Hello all...

I'm still new to all this. I need some help understanding propeller dimensions. I hear a lot of different terms tossed around. So could someone explain the effects of changing props.

For example. If I have an 11x6 prop. What are the effects going to an 11x7 prop?

Then, again starting with the 11x6, what would be the effects going to a 12x6?

If all things other things stay the same, what happens when only that 1 thing is changed. What will the impact on the engine? What will be the impact on the flying? Will it it gain in top speed? Or gain in acceleration? What will be gained? What will be lost?

Anyway... Thanks....


Lyndon

For example, take a 10x6. The first number is the diameter (10). The second is the pitch (6). The pitch is a measurement of how far the prop would pull in one revolution in perfect conditions (in inches).

Lower diameter, higher pitch props will fly an airplane faster but offer slower acceleration, like driving a car in 4th gear. A higher diameter, lower pitch prop will fly slower but offer better acceleration and better performance when flying straight up, like driving in 1st gear.

Typically if you increase the diameter you will have to lower the pich or vice versa. This is because you need to keep the load on the engine within a specified range. To find how much a prop will load an engine multiply the diameter by 2, then add the pitch. So a 10x6 prop will have a loading of 26. Let's say you wanted better acceleration in exchange for a slower top speed. You could install an 11x4 to keep the engine within it's designed RPM range (11*2+4=26, same load as the 10x6).

Like everything else with airplanes, it's a compromise. You can't have a bullet fast top speed with excellent acceleration. Find the loadings of the manufacturers recommended props with the formula. Then find other props in the same load range. Try the different props and stick with the one that you think suits your needs best.

not all props of the same size work the same so if you have the money try different brands as well.apc seems to do better than others.

APC brand props seem to yield the best performance, but are not the strongest. For beginners Master Airscrew props can't be beat. They don't have the performance, but are cheap and durable as heck.

This could bring in many opinions. it all depends on the type of plane and flying you want to do. many different combinations. Say you have a 40 size trainer and a 46fx. 10/6 propon the average. take thatsame engine and install it in a Shrike or Q500 plane or similar with a 9/7 prop and you now go from a 50mph plane to over 100mph.

Or 40 size engine, 10/6 or 7. .60 size engine, 11/6 or 7 .90 12-137/ or 8 but this can be wrong also if your into 3d where as you wil reduce the pitch to lets say 11/4. If you have a prop jet. Pitch is important for speed. You'l find that the pitch number gets close to the dia sort of.

Many variables. A very common question on the RCU is "what's the best prop for this plane or whats the best prop for this engine?" How are we suppose to know when you didn't tell us what type of engine or plane ? They say there is no such thing as a stupid question. Yes there is and this is it.

So since your new, ask away but give us details so we can give you better answers. LIke your original question of wanting to know how props work. Sounds to me like your leading up to something. Tell us what it is and we can better help you.

Thank-you all for your responses.. But they weren't exactly what I was looking for.


I intentionally omitted airplane specifics because I didn't want the conversation to drift away into "I've got this (which isn't what you have but it might mean something)" type conversation.

I wanted a more scientific discussion. So.. Assume I have an engine on a test stand. There is no airplane.

If I change from a 11x6 to a 11x7 prop, what is the performance difference? Does it have more thrust? Would it yield more acceleration? or more top end speed? If I change just that one and only one dimension. What happens?

Does that make sense?

On a test stand (or anywhere else), increasing the pitch and leaving the diameter unchanged will result in a potential higher top speed, but it will demand more power from the engine. Increasing the diameter, without changing the pitch will result in higher thrust, resulting in higher acceleration rates, again demanding more power from the engine. There is so much that an engine can deliver. That is why others have mentioned that you have to remain within certain boundaries. When you combine both effects mentioned above, you can decide what performance you want to get from your airplane.

I think I'm spot on with this explanation but the more experienced can feel free to confirm or deny my allegations

That's right. You can keep moving up in diameter or pitch to improve performance until you reach a certain critical point. At this point your performance will start to drop back off because you are overloading the engine.

For example, perhaps a 12x6 gives you 11,200 RPM which is where your engine will give best performance. If you switch to a 12x10 you won't get better performance because now the engine is only turning at 8,400 RPM due to the increased load. IF the engine could turn both the same RPM then you would see a large improvement in top speed.

Lower pitch = more thrust, low speed
higher pitch = less thrust, higher speed
More blades on a prop = less efficient
To add a blade without increasing load, lower the pitch by 1.
Larger diameter, lower RPM = more efficient

A prop is just an airfoil. It works off the same aerodynamic principals as a wing.

Another good way of understanding what propeller changes can do for you is to download and use Thrust HP. It's a free program that shows you the results you can expect from propellers, so it's a good way of playing "what if" without spending a lot of money on props.

[link]http://www.hoppenbrouwer-home.nl/ikarus/software/thrusthpv20d.htm[/link]

Ken

G'day Mate,
The best way to find out, is to try it for yourself.
Mount your engine, get a tacho, & some props, & start testing.

To keep engine loading about the same with different props, I take take the square of the prop length, multiplied by its pitch, and match that factor up with other prop sizes. With this equation, you are moving the same volume of air with each prop revolution for the different prop lengths. To first order, increasing the length by one, and decreasing the pitch by one is pretty close for most prop sizes.

So a 10x8 prop would give me a factor of 10*10*8=800. If I want a prop length of 12 with the same loading, the calculated pitch is 800/144 = 5.56, so a 12x6 would give you about the same engine loading. Note that just increasing the length by two and decreasing the pitch by two gives you the same result here.

I'll take a stab at this, too.

Your engine is going to have a certain rpm where it performs best. The idea is to choose the diameter/pitch that will put you at that rpm during your preferred flight profile. So there are 3 pieces to the puzzle that you are trying to optimize: engine, prop and airframe.

Your propellor's diameter is usually determined by ground clearance. You need to be able to take-off without digging holes in the runway. So for a .40-.50 sized plane you're diameters will be 10-12". The pitch is determined by engine loading and airspeed. The faster the plane moves the more the relative angle of attack of the prop blade changes. The net effect is that you need to add pitch as speed increases, but this increases engine loading so you should also reduce diameter. The pitch ends up being limited by flow separation. If you spin a blade with too much pitch too fast it will stall.

As a rule of thumb for .46 planes, a trainer would take a 11X5 or even 11X4. If you have a strong .46 you could probable turn a 12" prop, but I have found that my OS .46FX likes 11" props better, a sport plane would take a 11X6 or an 11X7 if it's real fast, and a 3D plane would take a 12X3.5. So you see, the faster planes take more pitch while the slow-flying 3D plane takes a real flat prop.

how would this affect for instance a hangar 9 j3 piper cub with an SK 50 2 stroke...I broke the engine in realizing that the prop was a 12x6 NOT the recomended 11x6...plane has 2 succesful flights on it with the 12x6 zinger prop...am I hurting the engine ?(am I doing any damage to it with the 12x6 (wood) prop on it)?...it has great torkie-like acceleration and takes off quickly...cruises nicely @ 1/2 throttle...anyhow , getting kinda' long-winded here...would the motor benefit from an 12x5 vs. the 12x6 prop...how would a 11x10 prop behave in my cubby?