d2180s

Hi,

Is anyone able to explain why a turbojet is known as a thrust-producer, but a piston-prop combo is known as a power-producer?

My ground instructor was unable to give a good reason, and although we don't need to know the reason why for ATPL's, there must be a good scientific reason why?

Perhaps turbojet thrust is linear for any given condition with throttle position, but piston-prop power is linear with throttle? I don't know, just guessing...?

Cheers.

Romeo Kilo

As a classmate of d2180s, add like to add to his question...

Within the context of climb performance, we have been shown the differences in terms of best angle/rate of climb with respect to thrust - a force; and power - rate of doing work, or indeed a force exerted over a distance per unit time.

It makes sense to me that, looking at range for example, whilst a speed that offers us minimum drag sounds efficient, we may able to travel further by sacrificing some drag in favour of true airspeed. So all of a sudden time is a factor, and we care about power rather than work, and speed rather than distance. Cool. I guess there's some sort of calculus going on there.

Why, however, is it that the best angle of climb in a propeller aircraft is derived from minimum power rather than minimum drag? When considering angle of climb, we think of it as a simple triangle, purely based on altitude/height gained per distance covered, which, at least in a jet engined aircraft, is directly linked to the geometry of the forces acting upon the aircraft. Why then, for a propeller aircraft, when we are not interested in the time taken to reach a certain height, only the distance covered, does power play a role?

I think that, essentially is what I'd like to add to d2180s's question. Its all very well being able to regurgitate the giff in the exam, but I get the impression this is some 'need to know' stuff. Pilot Sh!t, if you will.

Thanks for any help

keith williams

A turbojet engine is called a “thrust producer” because it produces thrust directly. By this I mean that it does not use any other devices to convert the engine output into thrust.

The output of a piston engine is not thrust, but takes the form of a torque applied to the output shaft. The power transmitted through a rotating shaft is proportional to the torque multiplied by the RPM. So the output of a piston engine is rotating power. We call it Brake Horsepower or BHP.

To convert BHP into thrust we must use a propeller. The efficiency with which this conversion process is achieved varies with factors such as RPM, blade angle and TAS, so there is no fixed relationship between the BHP and the thrust produced.





Best angle of climb for any aircraft occurs at the speed at which the excess thrust is a maximum. This is true for both jet aircraft and propeller aircraft.

JET AIRCRAFT
For ATPL study purposes we assume that jet thrust is approximately constant at all airspeeds. This means that maximum excess thrust and best angle of climb will occur at Vmd.

PROPELLER AIRCRAFT
The thrust produced by propellers is maximum before the brakes are released at the start of the take-off run. Thrust then decreases as TAS gradually increases. When we compare this gradually decreasing thrust with the drag curve we find that maximum excess thrust occurs at a speed that is slightly less than Vmp. The difference between Vx prop and Vmp is very small, so some texts state that they are the same speed. But strictly speaking best angle of climb still occurs at the speed at which excess thrust is a maximum.

In reality all of the above is further complicated by things like constant speed props, but these can be ignored for the purposes of your question.

Chu Chu

Power is proportional to the product of thrust and velocity. A jet engine can make roughly the same amount of thrust at any airspeed. Thus, it actually makes more power the faster the airplane goes.

The power made by a piston engine stays more-or-less constant as airspeed increases. This means that available thrust decreases as airspeed increases.

main_dog

Also, in a propellor-driven airplane, fuel consumption varies with power produced, whereas on a jet fuel consumption is basically proportional to thrust produced. And yes, on a jet the thrust levers essentially control thrust directly, whilst on a prop the power levers control, er, power. It thus makes more sense to think in terms of power when referring to props and thrust when referring to jets.

Both jets and props however produce both thrust and power, and in both jets and props best angle of climb will be the speed at which excess thrust is highest, while best rate will be the speed at which excess power is greatest.

The nature of the curves on the graphs (thrust remaining more or less equal over all airspeeds for jets and tapering off for props, and power increasing with speed for jets and remaining more or less steady for props) is due to the differing characteristics of the two types of powerplants as pointed out by Chu and KW. It simply means that Vx and Vy for props and jets will be at different points on the drag curve, higher up for jets, lower speed for props.

Agaricus bisporus

They are? By whom? Never heard of that myself. Sounds like a pretty pointless distinction anyway.

keith williams

We must be very careful in making this type of comparison.

Brake Horsepower is proportional to torque x RPM.
Propulsive power is equal to thrust x TAS.

If we use the simplifying assumption that jet thrust is almost constant at all airspeeds, then as we increase the airspeed the constant thrust multiplied by increasing TAS will produce increasing propulsive power. But the turbojet engine will continue to produce no BHP.

The BHP produced by a piston engine is almost constant at all airspeeds. But the thrust produced by the prop gradually decreases as TAS increases. So as we increase airspeed, the decreasing thrust multiplied by the increasing TAS causes the propulsive power to increase up to some maximum value then decrease as airspeed continues to increase. The propulsive power curve for a prop is a bit like an inverted drag curve.


So overall we can say that as airspeed increases:

PISTON PROPS produce
Constant BHP.
Decreasing thrust.
Propulsive power which increase up to some maximum value then decreases again.

TURBOJETS produce
Almost constant thrust.
No BHP.
Increasing propulsive power.

d2180s

Thanks gents, that is the explanation I was looking for. Probably explains why we consider thrust specific fuel consumption for jet engines and brake SFC for piston-props.

Agaricus bisporus,
The only on-line reference to it that I can find is here, pg5, bottom paragraph.
ftp://ftp.seu.edu.cn/Pub2/EBooks/Boo...on26/Ch177.PDF
I vaguely remember reference to thrust- and power-producers from my degree, but that was a while ago.

Brian Abraham

A formula to convert horsepower to thrust assuming 100% propeller efficiency

THRUST = BHP*325/Velocity (knots)

hawk37

How does that work Brian for a propellor aircraft at zero knots, engine run up ready for take off. Does your formula give infinite thrust?

italia458

This should help everyone get a clearer picture of what "thrust" and "power" really mean.

https://www.box.com/s/3302d62b8d269bfc96cb


hawk37,

Brian has just rearranged the equation. You should be able to see what's going on after you read the document I posted.

keith williams

The formula

Thrust = BHP x 325 / TAS in knots

is just based on the following:

1 hosepower = a work rate of 33000 ft lbf / min

1 knots = 101.3 ft / min

Thrust = work rate / TAS

So thrust per horsepower = 33000 ft lbf / min / 101.3 ft / min = approximately 325 lbf

So at any given TAS we get Thrust = HP x 325 / TAS in knots


You are correct in saying that this would give infinite trust when TAS = zero.

The problem is in the assumption that propeller efficiency = 100%

Prop Power output is in Thrust Horsepower or THP, which is thrust x TAS

Prop power input = BHP

Prop efficiency = Power output / Power input.

When TAS = zero prop efficiency = zero, so the 100% efficiency assumption cannot be used.

hawk37

Keith wrote I guess I'm having problems understanding what zero prop efficiency means. It seems a piston/propellor combination has the most thrust when velocity is zero, as Keith has said earlier

So....with a stationary aircraft and the engine at full power, we have the max thrust the engine/propellor combination can produce, but zero prop efficiency?

keith williams

A quick google search usually produces a number of documents providing conflicting definitions of the purpose of the propeller and the formula for propeller efficiency. Many of these documents include diagrams which show propeller efficiency of zero when TAS is zero.

In trying to resolve the problem we should probably start by asking the question "what is the purpose of the propeller".

Some people (and texts) would say "to produce thrust", while others would say "to propel the aircraft forward".

If the purpose is to produce thrust then:

1. We would probably call it a thruster.

2. Its output is a force (measured in lbf) and its input is power (measured in ft lbf / min)

If we use the standard convention that efficiency = 100% x (output / input)

we get the formula

Propeller efficiency = 100% x (lbf) / (ft lbf / min)

But this formula yields units of ( % min/ft)

We might well ask what exactly a (% min/ft) looks like (I do not know)


If the purpose is to propel the aircraft forward, then

1. We can continue to call it a propeller.

2. Its output is propulsive power (measured in ft lbf / min) and its input is power (measured in ft lbf / min)

If we use the standard convention that efficiency = 100% x (output / input)

we get the formula

Propeller efficiency = 100% x (ft lbf / min) / (ft lbf / min)

This formula yields units of %