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## PPL - Topic 14 - Propeller

• issaccph
• Topic Author

### issaccph created the topic: PPL - Topic 14 - Propeller

Hi,

I have a question while studying Topic 14 - Propeller in the PPL book.

Why doesn't propeller thrust act perpendicular to the airflow?

Thanks,
Issac

• Carello

### Carello replied the topic: PPL - Topic 14 - Propeller

"Why doesn't propeller thrust act perpendicular to the airflow?"
The simple answer is; because Thrust is not Lift.

When talking about a wing, it makes sense to talk about the forces acting perpendicular to the RAF (lift), and the forces acting parallel to the RAF (drag).

That being said, a propeller is a little different. To start with; it's not a wing. We can still talk about propeller lift and drag, but it makes more sense to think in terms of Thrust and Torque. The "Total Reaction Force" can be resolved into any two components we like. The choice of components will depend on what we are looking for.

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• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 876

### John.Heddles replied the topic: PPL - Topic 14 - Propeller

I'd take a slightly different perspective (but still much the same story) to that of my colleague, Carello.

If we have some fluid (in this case, air) flowing past a surface there will be pressures exerted on the surface and, as a consequence, the surface structure experiences and can exert a force.

It is conventional to consider the net force to be acting perpendicular to the direction of the fluid flow.

Usually, this force is not of great convenience for our desire to run sums to figure out what is going on and to produce useful data for engineering things. So, we resolve (schoolsciencedotcom.wordpress.com/2017/1...gle%20law.%20More%20) the net force into orthogonal (mutually at right angles) directions where those two directions are of more use to us for running sums. Just what directions we might choose depend on what we are trying to do.

Now, a propeller blade is just a (fairly specialised) wing (but, nonetheless, just a wing). The reason the blade looks a tad strange is due to the severe radial twist to accommodate the significant variation in tangential speed (velocity) as we move out radially from the hub to the tip. If, however, you were to untwist the blade it would look very much like a wing.

Both the usual wing and the propeller blade "wing" have fluid moving over them, creating pressure differences and, hence, forces.

The net forces are considered to act perpendicular to the local flow direction and, for convenience, we postulate an overall net force (call it whatever you like - total reaction, whatever) which sums all the local forces and gives us some data with which we can do engineering stuff.

We then, by convention, resolve this net force into convenient (orthogonal) directions so we can do useful sums. For the wing attached to the fuselage, those directions, generally, are vertical and horizontal referenced to the aircraft and, for the propeller blade "wing", generally fore and aft with respect to the crankshaft and in the plane of the propeller disc. No real difference, here, at all.

So the net force of each of these wings acts perpendicular to the local airflow which is the crux of your question.

When we resolve the net force into two orthogonal and useful directions we get two component forces.

For the fuselage wing, we talk of lift and drag, for the propeller blade wing, we talk of thrust and torque. Much the same thing, I suggest.

Engineering specialist in aircraft performance and weight control.