There has been a lot of discussion lately about prop thrust and how
it affects aircraft performance so I started a dissertation on
the subject.

It all has to do with the definition of HP.
In theory one HP could move a mountain.
Rather surprising result I would think.

The definition of HP is force times a speed.
Namely 550 pounds moving at 1 foot per second or 1100 pounds
at .5 FPS or 2200 pounds at .25 FPS or 5500 pounds at .1 FPS
or 55,000 pounds at .01 feet per second. You get the picture.

So if it is going slow enough the force could approach infinity.
The same thing applies to thrust.

In theory a 200 HP aircraft engine could put out tens of thousands
of pounds of thrust at 1 MPH. This would in fact happen
if we were feeding the engine HP into a gear box and used a super large
prop. In other words a hovering helicopter.


What happens in aircraft application is poor slow speed prop efficiency
rears its ugly head. More about that shortly.

I'll let you think about this while I work on refining this spread sheet.
The graph shows THEORETICAL thrust as a function of MPH for a 200 HP engine.
The blue curve is feet per second. The red curve is theoretical thrust.



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Part II

Propeller efficiency is characterized by a term called advance ratio.
http://selair.selkirk.bc.ca/aerodynamics1/Performance/Page8.html

Simplifying that so mere mortals can understand it is the ratio of what
the prop would do if screwed into wood instead of air.

A high pitch prop will screw into wood faster than a low pitch
prop at the same prop RPM.

In air, prop efficiency depends on how fast the airplane is moving. At static
speed or zero speed the airplane is not moving and the prop is slipping
100%. Theoretically the prop efficiency is zero despite the fact there is
a blast of air and thrust is being developed. We will ignore that for the
moment as it has no real interest for prop performance at speed.

In the real world prop RPM is constant more or less. Static RPM is only
a few hundred RPM less than cruise RPM so for all practical purposes,
and for the purpose of understanding what is going on, RPM can be considered
constant at 2500 RPM.

Let's just say we are choosing a prop that gives maximum efficiency at
a chosen cruise speed of 160 MPH. At lower than cruise speeds the eff. will fall off
and at speeds higher than cruise speeds the prop eff. will also fall of.

Here is a simple chart of the matter.

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Part III

I calculated the net thrust with the given fixed pitch  prop eff. curve
and plotted it. The light greenish blue line is the net thrust given
the prop eff..

Interesting. Rather unexpected as the net thrust is constant in the lower
speed ranges and begins to drop off at 150 MPH as pretty much expected.

The next step is plot the drag of the airplane and see where the drag
intersects the net thrust curve.

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Part IV

This is what the situation looks like with the drag curve added to
the chart. The top speed is the intersection of the drag
and the thrust curves. I choose a flat plate equivalent area
of two. You can change this to fit the estimated drag factor of your
airplane. (column F).

What is "flat plate equivalent area"? That is the estimated
drag factor times the frontal area of your airplane. It is published
for many experimental designs. Drag is MPH squared times the density
constant of .0026 times the flat plate equivalent area.

What is the "frontal area"? The frontal area is the area of a shadow
you get if you shined a spot light from some distance at your airplane parked
in front of a wall. It is not how pointy your airplane is. It is
the maximum cross sectional area of your airplane. Larger cabins
mean larger frontal areas.

If you change the pitch of your fixed pitch prop you will have to redo (column D)...
the prop eff. curve as a function of MPH. More pitch moves the curve break to the
right increasing the top speed slightly and less pitch moves the break curve
to the left improving the climb.

If you add or subtract HP you will also have to redo (column C).

If you study this spread sheet and learn how to us it you will gain a lot better
understanding of how to choose a prop and what you should be working on to improve
the speed and MPG performance of your aircraft.