I usually run a 9x7 on my OS25LA on a Stick type plane that weighs 3.5 lbs. It seems OK for the sport fun-fly stuff I do.

I went to fly it today, but was out of 9x7's and all I had was 8x6's. What would I expect to see with this smaller prop? Of course I expect a longer take-off, but how much vertical will I lose (not that I had that much anyway)?

Not knowing what to expect, I opted not to fly that plane today.

Quicker,
I think it's too bad you didn't fly it today because you would have liked that 8x6. I'd bet you would see better performance with that prop. The 9x7 is in my opinion too much prop for the LA.25. You might want to try a 9x4,8x6,9x5,and even an 8x5.
Tim

This plane would take off in about 15-20 feet on the 9x7. What could I expect with an 8x6? I would expect a higher top speed in the air...correct?

Quicker,
The first number is the length of the prop if measured from tip to tip. The second number is the pitch. The only way I know how to explain that is to say: In a perfect substance, say jello, one complete revolution of the prop would go through 7" of jello, if we are using the 9x7 for the example. Therefore a 8x6 would/should slow your airplane down as far as top speed is concerned. But I suspect the 9x7 is hampering (too big) the LA25 and not letting it rev to it's full potential. If you want speed then go with the larger pitch (second number) and smaller diameter prop, like a 8x7. You will have to try a few different props to find the one you like, good thing the little ones are cheap. Either way I'm sure you will like it a lot more when you get the right prop for the engine. Prop selection is dependent on what kind of flying you want to do, if you like to go fast use small diameter, big pitch. If you like to do fun fly or do aerobatics, go big diameter, small pitch. Big diameter, small pitch props will make the plane fly slower but have more power, like a John Deere. The opposite is true for the small diameter, large pitch props. Maybe some one else will explain it better than I can.
Tim

I like to run a, 8-5 on a .25, a 9-6 I prefer to run on a 32 to 36 size engine. Try the 8-6, you will probably like it.

AJC

OK...I just bored the holes out in the 3 8x6's I have to fit the shaft on my 25. I'll give them a shot when I get out this week.

Thanks, guys.

Hi
I have never bored any propeler.
How do you do it to mantain the hole perfectly centered?

Prop reamers usually do a pretty good job staying centered but I olways check the balence when enlarging holes to a much larger size.

Dauntae

Hand reamers work very well, but when I am in my shop, I use a "unibit", a stepped drill bit designed mainly for drilling sheet metal. They work much like a reamer but used in the drill press you can keep the prop exactly perpendicular to the bit, andit makes a super job. Also, they dont "grab" or go off center like a regular twist drill will.

AJC

Hi!
A 9x7 is a little too large for the OS .25 LA , though it could spin a 11x4 with ease if used on a light highwinged trainer ...on a low winged sport aircraft from which you demand a little more performance a 9x5 or a 9x6 APC is much better...that is if you fly at sea-level.

Regards!
JanK

Almost...the flatlands of Nebraska.

Well, I just got back from spending my lunch hour at the LHS talking with the seasoned verterans and they all recommended NOT going with a prop as small as an 8x6. After the many stories about the good 'ol days and all the 25 powered C/L and R/C ships they have had (these guys are all 65+) they said to stay around 9x5 to 9x7. Even though it's only a 25LA, they said I would be very disappointed in the performance of anything smaller than a 9x5, especially since this bird jumps into the air in 15 feet on a 9x7.

Obviously this seems to be less rocket science and more of a "feeling" thing. Hmmm......

I have run, with great success, both 8x4 and 8x5 props with my 25la. With both of these, but easier with the 8x4, I can hand launch my Ace Simple Series Ultimate Bipe. I'm not so concerned with top speed so I thought these props were great.

Mike

You got it. Finding the right propeller is not scientific at all.

I also think the 9x7 prop has a bit too much pitch, though a wood prop doesn't "dig" quite as hard as an APC.

Those older guys are used to running older cross-scavenged engines that produce a lot more torque than today's typical sport engine. The engines ran at lower RPMs, and could swing higher-pitched props without straining. Of course, they used a lot more fuel. I have an old Enya .19 on a combat plane that swings a 9x4 prop at 12,500, but uses over an ounce of fuel a minute.

A student of mine had a .25LA on a PT-20 trainer last summer. He started out with a 9x6 Master Paintstick. Frankly, with the 9x6, I thought the engine sounded sick. It wouldn't pull the plane out of the grass, and we had more noseovers than takeoffs. I had him try one of the 9x4 props I use for combat. With that prop, the engine spun up to a nice RPM, and yanked his plane off the ground like it weighed nothing.

The bottom line is, it doesn't hurt to experiment.

Thought I'd check back in with som flight reports. I tried a few different props with my 25LA and here is what I found:

8x6: What a dog! I barely got the plane in the air and I barely pulled off a loop. Not an option anymore
9x6: Again, not enough pulling power, plane felt sluggish
9x7: golden I really like this prop on this plane. Fit my flying style which is sport and sport aerobatics, not speed

I also just ran a 10x6 on it in my garage for the heck of it. I don't have a tach, but it diffinetly was not turning the RPMS of the 9x7, but WOW, what pulling power. I was holding the plane vertical and with full throttle I'd let go of it for a split second. It wanted to hover, but the little LA jus didn't have the power to pull it off. I think I'm gonna try to get out tomorrow night and try the 10x6 just for at least one flight, then I'll go back to the 9x7's.

Thanks all for the input.

My only experience to amount to anything with the 25LA was an a Sig LT-25. We tried all kinds of props before settling on an APC 10-3. It had very little thrust at idle for slow landings, but would let the engine turn up and generate plenty thrust without too much speed.
Second choice on that plane was a 9-6.

Larry

My opinion (as good as that is), would be to use a Apc 9x5 or Apc 10x4.

I'm suprised the La can even turn a 9x7 without bogging down, or going lean.

I might try the 10x4. I tried a 9x6 and was very disapointed with it, so I don't see where a 9x4 would better.

What brand/style props & fuel are you using and are you retuning the engine for each combo? What did the other props tach?

I'm using standard MAS props for all of these. I don't have a tach, so I don't know specific numbers. I'm just going off how the plane flies. Again, I'm not going for speed, I just want to do sport flying with sport aerobatics (just fun stuff...oops, etc.).

The plane in question is a Stick style homebuilt with a 40 inch span and weighs 3.75lbs ready to fly.



This is the 3rd plane that I have built off these plans, and all have been identical planes and have flown well with the 25LA on a 9x7 prop. I've got a 4th one almost finished that I'm going to put an MDS 40 with a 10x6 on it.

I lost the 1st due to a bad switch, and the 2nd my friend totalled (he's not allowed to fly my planes anymore ). It's based on the SpadStick from http://spadtothebone, but built with ordinary hobby foamboard from Hobby Lobby to keep it lighter for the 25 engine.

This all started because my LHS was out of 9x7 props...

Took that plane out today and tried the 10x6 just for the heck of it. As everybody said, it was way too much prop, but it made a cool sound

Actually, the plane took of very well (much better than the 8x6 or even the 9x6) , but it bogged the engine down if I tried to climb too fast, as expected. The LHS also recommended that I try a 10x4, but I ran out of time today as my wife was waiting for me to take her to Home Depot. They also said that since APC props are more efficient than MAS to try a 9x8 in that prop to continue my experimentations, so I picked up 2 of those with 2 10x4's. I still did not get any 9x7's because they got cleaned out before I got to them I think it's time to find a new supplier of them 'cause they are always sold out....

I'll post my experiences with the 2 new test subjects as soon as I can.

In my experience neither the .25LA or .25FP will pull a 10x4 or greater prop with any performance. I have bought just about every prop in its range and found the most thrust happens with a 9x4 or a 10x3. 9x5 adds a little speed and sacrafices thrust. 9x6 is borderline too much prop for performance but does quiet the rpms down. 9x7 would kill any bushed .25 that I have seen. You would really have to lean the engine out to get a 9x7 to pull and I think you would burn it up before the season was over. An 8x6 inch prop will work but it won't give you maximum thrust. If you really want to know how your engine feels you have to tach it. You can tell alot just by flying it but you won't be certain till you tach it. It wouldn't hurt to borrow someones temp gauge to see if its overheating with the 9x7 which can also cause premature engine wear. I typically like to see my 2-strokes in the 12k and up rpm range for bushed and 13.5K and up for ball bearing.

I believe the C/L people like the 10-4 and 10-5 for that engine. Performs similiar to the old iron .35 engines with those props. Kinda slow but still good pull in the loops.

I haven't flown this engine for about a year now. I was surprised to see this thread pop up again

I'm planning on putting this on a trainer for my 8 year old son this summer. I'm thinking a 10x3 or 10x4.

Just curious, like was asked before: "Are your retuning the engine after each prop change?"
You have to do that as the load to the engine changes. Your smaller props ought to really turn up.

Yes I was. I've never really been a set it and forget it kinda guy.