(Request) thrust of a B29A engine-propeller combination

[msxyz]

Does anybody have a figure for the maximum static thrust developed by a single R3350 (2200 hp) engine when mated to the 16ft propeller used by the Boeing B29?

Thanks!

 

[krieghund]

I think this will be hard to come by. The rule of thumb is multiply the take off horsepower by a factor of 2.5 to approximate static thrust which is about 5,500 lbs. To calculate an exact figure you'll need the propeller performance perameters.

 

[drgondog]

Krieghund is right. Simply stated, calculating 'thrust' equivalence with a propeller/engine system whose primary performance is horsepower is a formula based an propeller performance, Hp at a given set of atmosperic parameters of which altitude is the primary factor and the attained velocity of the system being studied.

Short answer - "It gotta move" to start the derivation of equivalent thrust based on the change of the mass flow rate through the prop/engine system in flight. The faster the system moves the less thrust is developed as the change in the mass fow rate from the conditions in front of the prop to that in the stream tube aft - is less.

 

[msxyz]

Thanks for the answers, that's why I specifically asked for the engine-propeller combination of a B29: knowing the horsepower of the engine is not enough to know how much force is developed.

Since propeller efficiency decreases as the aircraft to which is attached gains forward speed, would it be correct to say that 'static thrust' for a propeller driver aircraft is the maximum obtainable by such a propulsion system?

 

[Shortround6]

Propeller efficiency may not decrease as the plane gathers forward speed. It depends on the propeller. Fixed pitch props vary enormously in efficiency at low speeds (static) which is one reason for variable pitch props. Not all variable pitch props can go to zero pitch. Some had a lower limit of 20 degrees or so.

 

[drgondog]

Thanks for the answers, that's why I specifically asked for the engine-propeller combination of a B29: knowing the horsepower of the engine is not enough to know how much force is developed.

Since propeller efficiency decreases as the aircraft to which is attached gains forward speed, would it be correct to say that 'static thrust' for a propeller driver aircraft is the maximum obtainable by such a propulsion system?

Variable pitch Propeller efficiency increases in most cases to ~ .85 asymptotically as velocity increases up to a 'point'.

Thrust decreases with velocity. It is probably safe to say that static bench thrust is close to maximum attainable for a specific set of pitch/rpm criteria.

 

[wells]

best case ( standard sea level ), results rounded to nearest 100 lbs

0 mph ( static ) ~ 4000 lbs
50 mph ~ 3100 lbs
100 mph ~ 2400 lbs
150 mph ~ 1800 lbs
200 mph ~ 1400 lbs
250 mph ~ 1200 lbs

 

[drgondog]

best case ( standard sea level ), results rounded to nearest 100 lbs

0 mph ( static ) ~ 4000 lbs
50 mph ~ 3100 lbs
100 mph ~ 2400 lbs
150 mph ~ 1800 lbs
200 mph ~ 1400 lbs
250 mph ~ 1200 lbs

Wells - those number look reasonable but there is a curious anomaly - namely 2400 lbs at 100mph and 1400 at 200mph (instead of '1/2' the 100mph thrust results for '2x' velocity)

Did you get a variance of 'eta' between 100 and 200 mph to offset the linear relationship with inversely proportional speed?

 

[Aozora]

These might be of interest:
NACA report on the inlet design of the B-29 cowling
NACA report on cowling and flap design for B-29

Cheers.

 

[wuzak]

Those links don't work for me.

 

[wuzak]

Are these the reports?

Cowl Outlet Design
Cowl Inlet Design

 

[Aozora]

Are these the reports?

Cowl Outlet Design
Cowl Inlet Design

Yep: I haven't read them properly, but a quick skim through indicates they don't show the maximum static thrust figures that msxyz is looking for. Nevertheless they are interesting. Apologies for the garbled links in the previous posting. Cheers

PS: Hope one and all have a great Christmas!

 

[Zipper730]

To calculate an exact figure you'll need the propeller performance perameters.

Where would you find that sort of data?

The faster the system moves the less thrust is developed as the change in the mass fow rate from the conditions in front of the prop to that in the stream tube aft - is less.

The exhaust velocity gets closer and closer to the plane's maximum speed?

Variable pitch Propeller efficiency increases in most cases to ~ .85 asymptotically as velocity increases up to a 'point'.

And at that point it drops to a lower value by cruise speed?

Propeller efficiency may not decrease as the plane gathers forward speed. It depends on the propeller. Fixed pitch props vary enormously in efficiency at low speeds (static) which is one reason for variable pitch props.

So basically, it's like L/D ratio?

best case ( standard sea level ), results rounded to nearest 100 lbs

0 mph ( static ) ~ 4000 lbs
50 mph ~ 3100 lbs
100 mph ~ 2400 lbs
150 mph ~ 1800 lbs
200 mph ~ 1400 lbs
250 mph ~ 1200 lbs

How did you compute these figures?

 

[swampyankee]

Where would you find that sort of data?

First place to look would be at NACA. Do note, however, that it was quite rare for actual thrust to be measured, especially in flight. The next places to look would be in Boeing's archives or in the archives of the company that made the propeller.

The exhaust velocity gets closer and closer to the plane's maximum speed?
And at that point it drops to a lower value by cruise speed?

Thrust is mass flow rate times change in velocity across the propeller. The power required to produce that thrust is that mass flow rate times change in velocity squared; to a first approximation, piston aircraft engines produce the same power regardless of air speed. The exhaust velocity that counts here is that from the propeller, not the engine: whether the power comes from pistons banging back and forth, turbines spinning smoothly, or over-caffeinated squirrels isn't relevant.

So basically, it's like L/D ratio?

No. Lift is how the air gets accelerated; the thrust is coming from the change in the air's velocity.

How did you compute these figures?

Well, you could get a cfd program that can perform propeller analysis. I think VSAERO with the rotor code will do quite nicely. Failing that, use a lifting line or lifting surface program modified for propeller analysis.

 

[Zipper730]

Thrust is mass flow rate times change in velocity across the propeller. The power required to produce that thrust is that mass flow rate times change in velocity squared

So where do you get mass-flow figures are from the prop or the intake airflow to the engine?

Well, you could get a cfd program that can perform propeller analysis. I think VSAERO with the rotor code will do quite nicely.

Are any of these compatible with Mac?

 

[swampyankee]

So where do you get mass-flow figures are from the prop or the intake airflow to the engine?
Are any of these compatible with Mac?

VSAERO is sold by Analytical Methods, Inc. I don't know what it's been ported to. For the others, I'll point you to CFD Online

In forward flight, disc area time air density times airspeed will give massflow; at low speeds, this may not work as induced flow can be significant.

 

[Zipper730]

In forward flight, disc area time air density times airspeed will give massflow; at low speeds, this may not work as induced flow can be significant.

Induced flow is the airflow blown through the propeller right?

 

[Shortround6]

Induced flow is the airflow blown through the propeller right?

Induced flow in the air outside the propeller's diameter that is moved along with the airstream.

from fire engineering.

The fan at the bottom of the steps may move more air though the doorway than the fan adjacent to the doorway. The expanding cone of air from the fan will capture more air (induce) from the free air around it as it moves to the doorway and through it.

 

[Zipper730]

Like vortex effects?