# Calculating Prop Thrust

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#### Atomic.Sheep

##### New Member
Hello,

I'm learning about prop thrust, was wondering if you guys have some good info? I'm just googling at the moment.

#### Jan Carlsson

##### Well-Known Member
Hello,

I'm learning about prop thrust, was wondering if you guys have some good info? I'm just googling at the moment.
Hi,

Do you mean in flight or static?

European?

#### Atomic_Sheep

##### Well-Known Member
Pop, my head just exploded. Ever time I come here, I realise just how little I know!

What's the difference in a theoretical sense? Isn't the thrust being generated in both scenarios relative to the speed of the aircraft?

European????

#### Dana

##### Super Moderator
Staff member
Laden or unladen and European is a joke, a reference to Monty Python.

In flight and static thrust can and likely will be very different. Picture it: A propeller blade is a small wing, moving in a circle around the shaft. The blade angle gives it some angle of attack. When the aircraft is in motion, the air is coming in from the front, changing the blade angle of attack, thus the lift (thrust) it makes will be different. The blade angle is optimized for the chosen flight condition (blade AOA at or close to the airfoil's best L/D), so in a static condition it won't work as efficiently, and indeed part of the blade may be stalled. Thus, static thrust isn't terribly meaningful.

Dana

If you glue a piece of toast, butter side up, to your cat's back, and drop it from a high place, which way will it land?

#### tspear

##### Well-Known Member
Laden or unladen and European is a joke, a reference to Monty Python.

In flight and static thrust can and likely will be very different. Picture it: A propeller blade is a small wing, moving in a circle around the shaft. The blade angle gives it some angle of attack. When the aircraft is in motion, the air is coming in from the front, changing the blade angle of attack, thus the lift (thrust) it makes will be different. The blade angle is optimized for the chosen flight condition (blade AOA at or close to the airfoil's best L/D), so in a static condition it won't work as efficiently, and indeed part of the blade may be stalled. Thus, static thrust isn't terribly meaningful.

Dana

If you glue a piece of toast, butter side up, to your cat's back, and drop it from a high place, which way will it land?
Dana,

Why would static thrust not be useful? Seems like it could be important when helping to determine take off lengths.

Tim

#### Jan Carlsson

##### Well-Known Member
Dana,

Why would static thrust not be useful? Seems like it could be important when helping to determine take off lengths.

Tim
Actually static thrust is just as useful as fallos length competition on a ice cold, late Saturday night.

the static thrust have little to do with take off length, the thrust at lift off, means everything, you can half the static thrust and maintain lift off thrust, and take off length increase 5-10%

the thrust at speeds higher then static, is highest at or around lift off speed, then it fell off with speed.

Thrust is: power (HP) x efficiency x 75 kg (or 550 lb) / speed (feet or meter per second)

example 100 HP at 50 m/s (180 km/h)

100 hp x 0,85 x 75 / 50 = 127,5 kg of thrust

efficiency will fall off with lower speeds.
power will fall off with less rpm at lower speeds (fixed pitch)

Simple as that.

Ps Des Hart https://www.facebook.com/pages/Hart-Attack-Air-Racing/597452880343172?fref=ts won Silver final and took 5th in Gold, in F1 in Spain in his first year in air racing, with a pretty much standard Cassutt and standard 0-200 with a prop designed by me and manufactured by Rupert Wasey at Hercules propellers Hercules Propellers- High performance aircraft propellers (UK) the prop increased speed with 15 MPH straight and level on the race track

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#### Armilite

##### Well-Known Member
Actually static thrust is just as useful as fallos length competition on a ice cold, late Saturday night.

the static thrust have little to do with take off length, the thrust at lift off, means everything, you can half the static thrust and maintain lift off thrust, and take off length increase 5-10%

the thrust at speeds higher then static, is highest at or around lift off speed, then it fell off with speed.

Thrust is: power (HP) x efficiency x 75 kg (or 550 lb) / speed (feet or meter per second)

example 100 HP at 50 m/s (180 km/h)

100 hp x 0,85 x 75 / 50 = 127,5 kg of thrust

efficiency will fall off with lower speeds.
power will fall off with less rpm at lower speeds (fixed pitch)

Simple as that.

Ps Des Hart https://www.facebook.com/pages/Hart-Attack-Air-Racing/597452880343172?fref=ts won Silver final and took 5th in Gold, in F1 in Spain in his first year in air racing, with a pretty much standard Cassutt and standard 0-200 with a prop designed by me and manufactured by Rupert Wasey at Hercules propellers Hercules Propellers- High performance aircraft propellers (UK) the prop increased speed with 15 MPH straight and level on the race track

View attachment 32378
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Is there a Excel Spreadsheet formula to figure this out in USA spec's for the popular Rotax engines
277UL (26hp)
377UL (35hp)
447UL (40hp)
503UL (50hp)
582UL (65hp)
618UL (73.4hp)

Like a Stock 277UL Dynoed 25.4hp at 6000rpms. On a T-Bird I, it does say 65mph at max 6000rpms. Can you then figure the Thrust at the lower RPMs? Rotax parts and service Ontario 912 582 ultralights
 RPM BRAKE FUEL FLOW/HR B.S.F.C.​ CORR.HP​ CORE TORQUE​
 3500 9.3 16 6.8 2.35 6.9 10.4 4000 11.9 16 10 1.6 10.1 13.3 4500 12.8 17 12.1 1.41 12.3 14.3 4750 14.9 18 14.8 1.21 15.1 16.7 5000 16.8 18 17.6 1.02 17.9 18.8 5250 18.2 19 20 0.95 20.3 20.3 5500 19.8 18 22.8 0.79 23.2 22.1 5750 20.1 18 24.2 0.74 24.6 22.5 6000 19.9 18 25 0.72 25.4 22.2 6250 17.5 19 22.9 0.83 23.3 19.6 6500 14.8 20 20.1 0.99 20.5 16.5

Rich

#### Jan Carlsson

##### Well-Known Member
there is no units, but HP so?

#### TFF

##### Well-Known Member
Highest static thrust probably means once it starts flying it is already on the downward curve of performance. You want a propellor to have good static thrust but gets better in the operating range In a way it is like racing. Put an engine on a dyno, the race is with the dyno. Once the engine is in a car there are other factors like gearing, aerodynamics, weight, suspension that change how the power works the problem of going fast.

#### bmcj

##### Well-Known Member
HBA Supporter
European????
Obviously a reference to "Monty Python and the Holy Grail".

#### Atomic_Sheep

##### Well-Known Member
Are we talking about a fixed prop or a prop that can rotate around their axis? Can't remember what these latter props are called.

#### Jan Carlsson

##### Well-Known Member
Are we talking about a fixed prop or a prop that can rotate around their axis? Can't remember what these latter props are called.
If the propeller don't rotate the engine will.

#### Himat

##### Well-Known Member
Are we talking about a fixed prop or a prop that can rotate around their axis? Can't remember what these latter props are called.
Another variable, that is.
Fixed pitch prop vs. variable pitch propeller.
A propeller with variable pitch is often also called a constant speed prop as the engine rpm is keept more "constant".

##### Well-Known Member
Actually static thrust is just as useful as fallos length competition on a ice cold, late Saturday night.

the static thrust have little to do with take off length, the thrust at lift off, means everything, you can half the static thrust and maintain lift off thrust, and take off length increase 5-10%

the thrust at speeds higher then static, is highest at or around lift off speed, then it fell off with speed.
Enlightening post indeed, Jan Carlsson !
I'd have one more layman's question though - valid for a fixed pitch prop: what would be the WOT rev increase of the engine (in percents) while flying, compared with the max. "static" revs?

My experience being limited to 2-stroke RC-models, I wonder if the 8-10% rev increase (measured) would be also relevant for "real" airplanes.

#### Atomic_Sheep

##### Well-Known Member
Another variable, that is.
Fixed pitch prop vs. variable pitch propeller.
A propeller with variable pitch is often also called a constant speed prop as the engine rpm is keept more "constant".
Yep, variable pitch is what I was talking about, however I thought even the Cessnas have variable pitch props which can be controlled by the prop handle? I didn't think it was only constant engine RPM setups were the only cases of props being able to change in pitch?

#### akwrencher

##### Well-Known Member
HBA Supporter
What you are thinking of on the certified airplanes like Cessnas is, as Himat pointed out, called a constant speed prop. The reason is the use of a governor. When you adjust the prop pitch, you are actually just changing the setting on the governor. Say you have leveled off, and are ready to set up for cruise. You increase prop pitch. Now you can adjust your throttle. Unlike a fixed pitch prop though, your manifold pressure, but not rpm, will change. The governor automatically changes the prop pitch to keep the desired rpm. You don't really see them on less than 180 hp engines that much. They typically use engine oil pressure to operate the prop and have to have a hollow drilled crank. In the small side of the homebuilt world, you may find electrically operated variable pitch props on pretty small planes. These are not really "constant speed" props though, because they do not have governers (usually). You have to adjust the pitch yourself with each throttle change. No biggie, just different. Hope this helps clear up some of the confusion

#### jedi

##### Well-Known Member
Enlightening post indeed, Jan Carlsson !
I'd have one more layman's question though - valid for a fixed pitch prop: what would be the WOT rev increase of the engine (in percents) while flying, compared with the max. "static" revs?

My experience being limited to 2-stroke RC-models, I wonder if the 8-10% rev increase (measured) would be also relevant for "real" airplanes.
The rpm increase depends on how clean the airplane is and how the prop is pitched, cruise versus takeoff/climb prop. A clean airplane with a high cruise speed, Cassult racer for example, will gain much more % rpm than a draggy Steerman biplane.

It is not just small planes with little engines that have electric variable pitch propellers. Some high power planes had them too. One problem is the electrical slip rings. High power (and small ones too) electric props are prone to maintenance problems such as the famous Howard Hughs crash (I think) and perhaps the Boeing Stratocruiser (not sure which engines had the electric props).

"Another common type was originally developed by Wallace R. Turnbull and refined by the Curtiss-Wright Corporation.[8] This electrically operated mechanism was first tested in on June 6, 1927 at Camp Borden, Ontario, Canada and patented in 1929 (United States No. 1,828,348). It was favoured by some pilots in WWII because even when the engine was no longer running the propellor could be feathered. On hydraulically operated propellors the feathering had to happen before the loss of hydraulic pressure in the engine."

"Despite a [Boeing 377 Stratolcruiser] service record[5] remembered for one or two early disasters arising from the Curtiss Electric propellers fitted to early aircraft, the 377 was considered to be one of the most capable of post-war propeller-driven transports, and among the most luxurious. A total of 56 were built, one prototype (later reconditioned) and 55 production aircraft. 888 more of this general design, with differences, were built as KC-97 aerial refueling aircraft and C-97 military transports.[1]"

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#### Jan Carlsson

##### Well-Known Member
RPM gain with speed is as is said different from plane to plane.

If we say max is 2700, some thin can's have 2200-2400 static.
others like Aeronca and Cubs 2150 static to 2300 max

Talking Race plane. they can be >1000 rpm below max race rpm static.