You're welcome.

As to the pitch, the old wood props have a flat airfoil so it is a lifting section and the actual pitch is several degrees higher at each station of a pitch gauge. That being said, no decent model prop is the same pitch (so called "True Pitch") along the entire blade. That overly simplistic design just doesn't perform as well for a variety of reasons. More modern designs like the APC can use much better airfoils, tailored for the requirements of the position in the blade. An idea of the actual pitch might be closer to the mold parting lines. At the hub, the requirement is for strength and low drag, while further out the airfoil can be set up for the actual speed of the blade which changes rather a lot over the length of the blade. It would be quite expensive to do this degree of machining on each wood prop, but for a mold the time used in the mill is amortized over many thousands of props.


Rev-up's were usually slightly lower pitch than some of the competitors props. Machin knew that to get more power out of the engines, you needed more RPM. He kind of got started down that path (under pitch) because of Johnson combat engines were a little less torquey than some of the others back in the day. He also knew that people would not put a smaller prop on the engine to get it into the power band because of herd psychology, so he under pitched some props and put them into the field to great success.

Funny thing though, he once told me if I ever got the chance to bet on a pulling contest between two airplanes, just put the biggest diameter, lowest pitch prop on so I could drag the other airplane all over the field. This was decades before 3D, but I said he was a free flight guy which as he put it 'was just a vertical drag race'. He wanted to max out even if the wing fell off because the airplane would be so high.

Scientific Wild A** Guess. SWAG

They decide by whatever they believe in, what pitch number they feel like stamping on their products. I doubt any two use the same basis.

After some years of reading and recording the pitch values of props from different mfgs taken using pitch gauges, it became very clear there was no universally applied standard. I added that realization to what I found from testing props over the years.

Sorry Alisdair, I don't buy it. In 'a slight dive' T does not equal 0. T + gravity = total drag. So long as that prop is turning, then it is producing some thrust. Darts and 748's? Grew up on them, and F27's. Only from the ground perspective though. Again, with 'ground/flight idle' thing and single engine flight, so long as positive pitch is on that prop, there will be thrust. I think what you meant to say was that at some RPM and pitch, the disc drag of the prop was zero. The prop might not be adding any extra T to the airplane, but there will be some. Perhaps thats what the dive thing was meant to show, at some speed, with gravity assistance, the prop is neither providing all the T, nor is the prop disc providing any drag. That is not anything like T = 0. Again, I am unschooled, and might be wrong...
Evan, WB #12.

I get it. At the speed that the airplane reaches in its dive where prop thrust and disc drag are balanced, the crankshaft will feel no forward pull. Hmm, would have to be a pretty steep dive....
Evan, WB #12.

Actually there has been testing in GA aircraft doing Zero Thrust Glide testing. ZTG gives fairly accurate drag information at different airspeeds because the weight of the aircraft and descent can be easily documented. Here is an article on how it is done.

http://home.cogeco.ca/~n17hh/Models/...cleMarch95.pdf

Gentlemen, to throw a wrench into the discussion..........how about in a full size military cargo plane such as the C5B Galaxy the total maximum thrust from all 4 engines is 172,000 lbs. of thrust and the maximum take-off weight is 840,000 lbs. for a thrust to weight ratio of 20%..........or 1 lb. of thrust for every 5 lbs. of airplane. The takeoff roll though is 8,000 feet.....but hey they climb to a cruise altitude of 30,000 feet. Our RC airplanes for our yank and bank kind of flying really requires the high thrust to weight ratio that our model engines/motors provide with our balsa and foam airplanes. Never mind that in full scale airplanes...........they seem to be at first glance "underpowered"...........but hey they fly........but not in our model plane high energy loops and climbs. 'Nuff said. Larry.

That is not anything like T = 0.
disc drag

Pimmnz, You would need to explain what you mean by that, or better still take some more schoolin and learn what the rest of us mean.

Thrust is the force exerted by the air on the prop, in line with the propshaft. It depends on airspeed. At zero airspeedT is at a maximum. As airspeed increases the angle of attack of the blades reduces and so Thrust reduces. And as airspeed increases, Thrust just keeps on reducing (for an aeroplane in level flight) until Thrust equals Drag. In the diagram attached T is the thrust of the prop, D is the total drag of the airframe. The airspeed at which they cross is the maximum speed at which the aeroplane can fly in steadylevel flight. It won't go any faster, unless you dive. In a slight dive most prop planes will pick up some speed, the Thrust will reduce but now gravity is helping maintain the speed.
Gravity is powerful stuff, and if you use quite a lot of it (yes, quite a steep dive) the Thrust can go right down the curve and even below the line to a negative value. That is when we would call it Drag. It is additional to the airframe drag, but it is holding the plane back sonegative Thrust IS an addition toDrag.
This is not a regular part of flying, it is a bit extreme and probably beyond Vne. But it is part of what is possible, theoretically.

I should mention that a very coarse prop will have its blades stalled at low airspeed. At low airspeed the thrust will be poor, the purple line in the diagram, but once the blades are working properly with a bit of airspeed through them they will keep thrusting to a higher airspeed and produce a higher top speed. The only problem is taking off (and overshoot) and that's why variable pitch props were invented.

Disc drag = drag of the prop (Think FF rubber, windmilling on glide, vs stationary prop, vs folded etc) at some point the prop will be causing drag as the airframe speed 'catches up' to the prop. The point where the prop shaft feels no 'pull' is where the prop disc thrust and drag are equal. As you said, per the single engine training on the 748, 'a bit of extra idle so there is no drag from the prop'. Just balancing the prop disc drag with a bit of thrust, so, being pedantic, there is still thrust being developed by the prop, it's just not being transmitted to the airframe.
Evan.

Being pedantic in turn,
If it is not a force transmitted to the airframe, it is not thrust.

Ok, this has been a great discussion, but Stuffer, how did it fly? I for one am curious as to what actually worked or did not work....