Hi,

I would like to learn about props and what the different units mean.

For example, what does "8x6" mean when the plans for a plane call for an "8x6 prop"? What are the different parts to the prop? For example a wing has an LE, chord, etc. What does a prop have for its elements? How do you size a prop for a particular plane. . . . How do you compare props. . . . .

Are there any web pages that have this type of info?

Thankyou.
-Kevin

The first number is the length, and the second number is the pitch.
For an 8x6 prop, it is 8 inches long, and in a theoretical "perfect world", it would move forward 6 inches for every revolution.

The pitch and length work together to create the load for the engine, and each engine runs best within a given load range. For example, if you put a 12x6 on a .20 size engine, the engine would run poorly and overheat.
Putting a 9x6 on a .61 engine would not provide enough load, and would cause the engine to over speed, possibly damaging it. This becomes even more critical on 4-stroke engines.

So you have an engine that is happy within a given load range... let's go back to the .20, which is very happy with a 9x6 prop. What other props could you use, and why?

A 9x6 prop has about the same load factor as an 8x10, or a 7x16(!), and also a 10x4, or an 11x2.5

So what's wrong with the 9x6?
It depends on the application.

The 10x4 will provide about 50% more static thrust than the 9x6. This means that the plane will have better ability to pull vertical uplines, perform 3D "hovering" maneuvers, or simply "horse" your way out of a stall.

Well... if the 10x4 provides so much more static thrust, then why wouldn't you ALWAYS want it, or even go to the 11x2.5?

That thrust comes at a tradeoff of speed.
At 13,000 rpm, the 9x6 will be driving a reasonably slick airplane at close to 75mph. The 10x4, at the same 13,000, will be closer to 50mph.
Hmm... 50% increase in speed or thrust... take your pick.

Okay... what if you want to go faster?

Smaller prop... higher pitch.

The 7x16 will have the plane going close to 200mph... but offers LESS THAN HALF of the static thrust of the 9x6.
This prop/engine combination would have to be used on a VERY slick airplane with little drag... retracts and a low aspect ratio wing would be required. Even then, it would take a lot of runway to get off the ground, and take a lot of time to build up speed.
This is the extreme... a "speed prop" would actually be an 8x8 to 8x10 and offer just over 100mph on the right airplane.

Airplanes with a lot of inherent drag, such as biplanes, or trainers with flat-bottomed wings, will generally fly best with a prop that is larger in size and lower in pitch, but you will almost always want to stay near the middle of the recommended prop range for your engine.

3D type aerobatics are also better with long low pitch.

I base my prop selections on maximum expected airspeed of the model. If its expected to be cruising at 50 or less, I use the long low pitch. If its expected to cruise around 60 to 75 I use a medium/low pitch and medium/long prop. If I'm feeling the need for speed... I use the highest pitch I can get. (and with high pitch... I plan on needing a LOT of runway)

Hi guys,

Thanks for the great info. I also have another question. How does an 8x6 compare to a 9x7 on the same motor - a GWS "A" motor? (In terms of load and thrust.)

It will turn slower providing less speed and thrust.

A good response. I will only add that the pitch varies from hub to tip so that each blade element will be operating at the same angle of attack when the propeller is moving forward. Each blade station will have a different pitch. The propeller "pitch" is specified at a particular distance from the hub (such as 75%). While the distance for specifying is pretty standard, the actual "twist" of the prop from hub to tip may vary between manufacturers. That is one reason that propellers from different manufacturers with the same diameter and pitch specified may perform differently.

Thanks to everybody. It’s great to have the benefit of knowledge from experienced people. Now, some more questions for ya.

How do you calculate the load factor?

Using my example of an 8x6 vs 9x7, I see that my plane will go further on one rev of the 9x7 (from what RichLockyer stated).

Since the 8x6 turns slower and creates less thrust (Geistware above) is that because it is “working against” the motor more because the pitch is greater than the 9x7 and, therefore, the 8x6 would seem to create the most load?

But since the 9x7 goes further it would seem to me that it does more work and therefore, would “work against” the motor more than the 8x6. So which prop creates the most load?

(Is load = thrust? If that’s the case then from what Geistware told me I think the 9x7 creates the most load. Am I right? )

Me thinks you misunderstood the loading on the engine part.

"Load" is the amount of resistance the prop puts on the engine. How much effort is required to turn the prop. A high load prop will make an engine have a lower max RPM. A low load prop will increase the engine's max RPM.

"Thrust" is how hard the engine/prop can pull at max RPM. Thrust is very closely related to Pitch. A greater pitch means less thrust. Think of it like the gears in a car. First gear is like a low pitch blade. You don't get to go very fast, but you have good accelleration. Top gear - you can go fast, but have poor accelleration. Thrust is also related to diameter, a larger diameter prop means that you have more prop doing work, which means more thrust.

"Top Speed" - how fast can you go.. pretty obvious.

The problem, is that changing either pitch or diameter will change the load on the engine. Changing the load will affect the max RPM, so it all sort of comes down to percentages....

same-pitch, but larger diameter = more load.
same diameter, but more pitch = more load.

Therefore, 9x7 will have far more load on engine than 8x6. There is no question between which prop has produces the larger load on the engine, it is the 9x7. Thus, the 9x7 will spin slower than the 8x6, and it is possible that you will even get a better top speed with the 8x6 because the prop spins faster... if it spins 1/7 times faster (14%), then you will get enough extra RPM to make the 6inch pitch overtake the slower 7" pitch.

All pretty complex.

But, the manufacturers make it pretty simple by listing the recommended props for the engine, and each of them will have a similar load on the engine. This load is carefully chosen to be at the peak efficiency of the engine (optimal RPM). So, just choose one of the manufacturer's recommendations, and choose purely by pitch/requirement. For a high speed plane, shoose the largest pitch, for a trainer, choose a medium to low pitch, and for a stunt plane, choose the low pitch prop.

gus

I think you misinterpereted his answer... I know I did.
You asked about the 8x6 compared to the 9x7, and he gave you the parameters of the 9x7.
The 8x6 will probably spin about 2000rpm higher. Top speed would be slightly more, but static thrust would be slightly lower... BOTH of these numbers are very close though, so I'd say that both props would end up being comparable AS LONG AS you aren't overloading the engine with the 9x7.