LokiDog

I have gone to Tony van Roon's website, http://www.uoguelph.ca/~antoon/hobby/props.htm, dealing with props but I have more questions then he provides. I realize that the higher the number of the pitch, the second number, the faster the plane will go, but have less power for takeoffs and stunts. What happens when you increase the length of the prop? I believe the RPM's will go down but what else will happen? When the RPM's go down, does that mean less power and speed then?

da Rock

Don't try so hard to get simple answers. There are lots of things that narrow the answers, and often narrow the general answer right out of being correct for your particular situation.

Generally, more diameter will let the engine pull better. Unless of course, the engine doesn't have the torque to do it. And you know what..... if the engine does have the power to do it, the rpms probably will still go down. And yet the airplane will fly better.

What works better than having a pocket full of "rules of thumb", when it comes to choosing the best prop for your airplane, is to have a pocket full of props.

da Rock

We also have a problem with the numbers that're written on the props. There actually is no accepted standard throughout our model prop industry. Of course, the diameter number is going to be reliable for comparison, but the pitch number isn't even close.

Some years back, there were a couple of pitch gauges on the market. Back then a good percentage of the modelers in the hobby did a lot of things themselves. And reworking props, although not done by a lot of guys, was done by enough that the sale of pitch gauges was worth mfg'ing and selling them. Those gauges tell an interesting story. Turns out that a 10x6 from one mfg didn't have the same pitch (as measured by the gauge) as another mfg's 10x6 had. That actually isn't proof that one or the other mfg was wrong. It did prove however what a lot of knowledgeable modelers knew. There is more than one airfoil and AOA that'll produce a given performance.

Unfortunately, knowing that there are different ways to get six inches worth of performance out of a 10" prop doesn't stand as proof that one mfg or the other IS GETTING six inches of pitch results. It actually only shows that they're trying different ways. But sometimes it helped explain why one 10x6 didn't fly like a 6" pitch prop. Because we very often found that the mfg's printed pitch number really was not giving what it should, nor what other props of that pitch gave.

After all those words........ what's my point?
The guys who were seriously interested in pitch discovered that you could not bet on the advertised number. Truth is, to us it wasn't a biggie. We had learned to test and go with what worked and if the numbers were good, good for them.

Now, was there any value to all this "setup"? Hope so..............

contd.................

da Rock

Awhile back I was trying a few new props on a very tried and true model of mine.

And for the heck of it, I tried an 8" pitch prop in spite of the fact the model flew just about every brand of 6" I could find. Since you adjust your engine to the prop, and whatever rpm the engine settles for will decide the power output, and a torque curve is only so long, I really didn't expect the 8" was going to work. And I was expecting that the airplane just might not land as slowly as before, since the idle was about the same with the 6"s and that 8". And 7" pitch props had not been very good.

But that 8" worked better in the air than any of the 6" had. And approach speed was just as slow with the 8" as with the 6"s. And the idle rpm's tached within a couple hundreds.

Did the mfg of the 8" prop lie? or did they make a mistake? I'm guessing their test process tripped them up. I frankly didn't care a bit, why. It's happened before.

I've not put that magic prop on the pitch gauge to see what that will tell. That gauge hasn't been out of the drawer in years. I'm flying R/C now and for most of it, the details don't matter as much. So the gauge probably won't be out ever again. Unless I start racing, that is. That's about the only part of R/C I can see might benefit from that attention to detail. whatever.............

Don't bet on the pitch numbers.

da Rock

So what's the best way to pick a prop?

Depends on what you want to do. If the prop is a major player, quit reading right now and find an experienced modeler in whatever that kind of flying is. He'll save you lots of time and tears.

But for most people, start out with one of the engine mfg's recommended props. Start with one that has a larger diameter, not a smaller, for whatever you want to do. Want to 3D, then it'll be a large diameter prop to begin with. Going plyon racing? Find an experienced plyon guy and quit reading this mess. Otherwise, pick a larger diameter. And get a couple with different pitches. Best bet is to stick with one brand in these first steps.

Larger diameters often idle better. More mass in motion helps there. They also are more apt to give more uniform performance. They engine winds up having more constant load. That is, if the prop allows rpm that isn't on the low end of the engine's torque curve. Most mfg's won't recommend a prop that isn't safely on that curve. More mass and steadier load often restrict the engines ability to accelerate. But that won't matter much unless the pitch is low and you're looking for acceleration. If the pitch is one of the lower numbers, any prop/engine will usually be able to move up and down the rpm range quicker. That usually does give better acceleration and deceleration. Pattern flyers like to have deceleration, btw.

Test a number of different pitches and get used to the plane with that diameter. Now try out another brand of props. Try the same diameter and to save a bit of time and money, you can just buy one. Same pitch as the favorite from the 1st brand. Doing that might save you buying some worthless props. If the new brand prop's pitch is say a 7 and it acts like the 1st brand's 6, that info will let you choose the next test prop's pitch.

da Rock

One thing that happens with our glow engines is a useful guide when we're picking props. As the engine gets out of the middle of it's overall good performance rpm zone, the needle setting tends to get narrower and narrower.

When you're getting toward too much diameter, the needle gives you fewer clicks from peak rpm to running rich. When the engine has too little diameter or pitch, same thing.

da Rock

When you start from the middle of the engine's torque curve............

If you increase diameter..........
The rpm will decrease, the airspeed range will lessen and become more constant, airspeed might decrease slightly both high speed and low speed, idle might improve, prop should pull better through turns, acceleration and deceleration should improve but cover less speed range

If you decrease diameter..........
The opposite of the above

If you increase pitch.........
The rpm will decrease, the top speed should increase, speed at idle should increase, acceleration and deceleration should lessen, the airplane will probably lose speed in turns more

If you decrease pitch...........
The opposite of the above

For all..........
When you increase rpms by whatever means, engine heat should increase as should fuel consumption.
When you decrease, the opposite of the above.

flyX

Just get on a multi speed bike and try to peddle uphill and you're have a better understanding..this way your brain can really feel it.

-pkh-

My Feedback: (2)

Long length & low pitch is like low gear, pulls vertical better, flies the plane slower at full throttle, provides more drag/braking at low throttle.

Short length & high pitch is like high gear, gives you higher top speed, but vertical performance will suffer, less drag/braking at low throttle.

The prop loading factor is Pitch x Length^4. If you find a prop that gives you optimal RPMs, then another prop with a similar load factor should give you similar RPMs. This means if a 10x7 gives you optimal RPMs, then an 11x5 (11x4.8 actually) should give you about the same RPMs.

2slow2matter

My Feedback: (1)

What I really need to know is how do I get 10 inches of performance out of 6 inches of prop?
OK, sorry, I had too.
Generally speaking, you can go up in diameter until you reach a size of prop that is physically limiting the engine. i.e. your RPM's are so low that you are actually "loading" the engine too much, and you will do damage to the engine from overheating. As stated before, there are "rules of thumbs," but nothing beats real world results.
I would suggest you get a handful of APC props that you feel are within reason for your engine, and try them all. If you tell us the engine you are talking about, you can probably get some pretty specific suggestions to start out with.
Generally speaking, the pitch tells you how many inches that the prop will "screw" through the air with each rotation. Of course, these are ideal world numbers, and we don't live in an ideal world, we live in the real world. So efficiency really comes into play, and APC are great, affordable, efficient props for the average modeler, so therefore I recommended them. However, please realize these props are very sharp, and much less forgiving if you get your fingers into the prop disc than, say, a MAS prop would be. Of course they are both going to hurt, and send you to the hospital, but an APC is likely to send you missing some fingers all together.