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I'm reviewing training materials1 for straight and level flight.

The content focuses on drag types and explains that the required thrust curve is equivalent to the total drag curve. It then introduces the power required polar and states that velocity is related to available power.

Why not just say that thrust determines velocity?

What's the purpose of bringing in the power curve? Is it simply because Velocity = Power / Thrust?


1Page 17 under heading "POWER".

Also in an old printed manual from the CAA days. And also the Aviation Theory Centres flight training manual

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  • $\begingroup$ Also, page 150 of the FAA's Aerodynamics for naval aviators mentions power and not thrust for a piston/prop. I'm feeling particularly dense when it comes to this concept for some reason. I can't seem to practically grasp why we must consider power when it is thrust equalling drag? $\endgroup$
    – Crabsticks
    Nov 7, 2023 at 8:27
  • $\begingroup$ Interesting US Navy course: Thrust vs. Power $\endgroup$
    – mins
    Nov 20, 2023 at 14:43

5 Answers 5

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People, let's not confuse the newbie with endless comments, but write a proper answer!

Why power?

Power is reasonably constant over speed, so it is more useful than thrust when characterizing a propeller-driven airplane. The determination of thrust is much more complicated and error-prone. Power can be accurately measured on a test bench while only static thrust can be measured to an equal degree of accuracy (just tie the airplane to something really heavy and measure the force needed on those ropes). Thrust in flight (when it is really interesting) can only be approximated because so many factors will influence the measurement:

  • Propeller efficiency. This can vary a lot.
  • Airplane drag. What you can measure is either fuel flow at a trimmed speed or acceleration at full throttle. Both will only give you an indication of the difference between thrust and drag, but not drag directly. Good drag estimation is a very laborious task!
  • Slipstream drag. Even if you know the drag of the airplane in flight to a reasonable degree, the change in flow speed and direction within the propeller slipstream will add another drag component which also needs to be determined in order to calculate thrust.

Why not just say that thrust determines velocity?

You are right, thrust (or, more precisely, the difference between thrust and drag) does determine velocity.

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    $\begingroup$ Good answer. Thrust dictates the physics. Power is a model of how thrust will behave at different airspeeds. $\endgroup$
    – JZYL
    Nov 7, 2023 at 13:51
  • $\begingroup$ @JYZL -- no, it is equally valid to say that either thrust or power dictates the physics. The book "Gossamer Odyssey" about MacCready's human-powered aircraft goes into great detail about the power output limitations of the human pilot-drove the aircraft design process -- we can compute the required power needed to fly an aircraft directly from the no-power sink rate at the airspeed of interest, and the aircraft weight-- I don't see how one can say that power does not drive the physics-- $\endgroup$ Nov 12, 2023 at 1:28
  • $\begingroup$ @quietflyer JYZL only agreed that thrust is the force in the equilibrium, not power. And that power limitations drive the design process of airplanes is hardly a novel insight. $\endgroup$ Jan 31 at 19:45
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@Crabsticks Thanks for posting the page number to the passage that is confusing you.

It is simply because the airplane has a power lever -- it does not have a thrust lever. The propulsor (prop, fan, jet, whatever) generates thrust. It also does work on the air at the rate (power) T*V. In a helicopter (which is usually governed to constant speed), there can be a torque meter on the dash, so the flight manual there will be in terms of torque.

For all practical purposes, the terms thrust and power are interchangeable when we talk about how we fly an airplane. When you increase thrust, we also increase power.

Piston propeller airplanes generally have a maximum power rating. The shaft power the engine can deliver is pretty much independent of airspeed. We have instruments that report RPM and manifold pressure (and sometimes torque). So it makes sense to talk about operating the piston engine in terms of power.

Because the piston engine has a maximum power rating (maximum speed will occur at the engine's limits), a POH will talk about max speed occurring at max power.

The piston prop aircraft actually achieves maximum thrust at takeoff at sea level. Feel how much it throws you back in your seat when you push the throttle forward on takeoff -- lots of static thrust.

So, if you imagine there was a 'thrust meter' on the panel, it would be all over the place during flight. There is also no useful maximum allowable thrust that the pilot must watch -- like a maximum RPM, maximum manifold pressure, maximum exhaust temperature, etc. The engine instruments are all about not breaking the engine.

Because of the way engines and propellers work, it makes more sense to operate a piston/prop in terms of power.

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Just to add something to the other answers.

Why not just say that thrust determines velocity? What's the purpose of bringing in the power?

Using either thrust or power is really just a matter of ease of calculation.

An engine driving a propeller, delivers a power which is more or less constant with the speed. On the other hand, a jet engine delivers a thrust which is more or less constant with the speed.

So if you are analysing a propeller-driven airplane then it's easier to reason in terms of power, while if you are analysing a jet airplane then it's easier to reason in terms of thrust.

That's all.

Obviously you can always change from thrust to power and vice versa relying on the "definition" of power which is force*speed. So for example the power consumed by an airplane due to its aerodynamic drag $D$ flying at speed $V$ is $P_{drag}=D \times V$. The power consumed by a propeller creating a thrust $T$ with an efficiency $\eta$ at speed $V$ is $P_{prop}=\eta T \times V$.

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Power is what you put in.
Thrust is what you get out.
The opposed aerodynamic drag is what's left over onve you have got the power from the engine and into the air.

Engines are designed to provide a power curve.
Torque is proportional to Power per engine RPM - so you could if you wish deal with either, but power makes most practical sense.
An engine usually has an optimum RPM point, fuel supply, aspiration, air inlet and engine temperature and more. This leads into gearboxes and variable speed propellors and ... .

From there you go to how the power is converted to oppose the power generated by aerodynamic (and other) forces.
A propellor has it's own power vs RPM curve - and will have optimum conditions - air density, pitch, profile, ... .

The aircraft generates drag (and lift which is best stayed away from at this level :-) ). This will be frontal, generated by laminar flow & vortices at various places and ... .

Between power in and thrust there tends to be a lot of " ... " :-).

Overall the engines produces power which is used to develop the aerodynamic power required to overcome the drag forces (and ... :-) ).

A jet engine is more suited to being described by thrust as it is more fundamental to the description of the systems operation - whether pure jet / ducted / high-low bypass / ( ...) the engine tends to be more likely to be considered as a unit that generates thrust. (I've often seen thrust figues for jets but power figures are less commonly used.) Power for a jet is somewhat meaningless for a jet except at specified velocity.
Power = force x distance / time. Without the time component a jet delivers no power. An eg Saturn V Moon rocket, at the point of launch is developing maximum thrust BUT zero power as it is not moving. But not for long. Zero power at takeoff is true for all stationary vehicles - including rockets, Tesla's and Usain Bolt.

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Velocity is constant when Thrust = Drag

The purpose of any propulsion system is to produce thrust.

why talk of power with props?

This is a "left-over" from a time when early engineers, such as the Wright Brothers, put a propeller$^1$ on to any engine available to attempt flight. Engines are rated in "horse power".

Power is Force × distance/time

With the aviation propeller application, the power of the engine is absorbed by the drag torque of the prop: drag force x the distance from the center of the prop shaft$^2$

This definition can lead to some very embarrassing observations because:

Thrust is a function of power and propeller efficiency

A 2000 Horsepower engine can be churning away without producing any thrust if the propeller efficiency is 0.

The full equation for propeller thrust is therefor:

Thrust is proportional to (Prop drag torque) × prop efficiency

Since "power required" = "prop drag torque" ("power" is directly translatable to "throttle setting"), the usage has stuck, often as RPMs or % Power, but this only refers to power into turning the prop. Thrust output efficiency will vary with speed, especially for fixed pitch props.

variable pitch props have a far greater speed range of efficient thrust output until...

  1. Pitch to maintin optimal prop AoA to the relative wind becomes too great

  2. Incoming air velocity into the prop disk gets closer and closer to the exit velocity

This is why prop driven planes max out at around 400 knots, even with huge power plants and variable pitch props.

so how do you know how much thrust a propeller is producing?

By calculating the angle of dive in a glide for a given airspeed and multiplying the sine of this angle × the airplane weight.

This is now the thrust force required for level flight (for that particular aircraft).

If you want to know your thrust at full power and a lower airspeed, add sine angle of climb to your value.

Doing this, and plotting data points, can give some very good thrust data for a given aircraft/engine/prop combination.

and, of course, one can try various props for climbing or cruising needs.

$^1$ it should be noted that the Wright Brothers did make considerable progress in improving propeller efficiency.

$^2$ this can be 1:1 with engine crankshaft or it can be geared

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