For example, I've got a Midstar 40 with an OS .40 LA and 10x6 prop.

What does changing the pitch affect?
What does changing the length affect?

IE what would happen if I were to put a 10x7 on, or an 11x5-6?

more pitch, the second number means higher cruise speed less climb, flatter pitch means faster or better climb performance slower cruise generally.

Longer prop will give less RPMs and shorter more generally.

You put a longer prop on that engine it will be a dog. Might try a 9.5 X 5 or 6 for better performance. You won't notice it much on that plane though.

Well, I'm thinking along the lines of slowing it down a bit until I'm comfortable with it. What about a 10x5? Or should I just throttle back?

If you want to slow it down, put the 11 x 6 or 10 X 8 on it. That will slow it down.

Cutting the throttle helps too.

As with many things in life --- it depends. Prop related performance is an interactive process -- it is not always a clearly predictable case. The engine's performance is affected by the prop selected --- but the prop's performance is affected by the engine's performance characteristics, the airframe in use & by its own particular design characteristics.

In general (and working within the engine's accepted performance envelope) a steeper pitch means slower acceleration & climb, but (sometimes) greater top speed. An increase in diameter improves prop efficiency (usually) & increases total thrust (usually), giving faster acceleration, less loss of speed in turns, better climb & (sometimes) lower top speed.

In the relationship between pitch and diameter, an increase in diameter raises the power load (required power that the engine must produce), faster than an increase in pitch does. A 10-6, for example, is roughly equivalent to an 11-4 in power load, assuming both props are of the same design & made from the same material. However, the performance that the airframe derives from those two props can be very different & it is highly dependent upon the airframe itself. A light, clean, low drag airframe will go faster with the 10-6 than it would with the 11-4, but a large, draggy. &/or heavy airframe may very well go faster with the 11-4. In either case, the 11-4 will give better climb.

In the examples that you gave (10-6, 10-7, 11-5 & 11-6), you must also consider the power plant -- only the 10-6 is suited to your engine and airframe combo. The LA .40 can't effectively pull any of those other props, particularly the 11-6. It will spin the 10-6 (or 11-4) at ~ 12,000 - 12, 100 rpm (close to the torque peak at 12,700), but the revs will fall below 11,000 (say 10,800 - 10,900) with the 10-7 & ~ 10,400 with the 11-6 -- well below both the power & torque peaks.

After all of that -- keep the 10-6, or alternatively, use an 11-4.

BTW, a spinner will improve prop performance, but keep the spinner diameter within 25% of the prop diameter.

I just noticed your note about wanting to slow it down, & FLYBOY's suggestion of a 11-6 or 10-8. If you want to limit in-flight speed, use your throttle, that's what it is for. However, if you want to reduce the landing speed, you will get best results from a larger diameter, lower pitch prop (eg. 11-4).

Flyboy's suggestions will certainly reduce top speed, & the 11-6 will likely lower the landing speed, but the 10-8 will actually increase the landing speed, and both will absolutely kill take-off & climb performance.

very much so, I wouldn't actually want to put either of those props on an LA engine. Master airscrew 10 X 6 and learn to fly it.

different makes of props will affect it a ton too. APC props work great on my wild thing, where master airscrews make it just a dog. Masters do work better in a lot of situations. All in all, you won't notice a huge difference on your plane unless you just pull the power back.

An 11 X 4 would very likely slow the plane down quite a bit, and allow for slower landing speed. It also won't over tax that engine, where an 11 X 6 or a 10 X 8 would. (The 10 X 8 would definitely result in higher top speed and landings assuming they didn't cause too high a drop in RPM.)
Dennis-

I'll stick with the 10x6. Not gonna change anything until I fly it and see what I've got.

Doh! What was I thinking. You are correct. I went the wrong way! hahahaha. Which way did they go [&:]

Thanks for catching it guys!

No problem Flyboy, we all mess up once in awhile.

Multiplying the length with pitch gives you the prop load on engine.

Prop Load of 10 x 6 = 60
Prop Load of 11 x 8 = 88

Always be careful not to put extra load on your machine, or else it will give way to quickly.

If Any Expert here thinks I am wrong, please correct me.

Have a look at this article,
[link=http://www.aircraft-world.com/prod_datasheets/hp/emeter/hp-proptalk.htm]http://www.aircraft-world.com/prod_datasheets/hp/emeter/hp-proptalk.htm[/link]
which contains a lot of useful information about the effect of pitch and diameter changes.

/Red B.

Allow me to add my two cents worth (probably not worth much more).

Think of changing pitch as changing gears on your car. Lower gear on the car produces more pull but lower speeds. Higher gears produce less pull but higher speeds. Generally speaking, the props will perform the same way except lower pitch = more RPM and more noise, sometimes. Higher pitch produces less RPM and less noise.. sometimes. Again, as someone said, a lot depends on the design of the aircraft. But generally speaking, this is true. Diameter plays a role too.

Another consideration is noise. Most clubs have a noise factor in the use of various designs and combinations of engines, mufflers, and props. They limit the number of dB that a plane will produce at a certain height and distance. Larger diameter props, generally are more quiet, so, to limit noise, seek out the best diameter and pitch for performance leaning toward the larger diameter for a more quiet operation. Then govern your pitch according to your needs for speed vs. vertical performance.

If going vertical is not an issue, then opt for something that is quiet and has pull and speed.. however, you did say that your plane is a tad to fast. Ok. opt for lower pitch, good diameter, and back off the throttle.

Heck, the only time I am at full throttle is at take off, vertical climb, and on touch-and-goes (takeoff again), and once in a while on straight pass, I may open it up for a short time. Most of my flights are at half or slightly more than half throttle. That tames the speed down to something I can manage.

Hope all this helps. It takes some time to digest and to decide what exactly it is you want vs what you need. I have a small collection of props (as does most RC'ers) that I bring out with me when I fly. Mostly because I may ding a prop on a landing so I have a replacement for it (happens less and less now that I have been getting more flying time) but also to change characteristics of flight based on observations from my instructor and my wants and needs. It all works out, eventually.

Not bad Dick -- except that prop noise is greatly affected by tip velocity. With a larger diameter prop the tip velocity will be higher, unless the prop is turned at a lower RPM. For noise reduction, an increase in pitch will also drop RPM effectively -- in both cases performance can suffer. You have to decide how much priority is given to noise reduction vs performance. Another trick for noise reduction is the use of a multi-blade prop (3-b or even 4-b). The increase in the power load from the multi-blade prop necessitates a reduction in diameter, & hence, results in less noise -- but again, at a loss in efficiency & performance.

Noise reduction, & increasing performance are usually (but not always) mutually exclusive. Prop selection generally involves a lot of suck-it-and-see engineering, in addition to experience & understanding --- so bring a bag of props with you if you really want to get into this stuff.

You're right, Brit-brat. At times, it's simply a give and take issue, depending on if you are looking for reduced noise or improved performance. Right now, our field is located out in the boonies, no houses close by, so we re not really plagued by the noise issue, but we still practice 'safe noise' just to keep things in their proper perspetive. Of course, what happens if we visit one of the other clubs that may have a noise issue. Then prop selection based on noise becomes an issue.

Also, larger diameter props may turn at a lower RPM so increasing the pitch may return the power at the lower RPM with lower noise. It's not an exact science, that's for sure, with all the different engine types and power levels and aircraft design. I have one that seems to amplify everything. I put a soft mount on the engine, Tigre 75, and it got much quieter with the same prop. Go figure. The soft mount was to try to reduce the vibration at certain RPM's that the engine compartment cheeks were showing.. did that fine so (yep, the prop was balanced).

We (me and the "pro's" at the field) work together on that hoping that we can make things a bit more quiet but still get performance. A tough balance. That's why I keep several different size props in my box (and a few spares cuz I still ding them on landings, but fewer and fewer as time goes on and my proficiency improves).