Hi guys,

I went to the hobby shop earlier after pulling my prop off. I broke a corner of the prop. The size that nexstar recommends is 11x5. I was about to buy that, but the owner of the store brought me over to a chart, asked me what engine i had, then recommended a 10x7. Will work, but when i got home, I read the book some more to be careful, and i noticed that it says stay with the 11x5. GREAT... does it really matter? the engine is a OS 46fx.. any help would be wonderful.


Austin

10x7 will give you more speed, but less useable thrust (you'll find you have to rev the engine more to get the plane to move). Takeoffs will be longer. Personally, I'd stick with the larger prop/lower pitch, but give it a try and see how you like it!

since the nexstar has a tough time slowing down for landing, a 10x7 will make that even worse. a bigger prop acts as an airbrake when you chop the throttle. you can try it, but you may not like it. also, they will make your top speed go up also.

sean

Stick with the 11-5. It will give the best all around performance for the 46FX.

Put it on and try it. I like the 10-7 I'm using on my .46FX. It's a Master Scimitar. I've used an 11-6 and 11-7 APC also. I like them to. Basicly I use any prop that is recomended for the engine and it doesn't make a lot of difference on my LT40. You'll probably ding it up anyways but being a little shorter might help lengthen it's life..

I use an APC 11-7 and I love it on my alpha .40 I have a TT Pro .46 with that prop it is fast and does pretty nice aerobatics

It is interesting to me to read this thread.

I purchased a GMS .47 engine and got 2 11x7's and a 10x6. On another thread I was told to
use the 10x6 prop on this engine because the 11's were for .60 sized engines. But I see a lot of talk here about using 11's on .46 sized engines.

Reading this thread, now I am confused on what to do. [sm=confused.gif]

I have found out that I cannot use the props that I got with the engine were pushers so I can't use them anyway.

Was planning on getting a couple of 10x6's for the plane. Now I don't know.[sm=confused.gif]

-= EDIT=- This engine is going on my trainer.

Larry

That last bit of info helps. For a trainer you want lots of pull to get out of trouble and a bigger prop to act as a brake for landing. I tried different props and found that a larger prop will be able to pull the plane out of trouble easier than the shorter prop at lower rpms. It also works as a brake to slow the plane down for easy landings. A 10-6 is used where you want to let the engine wind up and stay at the upper throttle range. For training you will mostly be using 1/2 throttle or less. If you are flying from a grass field you will definitly want the 11" prop to haul the plane through the grass. For 60 engines an 11" prop is only used for break in on 60 engines. After that the suggested props are "Sport: 12x6, 12x7, 12x8, 13x6, 13x7" as quoted from the TH site. I tried a 13x6 prop on my OS 61 with my Sea Fury. The rpms wer much lower than with the 12x6 prop but the takeoff distance was much shorter. Hope this helps.

I'd swing a 10x7 for all around performance for a trainer. Lowering the pitch/diameter allows you to wind'em up as Fatsky stated, hardly what you'd want for training. Personally, I wouldn't bog them down with 11x7's.

Using a certain prop to slow a trainer down, is not necessary in my book. The ONLY time I would ever use a 12" prop on an ABC 2 stroker, is if I was flying a Cub....and that'll never happen.


Best thing to do is try a few different props, and use what you, your plane, and your motor like the best.

Gary

Larry,

A quick a dirty rule of thumb I use is to figure out the middle of the road prop for sport flying, for example a 10X6 for a 40 engine. This provides an optimized loading on the engine. So if you want more thrust, you increase diameter. However, doing this at the same pitch (i.e. 11X6) will increase the engine loading over optimal, but if you increase diameter and decrease pitch (11X5) then the loading doesn't go up quite as much. Therefore, you get more thrust, but slower flight. Same thing going the other way, trade a 10X6 for a 9X7.

This is not scientifically accurate and also varies by prop type, etc. However, it carries the message in an easy to understand way. Each engine has an optimum rpm where it produces max power. By juggling pitch and diameter, you can attempt to stay as close to that point as possible.

Also, try a few different prop types also. A 10X6 master airscrew is a much different animal than a 10X6 APC or wooden prop for that matter.

Duke

Thanks guys. I was looking at a 10x6 prop after I found out the ones I had can't be used. Sounds like I was heading in the right direction.

Larry

To more accurately figure the prop loading, cube the diameter and multiply by the pitch. The results can be compared favorably.

EXAMPLE:

9x7 prop: 9x9x9x7=5103
10x6 prop: 10x10x10x6=6000
11x5 prop: 11x11x11x5=6655
11x4 prop: 11x11x11x4=5324

If you stay in the same ballpark for total volume, the engine rpms will be very close. The more you depart, the greater or lesser the load on the engine.

Yeah, what he said!



Duke

Talking about props I did some experimenting and found out that 10x6 were not that great as it was really giving me a lot of RPM but little more speed. I feel that a 10x7 is better suited for a 46 2C. I have tested out a 46 with a 12x6, that was fun ... you get decent performance in the mid range but once you go to full throttle, the engine is over loading and there is not much more speed. For me I go for the Bolly 10.5 x 7, those props are nice. The Master Airscrew 11x6 is pretty decent too. For a decent amount of torque I will still stick with your APC12.25x3.75, been running it on my Sukhoi 31 and its fun to pull nice big loops (I don't know how to 3D, hehehe).

Again, all this is subject to the plane design, fuel and engine tuning. A tach is always useful.

11x5 and bigger wheels

Ausitn,
Re-read Dukes first paragraph. That's a good rule of thumb, (and Larry) it will help explain why an 11 X 7 is being considered too much prop for your engine, but why an 11 X 5 isn't.

Remember, the Diameter of the prop determines how much thrust you're going to generate (as in vertical pull and the ability to take off more quickly), while the Pitch determines your overall speed.

ie. A 10 X 7 on a 40 or 46 engine will produce more speed than an 11 X 5, but take off runs will be longer and the plane will not slow down as well for landings.

Selecting props IS a bit of a black art, but the best way to do it is to look at what the engine manufacturer recommends, then selecting for either speed or thrust.

One other point of interest. The pitch of the prop is theoretically the distance in inches that the prop will move forward with each revolution. For example, a prop with a 7 inch pitch will move forward 7 inches with each revolution (or 5,833 feet per minute), while a 5 inch pitch will move 5 inches (4,167 feet per minute). Of course that doesn't take slippage, drag, and friction into account, but it does giv a comparison.

Good luck, have fun, and welcome to RCU!
Dennis-

Last tiime out we tried several props on my buddies Nexstar. The 11x5 that came with it was ok, but very fast and hard to slow down.
12x4 was much better but the pull seemed a little week.
12x5 was just about right
Just for informative purposes I put a 11x3 on it to show my buddy how a prop can make such a differance. That little engine was screaming but the plane was pokin along. Acted very tail heavy even though it is actually a bit nose heavy.

So my $.02 worth, 12x5 or the stock 11x5 if you don't mind the speed.

My standard answer for prop questions:

That's a neat chart. Hope you don't mind that I saved it to my hard drive.

JettStarBlue

Where did you get that chart. I would like one, but can't clearly read the one you posted.

Larry

Larry,

Save it to your computer, in "MY pictures", (download takes a few seconds, it's not a big file) then you should be able to see it perfectly. You can also print it out from there, and hang it in your shop.
As an alternative quickie, you can click on it, then when it loads, and gets small and blurry, put your cursor on the right bottm corner of it, and after a second, a funny looking box with arrows will appear. Just click on that, and it will be full sized.

Thanks.

Larry