James Watt came up with the Horse Power measurement in order to sell his steam engine to mine owners who used small horses to pull mine carts full of coal up hill on narrow gauge rail tracks.
So, Horse Power is a measure of WORK.
The Watt used to measure the consumption in electrical devices, is the same James Watt.
So, Horse Power can be converted to, or expressed in Watts.
So, one (small) horse could lift 33,000 pounds of coal 1 foot in one minute.
So, one “Horse Power” is then 33,000 pounds lifted one foot in one minute.
So you need a time (one MINUTE) a distance (one FOOT) and a FORCE to lift (33,000) pounds.
A single cylinder 2 cycle diesel with a stroke of two feet (one foot lever to make calculations easy)
and a piston the size of a picnic table might be able to do this at one RPM, so, it would be a true
one Horse Power engine. It would weigh maybe 7,000 pounds in cast iron. A long way to go for one HP.
However through the magic of mathematics (see Ed Anderson) we can build more useful engines, and just measure their effectiveness in James Watt's Horse Power.
By juggling bits of formula around we can come up with one even I can figure out.
One Horse Power is equal to Torque, a twisting (FORCE) At the end of a (ONE FOOT) lever times
Revolutions per (MINUTE). Revolutions gives you the distance, because the torque is at the end of a one foot long lever, or one revolution is then one foot times Pi. D (the diameter of a circle with a one foot radius) or two times Pi. Or 6.26 feet. So we divide the outcome by 5252 (a constant because the little horse cheated by dragging the load up a long hill not actually lifting it. So a bit of a fudge here ) and we have all three items needed to replicate the output of James Watts little horse or his steam engine.
Instead of steam to push our pistons or rotors about we use a vapor of gasoline in little explosions.
How much vapor determines the FORCE of the explosion. So more vapor per explosion means
more FORCE against the piston or rotor, and that means more torque. And that means more HP.
If we move the end of that one foot lever further in one MINUTE, that means more HP. (WORK)
So more work can be had by increasing the TORQUE and by increasing the RPM, or both.
In a supercharged or turbo charged engine we just jam in more vapor to generate higher TORQUE per REVOLUTION and we have instant success with more HP (WORK).
We can increase the RPM which is the DISTANCE we push the end of that one foot long lever in one
MINUTE, and again we have instant success with more HP (WORK)
The best (for high HP) induction system for the rotary is the Periphery Port.
The factory made rotor housings for this application, but they are ridiculously expensive, so many folks manufacture their own Periphery port housings.
The object is to maintain high velocity flow, so as the RPM goes higher, and the time available for the
vapor to flow into the passing low pressure void created by the rotor move away from TDC becomes shorter.
A shorter time at any flow rate means a smaller volume per unit of time, of vapor will be inducted.
Turns and other shapes impinging in an induction system cause energy to be lost. The lost energy appears as increased heat, and lower velocity. The stock or Side Port engines have a turn in the cast iron just before the vapor enters the engine. So that turn reduces velocity right where we want the highest possible velocity. In addition, the ports in a side port engine are actually closed off by the side of the passing rotor, so, for any RPM, the Periphery port engine has a massive advantage over the side port engine, because the Periphery port is NEVER closed.
Porting a side port engine is an attempt to increase the time the port is open at any RPM, so as to induct
more vapor per revolution. Longer time=more vapor
This increase of the port opening in three directions also reduces velocity, so at lower RPM it reduces the amount of vapor inducted. So, ported side port engines should have reduced low RPM performance.
And they do.
The later closing intake port extends the closing point well past Bottom Dead Center, where the volume of the working chamber is getting smaller, and the pressure is going up. At low speeds part of the vapor in the chamber will be pushed back into the intake. At high speeds the mass and velocity (Over 350 miles per hour) of the intake vapor can overcome this pressure and more vapor will enter the chamber before the port closes.
But we want best power from about 5,000 RPM on up, so for most applications the low RPM poor performance is not a factor.
The STREET PORT is just an increase in open point and a delay in closing point. A typical closing point for a stock engine might be 50 degrees After Bottom Dead Center, and a in a street ported engine it might be 60 to 65 degrees ABDC. Better top end power, and some loss in low RPM power. It is called a Street Port because it is mild enough to use in a street driven auto.
A big street port might move the open line so far as to un-support the trailing end of the side seal.
The closing line might be moved to 70 degrees ABDC.
Much improved top end power. Obvious loss in low RPM power.
A small Bridge Port might be of some value in aircraft use if kept above 5,500 RPM.
A “J” bridge port would be of no value in aircraft use, because it has no power below 5,000 RPM
And without a controllable pitch prop, might not even be able to pull a prop load into a usable RPM.
Converting Horse Power into thrust with a propeller is best explained by Mr. Anderson.
However I have some thoughts.
When reved up from idle the prop is facing a mass of air that is barely moving, so the fixed pitch propellers effective pitch is close to advertised. The prop tips may be slightly stalled.
As the mass of air in front of the propeller starts to move through the propeller, the effective pitch
Or, the pitch relative to the moving air mass is reduced, and that un-stalls the tips, and that allows the tips or outer part of the prop disc to become more effective at moving air and that is WORK, and the added work load slows the engine slightly.
As the aircraft accelerates, the effective pitch relative to the air stream is reduced, and this unloads the engine slightly, and the engine RPM increases slightly. Once propeller thrust is equal to total drag
engine RPM will remain constant.
You can only go faster if you increase RPM in order to increase thrust. Or, increase relative pitch to increase thrust. Or, both.
Advertised pitch, is that blade angle relative to the center-line of the crank shaft. Effective pitch is the angle of the blade relative to the blade,s path through the airstream.
Propeller blades and wings are the same thing. So, they both operate in relative wind.
Thus the controllable pitch propeller is the gold standard and allows the engine to be used at its best power RPM for any airspeed.
The fixed pitch propeller is limited to a small range of RPM, that only suits the engine's best power in one place along that RPM range.
The same as driving your car all day in 2nd gear (for a fixed pitch prop) compared to driving all day with an automatic transmission that keeps changing ratios based on load and throttle setting. (For a variable speed, or controllable pitch propeller).
Thrust (FORCE) from the propeller (MASS times VELOCITY) is the WORK done by the engine.
Or, I could be completely wrong.
Lynn E. Hanover