Physics of prop flight

[jerkins]

I was thinking, why is it that a planes reaches a top speed, what is the limiting factor? 

It comes down to drag from air resistance, when the thrust force and the drag force are equal and opposite, the plane must stop accelerating; therefore, it has reached its top speed. 

But does the thrust force the plane is capable of decrease with speed?  I know this must vary depending on the prop, but in general.

If the maximum thrust force does in fact decreas with speed, I picture it like this.  If you plotted thrust versus speed, you would end up with a line that starts high (speed 0) and it would slowly decrease to some number (at high speed) maybe even theortically hitting 0.

Now plot the drag force with speed and I picture the force starting at 0 (speed 0) and increasing with speed, theoretically this could reach infinity.

So no if you were to plot both the thrust vs speed and the drag vs speed on the same graph, the point where the lines intersected would be the maximum speed achievable in level flight.

Is this correct, can any enlighten me?

Does the amount of thrust possible decrease with the speed of the plane? (not talking about rpm or engine related, I mean linear motion of the plane)

p.s. Obviously I didnt consider altitude air density fluctuations, but if you are in level flight (constant altitude) they would not have an affect.

[Karnak]

Yes, props become less efficient as speed increases, reducing the amount of thrust produced.  As speed increases parasitic drag also increases.  Top speed is reached when thrust and parasitic drag reach equilibrium.

[Badboy]

I was thinking, why is it that a planes reaches a top speed, what is the limiting factor? 

It comes down to drag from air resistance, when the thrust force and the drag force are equal and opposite, the plane must stop accelerating; therefore, it has reached its top speed. 

But does the thrust force the plane is capable of decrease with speed?  I know this must vary depending on the prop, but in general.

Yes, in general thrust is higher at low speed and decreases as speed increases and is inversely proportional to speed over most of the envelope. The amount of thrust in lbs is given by this relationship, Thrust = 375 * Prop_Efficiency * Brake_Horse_Power / Velocity.  That relationship is good for the range of speed we are interested in, but it breaks down at low speed.

If the maximum thrust force does in fact decreas with speed, I picture it like this.  If you plotted thrust versus speed, you would end up with a line that starts high (speed 0) and it would slowly decrease to some number (at high speed) maybe even theortically hitting 0.

Now plot the drag force with speed and I picture the force starting at 0 (speed 0) and increasing with speed, theoretically this could reach infinity.

So no if you were to plot both the thrust vs speed and the drag vs speed on the same graph, the point where the lines intersected would be the maximum speed achievable in level flight.

Is this correct, can any enlighten me?

Yep you basically have the right idea, but with Prop aircraft it isn't normal to plot thrust and drag directly, we use something called a Penaud diagram like the one shown below. Notice what happens to the shape of the propulsion curve at low speed, as I mentioned earlier.   

Does the amount of thrust possible decrease with the speed of the plane?

Yep, providing we are only talking about the typical WWII prop fighters.

Hope that helps...

Badboy

[Yossarian]

www.av8n.com/how

This site is superb.

<S>

Yossarian

[jerkins]

Thanks for that great info BadBoy.  As a matter of fact that is exacly how i pictured the propulsion curve to look.  Only difference is that in your figure, the thrust is nomalized by the weight of the aircraft in order to put mutliple planes on the same playing field so to speak.

Also I am interested in the thrust equation you mention.  I will look into it further.

Thanks yossarian for the site, I havent looked at it much yet, but I will.

[goober69]

interesting stuff guys, my question that i have is, what was it called that they did to the props to make them more efficent, wasnt it that they had variable angles of attack and a govener on the rpm? what were the benifits of this design and when was it first implemented in american aircraft?

and also what were the posible outcomes of a prop running away ungoverned? would it just bind the engine up or break the driveshaft?  either way very bad day.

[Badboy]

interesting stuff guys, my question that i have is, what was it called that they did to the props to make them more efficent, wasnt it that they had variable angles of attack and a govener on the rpm? what were the benifits of this design and when was it first implemented in american aircraft?

Check out this link for further information:

Development of variable pitch propellers

Badboy

[CAP1]

DAMN........

look through the links

there's some pretty informative stuff there.

[goober69]

Check out this link for further information:

Development of variable pitch propellers

Badboy

wow thats great stuff.  a little out of date, as far as i know the new F22 raptor encorporates fully swingable wings from 16 degrees to 72 degrees. it s a sweet bird.  i was looking at varibable wing technology (seems these articles were publised 2003 ish)