RS2K

My Feedback: (8)

Here's what I got:

An 18 X 10 APC at 8000 puts out about 28.22 Lbs of static thrust.

An 18 X 16 APC at 8000 puts out about 28.22 Lbs of static thrust.

Only difference is the speed and HP.

This doesn't make any sence to me. The way a helicopter blade makes more lift/thrust is by increasing or decreasing the pitch. It only seems logical to me that an 18X10 would put out MUCH less thrust than an 18X16 at the same RPMs. This makes me wonder how accurate thrust HP is. Please clarify.

Thanks.

Spaceman Spiff

My Feedback: (1)

Just a guess, but i doubt either of those are even close to real world.

mtthomps

My Feedback: (2)

You are misunderstanding the meaning of "static" thrust. Static thrust is that which is produced when the engine is held motionless on the ground. The reason why the 18x16 prop has a lower value is that the pitch (prop angle of attack) is not the most efficient in a static configuration. Once the airplane is moving, and there is an airstream moving past the prop (no longer static), the angle of attack of the prop blade will be less than a 16 pitch. At a certain optimum angle of attack, the prop will generate the most forward lift (thrust). This optimum angle of attack is reached at a higher airspeed for higher pitched props, because they start from a higher angle statically. This angle of attack is reached at a much lower speed for lower pitched props. The optimum angle of attack will be at a different forward airspeed for the 16 pitch prop as opposed to the 10 pitch prop. Once the optimum angle of attack is reached for either prop, at whatever forward airspeed that is, they will produce the exact same amount of thrust, being that they are both 18 inches in diameter. Props can be thought of as gears in a manual transmission. If you want a lot of thrust at a low speed , but no thrust at a high speed, (1st gear), use a lower starting pitch and a higher diameter. If you want the thrust to peak at a higher speed, (3rd gear) use a lower diameter, higher pitch prop.

RS2K

My Feedback: (8)

I understand what static thrust is, but a heli gets MORE static thrust when you increase pitch while keeping RPMS and blade leanth the same, that's why it doesn't make sence to me.

With a Funfly/light 3D plane you want maximum static thrust.... What happenes when you get to 50 MPH on 20X6 at 6300 RPM? Thrust HP shows maximum speed as 34 MPH at that RPM, that seems pretty slow, though.

Rudeboy

I use that applet as a pitch speed calculator... nothing else... I can do it by hand, but this is faster.

I put in numbers once for an APC 16x8 and a Master Airscrew 16x8 at 9000 rpm... the program always predicts higher thrust from the an APC because they are more efficient, yet in reality the Master Airscrew gives a lot more thrust than the APC, probably because of the difference in blade width and tip pitch...
A Master Airscrew puts a much heavier load on an engine compared to an APC of the same size... (in reality, not in Thrust HP)

A program this simple to compute something this complicated and complex is crap IMO...

And I don't understand why engine manufactures with supposedly good reputations do hp and thrust claims, backed up by this program...

RS2K

My Feedback: (8)

It's all in the numbers... To make the numbers look good.

mtthomps

My Feedback: (2)

I also do not think that thrust hp is that close to the real world. In your example, you will never reach 50mph on a 20x6 at 6300 rpm. The effective angle of attack will have reached zero and actually be negative (pushing) after going over 34mph. That is what thrustHP is telling you with a max speed. I did a small calculation and determined that after such a prop, spinning at that speed, reached 35.795 mph, it would be producing zero thrust. In the real world, however, the prop would encounter less drag the closer it got to this speed, so the rpm will climb. The top theoretical speed with the rpm increase would be somewhat higher.

JimTrainor

Read ThrustHP's help documentation (included with the program). It has a comment about pitch. The forumula's it uses were published in AMA magazine in the 80's.

I've been using the spreadsheet below recently, it seems to be more reasonable, and the thrust estimate varies with pitch.
http://home.wanadoo.nl/pereivers/Goodies.html
Download the "prop-power.xls" spread sheet.