I am posting a question about propeller choice. At first look, it seems that the best way to choose a prop is to first decide how fast you want the plane to fly, and choose the pitch that gives you that speed for the rpm you want to run your engine. Then increase the diameter until the engine runs at the rpm you selected. That way you get the right speed and the most thrust.

Say I want my Conquet to fly at 90 mph top speed and I think the engine will turn 10,000 rpm. I would choose a 9.5 inch pitch. Then I would have to go up to 13 or 14 inch diameter prop to slow the engine down to 10,000 rpm.

In reality, most pilots use an APC 12 x 12 for the Hanno. I also found some words from Dave Patrick, who designed the Conquest, saying he used a 12.5 x 11 APC prop. (and probably a YS 61 AR, which is similar to a Hanno)

Why is my logic wrong? I know there is some slip, but I ahve read that the slip is minimal with APC props and a clean airframe like a pattern plane. Do we incease the pitch to keep a load on the engine and avoid significant rpm changes with speed?

I do my selectin the other way around.
I look for a propeller that will give me about 1.5 thrust to weight, then I select a pitch that will give me the RPM I want.

Prop selection is very model dependent. It has a little to do with the motor you choose. It has a huge amount to do with the way the air flows around the model. They call this Spiral slipstream and this effects lots of factors int he model...like thrust lines and so on.

Now you need to be in the best rpm range of your motor...I ran the Hanno's on a 12-12 APC on 25% nitro and turned 10,500rpms. This was a really good combo. Run the Hanno above 10,000 in my personal opinion and not above 11,000.

On the YS 140's you want to be 8300-8600rpms what ever props get you around 8300-8600 then try the performance and see what they do to the models flight performance.

Back to the Hanno: the differences in 12-11, 12-12, 12.5-11 ar all very small. Its a try and try again thing.

I find a prop that I like and I tend to stick with it for a while, Then I try another one and wow its such a different feel I stick with it a while. Sometimes its hard to tell a difference and other times its just land and pull it off the model flies crappy with that prop. I know your running the Hanno so I'll tell what my experience was....I ran the 12-11 and 12-12 APC on most everything at the time. If you have an exceptionally light model...like 7.25Lbs you can run a 13-10 and it will be a really good prop. Above 7.25 to 7.5lbs the 13-10 is not so good. But if you are the typical 7.75-8lbs the 12-12 or 12-11 will work the best for you. Now try the others and see what happens. I'm always trying props some are slower and some are faster. Some are better speed control and some are better vertical pull....

Its all a matter of preference and what you think the model should perform like.
Remember just because you have X motor doesn't mean that Y prop is the best for it.


Troy Newman
Team JR

Right now, all I lack to finish the Conquest is the fuel tank, battery, and receiver install. I weighed the plane with this stuff, and I am just under 7 lbs. I used light components and the wing cores were lightened before sheeting. That 13 x 10 may be the ticket, since the plane is light and has thicker airfoils than most other 60 pattern ships. I am sure Troy is correct that flight testing is the only way to get the right combination.

I like to use engineering theory to get close, then refine with testing. Just another way to enjoy the hobby. I started this thread hoping to understand why pattern pilots use so much pitch. It may boil down to quiet operation requiring lower rpm, more than anything else.

So much pitch?

the 12 pitch is not a bunch....We are not doing flippity floppity stuff. The prop selection has to do with the acceleration and deceleration of the model. When you fly the really flat pitched props like on the fun fly models the planes don't respond to throttle....The whole purpose behind the flat pitch props is to decrease the sensitivity of the throttle so that you can make bigger stick changes without the model responding instantly. With a pattern model you want instant response....

Take a horizontal snap roll followed by a rolling component say 1.5positive snap-4/8pt opposite direction....The model needs to be stalled in the snap...Then you want it to recover and instantly be tracking to perform a point roll. If you use the big flat pitched props the model will die in the snap and it takes too long for the model to recover.

Also the higher pitched props will tend to be more constant speed. The difference between upline and downline speed closes...and the throttle becomes easier to manage. Sometimes a higher pitch prop will be a slower downline than a flat pitched prop...Example the in some cases the 16-13 has better brakes on the down line that the 16-12 or 16-11. Usually Diameter helps more with the brakes than the pitch does.

On your model the 13-10 may work out well for you....but I would venture to say the 12-12 or 12-11 might even be better at controlling the speed of the model.

Good Luck with it

Troy

And what about the best choice for a big 2 stroke like 1.40 RX or 1.60 FX or 1.40 FI, what is your recomendation: 16X14 Or 17x12, In deep, what is the diference?

Thaks.

Jorge Lozano

Thanks Troy,

That is the information I am looking for.

Lem

Part of the reason pattern fliers use such high pitches is to absorb the power of the engine and keep the tip speed down with smaller diameters. It's all about noise control. I'm using a 17-12 on my current OS1.40, but used a 17-10 and 18-8 succesfully on my ST2300 powered Arch Nemisis. The 2300 didn't have quite the HP of the OS, so noise with the lower pitch props wasn't a big factor. I flew a Runaround for 4 years with an OS91 four stroke using an APC 14-8. THe lower pitch props will work fine in pattern, just have to get used to the different feel. And it depends on the class. The lower classes have no snaps, so everything is about smoothness and placing the plane. The upper classes with snaps have entrance and exit speed demands, so mating the right prop to the plane becomes more critical.

Jeff; You answered my question before I even asked it. Just started flying a pattern ship.
A Swallow-90 with a OS 91 4 stroke and I was wondering what prop would be good. Right
now I have a 14/6 on it. But seeing what you had to say about your experience with your 91
I'm going to try a 14/8. Thanks.