Looking for help... propeller sizing
 

Hey everyone, beginner here with a few questions about propeller sizing. Here's a few details about my setup...

The engine is the 1.40 RX from O.S. engines. The aircraft weighs 30-35 lbs (depending on payload). My current prop is a 13.8 x 10, 4 bladed APC prop.

I'm trying to get as much static thrust out of the engine as I can, so I'm looking for suggestions for different propellers. Obviously, this is to help decrease the take-off distance, so I need to get as much power as I can to get the aircraft up to speed faster. I'm looking for good pulling props and I'm not that worried about sacrificing in flight cruise performance. Here's some of the recommended props...

16x14 (2-blade), 15x12 (4-blade), 15x13.5 (3-blade), 15x14 (3-blade)

If anyone needs more details, I'll try to tell you what I can. Thanks for your help.

RE: Looking for help... propeller sizing
 

Welcome to RCU! I know nothing about props for a plane that size, but I am curious what kind of plane it is and why a beginner is starting with something so large. Big planes and carrying payloads are not something a beginner should typically be attempting.

RE: Looking for help... propeller sizing
 

Hey, thanks for your reply. I wish I could tell you more, but let's just say it's "home-built." It's a school project and we have an experienced pilot on our team, and all of us have some pretty significant background in aircraft design.

I guess I should have put this in the fuel engine section, but I saw beginner and thought I should go there. Perhaps if someone could move this thread for me, that would help.

If someone would be able to explain the effects of changing the propeller size and pitch, perhaps I could figure out what I needed. I read in another thread that decreasing the prop diameter would increase the ability of the engine to spin it at a higher RPM. Same goes for decreasing pitch: gives faster RPM. Does this faster RPM mean I'd be getting more thrust/pulling power out of the prop?

How does going with a larger prop or higher pitch effect the thrust? Would a larger prop mean it can pull more air, therefore more power? Would higher pitch also mean the same thing?

RE: Looking for help... propeller sizing
 

Thanks for explaining the background, it helps to know there's an experienced pilot involved.

In simple terms...

A larger prop of the same pitch can move more air, if it's spinning at the same speed. However, since moving more air requires more power, the larger prop will turn slower.

A prop of the same diameter, but larger pitch will move air faster, if it's spinning at the same speed, however, this will also require more power, so it will turn slower.

A smaller pitch, but same diameter, will require less power, so it will turn faster. Same goes for a smaller diameter, but same pitch.

A smaller prop with a larger diameter, for example 10x7, may end up using about the same power as something like an 11x6. In this case if you wanted a faster plane you'd go with the 10x7, but if you wanted better pull during takeoff, you'd go with the 11x6.

The "art" is in finding prop sizes that allow your engine to turn in the range that produces the best power. If your engine develops peak power between 12 and 13 thousand RPMs, look for props that let it run in that range. Going back to the example, perhaps the 10x7 and 11x6 are in that range. If you try an 11x7 and discover the engine only turns 10.5k, you probably wouldn't want to use it. Usually, the engine manufacturer has done some of this discovery for you and lists the props that work well for the engine.

Once you know what props work best, you can then choose the right one for your application. In general, the smaller pitches, perhaps with a larger diameter, will be like being in a lower gear in a car. You'll have more pull, but less top end speed. A prop with a larger pitch, but perhaps smaller diameter, will give you less pull, similar to being in a higher gear in a car. This means it can take a bit longer to accelerate, but you'll have a higher top end speed.

RE: Looking for help... propeller sizing
 

Thanks again for your help. Your post along with this article..
http://www.masportaviator.com/ah.asp...&ID=72&index=0

has helped me out a lot. I think I know where to head next. I just have one more quick question about number of blades. We've been using a 4-bladed prop, but diameter and pitch choices are limited. I remember from one of my classes that more blades can provide more thrust, but they also cause more interference with each other, making the efficiency worse. Does the number of blades affect the performance significantly, or is it merely for looks/show.

RE: Looking for help... propeller sizing
 

I hope this plane is a multi engine aircraft. At 30 to 35 pounds i highly doubt this plane would fly with that engine(i could be wrong). If you need more static thrust go with a larger diameter prop with lower pitch. If you need higher speed go smaller diameter with higher pitch. Props that have more than 2 blades tend to produce less static thrust. Usually if you take a prop that the pitch is half of the diameter, will work more efficient and give you best overal performance than high or low pitch props. It all really depends on several factors like weight, drag, wingloading and much more. The only true way to find the right prop is to grab one and try it.

RE: Looking for help... propeller sizing
 

Lol, I guess my memory from class was a little wrong. That's probably why I'm having troubles now. I guess I should go back and check my notes.

EDIT: I was correct. More blades will give you more static thrust; however, the engine will turn at a lower RPM and the propeller is not as efficient.

Btw, the plane did fly at 25 lbs, but we didn't have enough elevator control power to rotate at 30 lbs. That issue is being resolved by someone else. At 30 lbs our take-off distance signficantly increased, which is why I'm trying to get more power out of the engine, since we will ultimately be flying at 35 lbs.

RE: Looking for help... propeller sizing
 

jaff,

What you are trying to do throws the normal wisdom off a bit. The heavier you load the same airframe, the faster it must go to achieve the necessary lift. Therefore, you will need to experience a much longer takeoff run. Remember the drama of Lindberg's takeoff for the Paris flight.

There are formulas ( formulae) to calculate airspeed using the RPM and pitch.

You are not running out of elevator (unless the load is causing the plane to become nose-heavy), you need more airspeed before being able to lift off.

Think of the times you have been in a commercial airliner. The more empty seats, the shorter the takeoff run and vice versa.



Good luck,

Ken, AMA 19352

RE: Looking for help... propeller sizing
 

Right, I know that increasing the weight will cause the ground roll to be longer. I'm trying to reduce the ground roll by getting the airplane up to speed quicker.

There was a problem with our plane being a little nose heavy, but also our elevator deflection wasn't nearly enough to be able to lift the nose. It wasn't an issue of not having enough speed to get enought lift out of the wing. Our stall speed is around 30 mph and we were traveling over 40 mph when we had to call it off before we ran out of runway. I'd like to get the speed up even more though, since we still have to add another 5 lbs, that's why I'm looking for the best take off propeller.

From what I've read, it looks like I need a larger diameter prop with as low a pitch as I can get. Our cruise and possibly landing performance might suffer, but we have flaps to help with that.

RE: Looking for help... propeller sizing
 

A thought based on past experience working with naval aircraft. (ie.. carrier take offs)
A catapult would most likely be out of the question not only due to logistics but stress on the airframe would be tremendous but a blast deflector at the beggining of the runway would give your prop wash something to push against and may help get you up to speed faster.
Just a thought, it would be interesting to see how much difference it would make?

RE: Looking for help... propeller sizing
 

Consider that stall speed increases with increased wing loading.

As you ran out of runway before being able to lift off, you are correct to try a longer, lower pitched prop. It would be helpful to find a review of that engine, to see what prop gave the most thrust and start from there. You will arrive at the max speed for that prop the quickest. If that speed is not enough, go for the one with next highest thrust, which will most likely be one with higher pitch or RPM.

Ken, AMA 19352

RE: Looking for help... propeller sizing
 

We did consider rocket assisted take-off in our initial design, but determined that it would be too risky and that the engine would be able to provide enough thrust to meet the take-off requirement. That's why we decided not to go with that.