How does size and pitch of a prop affect flight, I have an LT-40 Trainer with an Irvine .46 (My first plane) and the recommended props are a 10x7 - 11x8, I know that that is the diameter and the pitch of the prop, but what does changing those values do ? What effect does increasing or decreasing size or pitch (Within the recommended range of course) have on flight behaviour.

Marcel

Marcel, to keep it in simple terms a small pitch gives you more "out of the hole" power while a higher pitch gives you more top-end speed given that both props have the same diameter. Now, when you have combinations of diameter and pitch its a bit tricky. An 11x8 increases the pitch and the diameter from the previous one, the increase in diameter makes the max rpms come down some. Then you add the increase in pitch and you are loading the engine even more so expect more rpm reduction. If your model is in a slight dive its top speed will be much higher but at the cost of a sloppy performance while level flight. If you are looking for high speed flight you go down a diameter # and increase the pitch.

Phuzz.
As this is your first plane speed should not be an issue. I would suggest an 11x5 or 11x6 with your LT-40 Irvine combo - a good compromise 'tween power and speed. Good Luck

I would also like to know the what effect a larger prop diameter has on flight characteristics. For instance, many 4-stroke engines can push a much larger diameter prop. Why would this be desireable? Would a larger prop help slow the plane down when landing? Would it provide more low rpm performance? Is there any benefit for going to a larger diameter but a smaller pitch or < diameter / > pitch? I am interested in details. If this is not the appropriate forum for this question, please advise.

Thanks,
Danno

Well, it's the diameter that gives you "thrust", or that "out of the hole" performance as cappio called it, while pitch is the speed end of things.

Therefore, a larger diameter prop will do a better job of giving you vertical performance, and will tend to slow the plane more at idle, as say, in the down leg of a maneuver or when landing. The "flywheel" effect of the larger diameter will also make the engine idle a bit more reliably. (OR a bit slower.)

Let's look at a couple of situations; as in Marcel's recommended prop range.

A 10 X 7 is the small side of the range for both diameter and pitch. The smaller diameter WILL allow the engines rpm's to increase somewhat, as will the smaller pitch. So with this prop the engine will be lightly "loaded". The 11 X 8 on the other hand will load the engine more, and reduce rpm. BUT! the higher diameter should provide a bit more "Torque" while the larger pitch gives a bit more top end speed.

Now look at Vinnie's recommendations. (Good ones by the way.) The 11" diameter will provide an increase in thrust, or torque, which helps the plane take off with a bit shorter roll, helps the plane climb a bit better, as well as slow down a little more when landing, and should help the idle a bit. And, the smaller pitch allows the top rpm to remain a little higher so the engine isn't loaded down. It will also keep the top speed down a little, which may be a benefit when first learning.
(Danno, four strokes do better with a larger diameter because that's where there "sweet spot" is when running. 4 Strokes do not do well at high rpm, but do produce more torque at lower rpm than 2 strokes. a 2 strokes "sweet spot" will be on the higher end of the rpm range.)

Let's look at another example. Once you're use to flying an landing and all that, you may decide you want to fly faster. Then try dropping the diameter to 10", and increasing the pitch to say 8". You should see a pronounced increase in top speed. But, you'll also find the plane doesn't climb as well, and that it will require a much longer take off roll.

It's fun to experiment though. So buy a wide range of props and find out what works best for you.

Oh, and a general rule of thumb. For every inch of diameter you increase, decrease pitch by one inch. That should always keep the engine at the middle of the recommended range. ie. 9 X 8 = 10 X 7 = 11 X 6 = 12 X 5 (Obviously, there's room on both sides of the pitch or diameter to play.)
Dennis-

Thanks for the advice guys, I had a look at answers to similar questions posted and someone also decribed it like the difference between 1st gear and 5th gear in a car. Pulling power vs top speed if I may. Once I learn how to fly the plane I can imagine it being quite interesting trying out different props to see wich does wat :-)

Marcel

Very helpful and informative - Thanks again