How do props work? For example...what would be the difference in a 12x6 prop vs an 11x7 prop. I understand the diameter and pitch...but what difference would there be in performance between the 2?

the 12x6 would have more down line breaking and more pulling power over the 11x7 which would have less breaking on the down line and faster speed in most applications.

kc

Choose the 12x6 if you want a climbing prop. Short takeoffs and the ability to go verticle.

Choose the 11x7 if you want a high top speed.

3-D planes may choose something like a 13x4 for hovering and the ability to accelerate out of a hover. Top speed will be limited just like a car that is stuck in low gear. These props also brake well when the throttle is closed. They brake so well that you often have to land with the throttle open a little so the plane doesn't just stall and bounce on the runway.

200 mph pylon racing planes might use something like a 7x9, 8x9, or 8x10 or something close to that based on a bunch of eperiments. These guys often carve their own props.

most use an 8.8x8.75 but they aren't 200mph, those rumors seam to fly all over the forums 160-170 is ballpark for the Q40 and quickie 500s. the only 200mph quoted planes that actually were able to fly that through the trap were turbine driven jets, all those pylon racers and delta wings seam to loose roughly 40mph when they fly the trap for some strange reason

kc

I'll take your word for it, still, thats a pretty long pitch for a prop of such a small diameter.

Don't take my word for it when a pic is worth a thousand words

FWIW i use this on my Jett 60LX turning it at 21 grand on a Diamond Dust, i also use the 8.75x9w at 19 grand, all the 'Pylon' series APC props have a D hub which is much thicker then the regular 'sport' APC props so they handle the high RPM without slinging a blade.

kc


EDIT: found a package for the 8.75x9w and added the pic

My only experience in pylon was sitting in the cage and calling cuts for Quicky 500, Q-40, and F-1 classes. Most of the F-1 flyers had hand carved wooden props small enough to let the engines turn 24-28K. Those diameters and pitches I quoted were ball-park guesses to make the point about how a prop's dia/pitch is determined by the airplane's task.
F-1 is no longer an AMA class.

You are probably right about the speeds, if they really went 200 mph, they would finish the 2.5 mile race in 45 seconds. Typical winning times are in the 1:04 range.

I think they did away with the cages too. Now they call cuts from the sidelines.

Perhaps the easiest way to put it is the way I understand it.
The second number in the prop size is pitch. Theoretically, the plane will travel that many inches per rotation of the prop. So say if I've got a 10x6 prop, the plane will travel 6 inches per rotation. Of course other factors come into play since your plane is not and never can be aerodynamically perfect.
The length of the blades determines how fast the prop will rotate. Longer blades also have more surface area, and thus more pulling power. That's why the 12x6 blade that BLE mentioned above has more hanging power than the shorter 11x7.

Here's a handy speed calculator I found. Measure the maximum rpm with a tach and then plug it in, along with the prop pitch and it will tell you the maximum theoretical speed (in either MPH or KM/H) that your plane could travel.
http://www.missionwings.com/airspeed.asp

the problem with theories are they seldom hold true in real world applications. I've had several planes that would go faster then the theoretical max speed the prop would pull it doing the pitch x rpm equation, and that was even using the true in flight RPM.

kc

Model Airplane News published the results of some measured true airspeeds and in-flight rpm in a model airplane. They found that the plane's speed exeeded the theoretical pitch speed by a significant percentage. The reason is that a props pitch is usually defined by the angle a straightedge would lay on the back surface of the prop blade. Flat bottom airfoils create lift even at a few degrees of negative angle of attack.
If the pitch of a prop was determined by the airfoil's zero-lift angle of attack, then your true airspeed would always be lower than the pitch speed.

Another surprising finding was that the highest engine load did not occur while static on the ground but at low flying speeds like the top of loops. As your plane accelerates during takeoff, the prop first loads the engine more as the blades "hook up" because they are no longer in a partial stall and then as the speed increases more, the prop unloads as the air speed catches up with the prop's pitch speed.

What would the end result be in engine rpm ? If you ran a 12x6 and changed to a 11x7 would rpm stay the same ? Have a lt-40 with OS .40 with 10x4 prop running 12700. What prop should I change to to increase rpm's or maximize power. Spec's say the motor should run up to 15,000 rpm.
Thanks in advance.

a 10x4 seems a little small for a .40, im running a 10x6 on my MDS .40 and it sounds very scale (i.e. not a screamer) and has a surprising amount of pull, it will make your landings a bit faster though, but thats fairly easy to counter by cutting the throttle a little earlier

No, the 11x7 would be less engine load and that would allow the engine to produce more RPM.

on the prop selection for your engine, there is only one way to know what works best with your engine/plane setup and that is real world testing.

kc