Hi guys
Is there any general rule about what kind of propeller matches in a specific engine or i'll have to experiment with several sizes?

Read the manual, it will give a range of prop sizes for your engine. These props will satisfy the engine requirement to run at the optimal max RPM (i.e. they will not overload, nor over-rev the engine).

The range will be a combination of prop size and pitch. The smaller the prop size, the larger the pitch will be in the range. For example, and OS.46FX has the recommended prop ranges: 10.5x6 through 12x7...

If you want a FAST plane, go with something like a 10.5x8, if you want a powerful plane, go with something like a 12x5

gus

the manual is a good source of information,lot of fliers pick a prop them never change but if you'll experiment you'll probably find one you like the best.

I must admit, I do not know what thread it is on, or where it is located, but there is a table that you can print out. I will look and see if when I printed mine it kept the URL. Maybe someone else knows where it can be found. You might do a search here on ..prop table, prop graph..or something similar. And as mentioned, the engine has a manual with it and the prop recommendations.

You mean this?!

(credit to top flight, ECMC.com, and all the others who have it posted on their site!! )

That is a nice chart and should be very handy. One thing i didn't notice on the chart was the "prop factor" description.

prop factor = diameter times 2 and add the pitch...for instance a 12x6 prop ...12 times 2 =24 plus pitch of 6 = 30.

12x6 = 30 pf
13x4= 30pf
11x8 = 30pf

All props with the same prop factor take basically the same horsepower to turn the same rpm (there are slight differences between manufacturers). Witht the props ive mentioned if you want a speed demon use the 11x8 or if you want a stump puller use the 13x4 the motor wont know the difference.

If you use this formula and use a prop factor from the recommended props from the manual you should always be in the ball park.

[sm=spinnyeyes.gif]
Thank you guys,you have been very helpful.Nice chart dr-wogz, i am
gonna use it.Prop factor?Hmmm [X(] I am learning KENNYROY ,i am
learning.

"prop factor" is a good quick guestimate, but it does break down when you start moving more than 2-3" in diameter away. Also, I've seen a couple of different formulas, Kennyroy's being one of them, and plenty of people swear by their formula. It's an inexact science at best.

That chart posted looks pretty good, better than some that I've seen. Though I will say that I went and checked some engine/prop combinations I use, and I found them outside that chart's range. Not a big deal, of course, the chart is a good place for a start, and you won't blow your engine up with it. The chart also doesn't seem to show some prop sizes that you might encounter, such as the ones in the 12x4 range the 3D guys like to run.

Also, as I think someone else mentioned, differnet brands of prop in the same size will often pull very differently in the air. So sometimes you can change performace just by changing prop brands.

Another thing... match the prop to the airframe.

A .46 on a Sig Kougar needs a 10X7, as the Kougar is meant to fly fast. That same .46 on a 4*40, you want a 11X4 or 12.25X3.75, as the 4* is a relatively slow flying design. (too many try to fly it too fast... not good for it.)

Most .40 size trainers... 11X4 (using a .40 or .46) Again... you are not after the high top speed from the shorter, higher pitch prop.

Low pitch and long blades means more pull at low arspeed at high throttle. Prop blades can stall just like your wing. (they are rotary wings...)

Prop Chart For Two-Stroke Engines

Alternate Propellers Starting Prop
Engine Size

5.25 x 4, 5.5 x 4, 6 x 3.5, 6 x4 , 7 x 3 6 x 3
.049

7 x 3, 7 x 4.5, 7 x 5 7 x 4
.09

8 x 5, 8 x 6, 9 x 4 8 x 4
.15

8 x 5, 8 x 6, 9 x 5 9 x 4
.19 - .25

9 x 7, 9.5 x 6, 10 x 5 9 x 6
.20 - .30

9 x 7, 10 x 5, 11 x 4 10 x 6
.35 - .36

9 x 8, 11 x 5 10 x 6
.40

10 x 6, 11 x 5, 11 x 6, 12 x 4 10 x 7
.45

10 x 8, 11 x 7, 12 x 4, 12 x 5 11 x 6
.50

11 x 7.5, 11 x 7.75, 11 x 8, 12 x 6 11 x 7
.60 - .61

11 x 8, 12 x 8, 13 x 6, 14 x 4 12 x 6
.70

12 x 8, 14 x 4, 14 x 5 13 x 6
.78 - .80

13 x 8, 15 x 6, 16 x 5 14 x 6
.90 - .91

15 x 8, 18 x 5 16 x 6
1.08

16 x 10, 18 x 5, 18 x 6 16 x 8
1.20

18 x 8, 20 x 6 18 x 6
1.50

18 x 10, 20 x 6, 20 x 8, 22 x 6 18 x 8
1.80

18 x 10, 20 x 6, 20 x 10, 22 x 6 20 x 8
2.00


Return To Top of Page


Prop Chart For Four-Stroke Engines

Alternate Propellers Starting Prop
Engine Size

9 x 5, 10 x 5 9 x 6
.20 - .21

10 x 6, 10 x 7, 11 x 4, 11 x 5, 11 x 7, 11 x 7.5, 12 x 4, 12 x 5 11 x 6
.40

10 x 6, 10 x 7, 10 x 8, 11 x 7, 11 x 7.5, 12 x 4, 12 x 5, 12 x 6 11x6
.45 - .48

11 x 7.5, 11 x 7.75, 11 x 8, 12 x 8, 13 x 5, 13 x 6, 14 x 5, 14 x 6 12 x 6
.60 - .65

12 x 8, 13 x 8, 14 x 4, 14 x 6 13 x 6
.80

13 x 6, 14 x 8, 15 x 6, 16 x 6 14 x 6
.90

14 x 8, 15 x 6, 15 x 8, 16 x 8, 17 x 6, 18 x 5, 18 x 6 16 x 6
1.20

15 x 6, 15 x 8, 16 x 8, 18 x 6, 18 x 8, 20 x 6 18 x 6
1.60

18 x 12, 20 x 8, 20 x 10 18 x 10
2.40

18 x 10, 18 x 12, 20 x 10 20 x 8
2.70

18 x 12, 20 x 10 20 x 10
3.00


i hope this helps.

David D

Where'd you get that from?

There's a few on there that I wouldn't do, and a few combinations I use all the time that aren't on that list. But it's not a big deal.

If you're just learning to fly, it's not critical, but it's a good idea to try a few differnet props.

If you are looking for performance, you pretty much HAVE to try several differnet sizes in differnet brands to see what works for you. As FHHuber said, airframe does play a part. So does your fuel, and where you fly, etc.

I find that prop selection is very inexact. Many factors change your prop needs. Examples are one engine having more power than another. One engine makes it's peak power at one rpm and another at a different rpm. Flying styles make a difference. Even the fuel makes a big difference as well as how far above sea level you are.

Having said that you should have a selection of props. Look in your manual and find the reccomended sizes. Buy a range of them on either side of the reccomended. And remember this simple addage:

Diameter=Power Pitch=Speed

You just need to use a few on your combination og engine and airframe until you find a combination that pleases you.

Mark Shuman