around 1. Even if I factor the thrust of the J31 into the equation (~2,000 lbf) that would give the prop about 4.5 pounds of thrust per horsepower for it to be around 1:1 with a very low fuel load.

4.5 lbf/hp seems pretty high to me, but I am open to seeing hard numbers.

BTW, the J31 jet engine (aka GE I-16) was 1600 lbf static thrust, not 2000 lbf. This might make the 4.5:1 prop thrust value on the conservative side.

You have your work cut out for you Jane coming up with all the figures you request.

The conversion from Thrust to Horsepower, and visa versa, is not a straight conversion of units, such as Kilometres per Hour to Miles per Hour, it depends upon the Thrust and the speed of the aircraft, as derived from the basic relationship where -

Power = Force X Velocity.

As a straight conversion to eliminate the constants of the various units used, the following formula applies -

Pa = Ta V / 325, where -

Pa = Propulsive Power available in Horse Power, Ta = Thrust available in Pounds, and V = Velocity in Knots (if working in MPH use 375 as the constant rather than 325).

Taking the RR Merlin engine as an example, at a given horsepower setting the thrust produced produced would be vastly different between a Spitfire fighter and a Lancaster bomber.

barit1

Ryan FR-1 Fireball

Weights
OEW: 7,690 lbs
TOW: 11,651 lbs

Engines:
1 x R-1820-72W or R-1820-74W = 1,350 to 1,500 hp (respectively
1 x J31 = 1,600 to 2,000 lbf

If I recall correctly the early variants of the J31 produced 1,600 with later models producing around 2,000. Could be wrong though.

Does the fact that the FR-1 was only able of doing around 275 mph on piston propulsion alone have anything to do with such a high prop-thrust?


Brian Abraham

Understood

Okay so

1.) Pa = Ta V / 325
2.) (1,350) = (x)(132)/325
3.) 1,350 = 132x/325
4.) (325)(1,350) = 132x
5.) 438,750 = 132x
6.) 438,750/132 = x
7.) 3,323.864 = 325?

Different speeds, different propeller used?

Different reduction ratio, different prop diameter I'm sure.

Can't help with your problem Jane, but I seem to remember reading that after visiting Lutterworth to see Whittle's first flight engine - as opposed to the rather horrible experimental engines - Stan Hooker told Lord Hives (Chairman Rolls-Royce circa '41-'42) that Whittle's jet produced 1000 lbs static thrust.

Hives was unimpressed, but asked Hooker how much thrust a Merlin produced at 350mph. Hooker lit up his slide rule and did the calculations.

"About a 1000 lbs of thrust." He told the chairman. Suddenly Hives wanted to see a jet engine for himself. :)

Roger.

My point is perhaps a little misunderstood.

The Merlin XX, for example, was used in,

Beaufighter II
Defiant II
Halifax I/V
Hurricane II/IV
Lancaster I/III
Mosquito I/II/IV/VI

The Merlin in the Mosquito was actually designated as a "21", but the only difference to the "XX" was the coolant flow direction was reversed.

Its rating was 1480 horsepower at 3000RPM. Critical altitude in low blower 6,000 and 12,250 in high.

There was a great disparity in speed between the Defiant and Mosquito.

It is extremely difficult to compare the performance of each of the aircraft at identical power settings, as the flight manuals of the day didn't go into such matters in depth.

Brian Abraham

Okay so you're talking about thrust-to-weight, thrust-to-drag, and lift-to-drag figures?

No, thrust versus horse power.

For example, the Mosquito I is quoted as having a maximum speed in the order of 370MPH, and the Lancaster I about 270MPH, both using the same engine giving 1,280 horse power. Given the paucity of exact figures these are ball park and will do for illustration.

Using the formula thrust=horse power*375/velocity

Mosquito thrust=1280*375/370 = 1,297 pounds/engine

Lancaster thrust=1280*375/270 = 1,778 pounds/engine

Brian Abraham

Then the issue seems to come down to different prop-diameter/blade-geometry, different gear-ratios and so forth like barit1 stated?

The question you asked in your OP wasThat is what I have been addressing ie the relationship between horse power and thrust.These elements have nothing to do with the basic horse power/thrust relationship. They do have a role though in optimising the engine/propeller match.

The horse power produced by an engine is given by the formula,

brake horse power = brake mean effective pressure*engine displacement*RPM/792,000

For static thrust try NACA Report 447
Fot thrust with forward speed NACA Report 1012

Both are available on line

This dummy can't find them Clive. Have links? Thanks.