chall

I've just bought a used plane. It came with an .35 size engine and a 10x4 prop. The plane manual suggests an .20 to .30 engine (glow), and the engine manual specifies a 10x5 prop for break-in and 10x6 thereafter.

The plane flies fine, but I feel like I should put the specified prop on it, a 10x6. What do you think will happen?

The previous owner likened using a low pitch prop to having a low gear on your car gearbox, but the more I think about it the more I think this analogy is bogus. It only helps a car to have a gearbox so it can run at a high RPM without stalling. I can run at high RPM regardless of the prop.

If using a 10x4 lets the plane fly slower, than I'll keep it. If it's just making the engine run hotter or something, I'd like to change it.

Any ideas?

Thanks!

Charles.

aerowoof

the previous owner is correct.the 4 means the pro will move 4"every full revolution a 6 means 6" every revolution.with the 10-4 you have less speed but mor thrust thrust = vertical performance 10-6 gives you speed but less thrust.looks like he was trying to get the plane to fly in the speed envelope of the recommended engine with out putting extra stress on the airframe.you could use the 10-6 on the .35 if you know how well built the plane is.nothing worse than folding a wing from increased g's from the extra speed in a dive when trying to pull out

flyn_colorado

Excellent Explanation!

Jimmbbo

Charles,

aerowoof hit it right on the nose...

Just as you wouldn't try to start off from a stop sign in 4th gear, and you wouldn't try to go 55 in 1st, each regime of flight in a propeller driven airplane has an optimum propeller pitch. With the prop on your model, you start with the manufacturer's recommended prop range, experiment and find the one that best suits what you want to do with the airplane, knowing that you will compromise optimum performance in some other flight regime.

FYI, in FS airplanes, propellers are available in a range of pitch settings, similar to those for models. Your 4" pitch would be called a climb prop, which allows the engine to develop more RPM and more thrust for takeoff and climb at the expense of cruise performacne. The 6" pitch prop would be called a cruise prop, where less thrust is required and since cruise speed is faster than climb, the bigger bite of the prop is better suited for cruise, at the expense of takeoff and climb performance.

To eliminate those inefficiencies, engineers developed the adjustable pitch propeller. In such a design, the propeller is set to the lower pitch (4 in your version) to develop maximum thrust for takeoff and climb. When in cruise flight, the pilot reduces the throttle and adjusts the propeller to a coarser pitch which slows the engine RPM, getting a bigger bite, which provides more efficient cruise.

Cheers!

Jim

ChuckW

All of the explanations are right on the money. A low gear ratio in a vehicle is great for quick acceleration & climbing/pulling ability and a high gear ratio is good for top-end speed. Props are the same way. A low pitched prop will often get the plane in the air quicker and climb better but will lack top-end. The higher pitched prop may require a little more take-off role but will have a higher top-speed. It's best to just experiment with various props and find one that allows the engine to make good RPM and makes the plane behave the way you like to fly. Another advantage of a low pitch prop is that they act as sort of an air brake which can help reduce speed in dives. If you go with very low pitch, it's sometimes a good idea to go up in diameter too, this provides maximum thrust. For instance, I'm finishing up a Morris Profile Hots with a Super Tigre .45 2-stroke. I want the plane to hover well so I'm using a low pitch prop with a large diameter, a 12.25x3.75. I wouldn't think of using anything larger than a 10 or 11" if the pitch were higher; the engine would have a harder time turning it if I did.

Red B.

A large diameter, low pitch prop is good for take off, climbing and 3D-flight (hovering etc.).
By increasing the pitch, and probably reducing the diameter in order to let the engine operate in the proper RPM range, you will trade sustained climbing performance for speed. This is what others have already stated. What has not been written is that the choice of propeller has other effects on the behavior of the aircraft.

My daily "hack" is a SIG Somethin Extra (SSE) with a .46 engine.
With an APC 12.25x3.75 propeller I have good static thrust and the plane will hover and climb vertically "out-of-sight".
For traditional aerobatics (loops, rolls, cuban eights, knife edge...) I have found this propeller less than satisfactory. Big maneuvers are performed at too low airspeed, making it difficult to perform the maneuvers accurately. At the low airspeed the SSE operates at large angle of attacks, making it sensitive to wind gusts and requiring large control inputs and corrections.
By substituting an APC 11.5x6 propeller the SSE is transformed. Airspeed is increased significantly, and I am no longer able to hold it in a sustained hover. On the plus side however, it becomes much easier to perform traditional aerobatics. Maneuvers are performed at higher speeds, and by trading entrance speed for height I find that it is possible to perform maneuvers that are at least as big as with the 12.25x3.75 propeller, but with much less control inputs, less corrections and less sensitivity to wind gusts. In short, I look like a better aerobatic pilot when I switch from the 12.25x3.75 to the 11.5x6 propeller.

I don't use either of the propellers exclusively, but rather switch between the two depending on what kind of flight I want to perform.

My point is that often pays off to experiment with different propellers. It has a lot of influence on the behavior of the aircraft.

/Red B.

ChuckW

Good point. I always flew my Somthin Extra with an 11x5 and occasionally an 11x6 with a 2-stroke .45. I also have a Hanger-9 Twist with a 4-stroke .70. With a 12x6, it is fairly fast, stable and does nice maneuvers. With a 13x4, it takes off almost instantly and climbs well but is a little less stable. This is fine for trying to hover and to fly around lazy but not ideal for other stuff.