Prop Question

[Randy Ryan]

I took this off SSW because I thought it was a good question, one I surely can't answer. I do know I get really good performance with APC narrow blade props though, I just have no idea why. This is not my question, but I'm interested in the answer.

Quote:

"Recently I started using some of the Top Flite Power Point props, they work pretty well,(and they are available at the local hobby shop), they have narrow bldes as compared to the old Rev-ups or even the Zinger props. Does anyone have an opinion what might be better, wider blades, or the thinner type such as the Top Flite variety? Any reasons why one might be better than the other?"

[Randy Ryan]

No takers huh?

[Jim Pollock]

I always liked the Rev-Up 10-6 EW's best back in the late 60's and early 70's

Nowadays I kind of like Eather and Bolly 3 Blade props.

Jim Pollock   

[Brian Massey]

I think it's a great question; but I'm in the dark on props. I broke my 3 blade 10/7  and replaced it with a 2 blade 11/6. Left the rpm's about the same thinking "it will fly almost the same". Not, that switch took .7 seconds off my lap times!  I went from turning 4.9-5 second laps, to 5.7 laps. On the next flight I turned up the r's by 400 thinking "it will fly almost the same" . . . I only picked up about .2 seconds. What surprised me then was even turning a 5.5-5.6 lap, the plane would pull over the top. Not with all the line tension I feel comfortable with, but it made it. The 3 blade would not have at that speed. I have a lot to learn about props.

Brian

[Jim Thomerson]

Here is my reply from SSW.

Let's cut to the chase. Go out and buy as many different pops as you can afford, of around the size you think will work. Fly them all and see which one or two work to suit you. Fly those props and save the rest because one or more of them will likely be the best on the next airplane/engine combination. You may be very surprised by what works best for you on a particular airplane/engine combination

[Randy Ryan]

Here is my reply from SSW.

Let's cut to the chase. Go out and buy as many different pops as you can afford, of around the size you think will work. Fly them all and see which one or two work to suit you. Fly those props and save the rest because one or more of them will likely be the best on the next airplane/engine combination. You may be very surprised by what works best for you on a particular airplane/engine combination

But that's not the answer to the question Jim. The basic question is "what makes certain props behave certain ways". We all do the "swap-a-prop" dance.

[Mark Scarborough]

Thats exactly the problem, to answer this, you need to read several legitimate publications, do a serious amount of experimental testing, and then generate a report. Pretty much, the best way to approach this question is to find other people flying similar setups to what you want, then see what they are using and ask why. Then go do the tests yourself.
I have prepared over a dozen props to test for the Avenger when I maiden, and start trimming it. I have several options that have been suggested by people who fly the engine I am using on similar sized airframes. So, I have broken them down into 4 basic groups, with about three selections in each. major divisions, diameter, pitch, then blade area, then blade thickness. I will start with one, if its ugly, then I will select the next one based upon whether the one I tried was slow, fast, low tension, or whatever. Its not an exact science. There are general things but again, it would take a book. do a search for props on here, and you will find enough info to get started with understanding. THEN go out and try different stuff

[Randy Ryan]

All true Mark, but it still bypasses the root of the question.

[phil c]

I took this off SSW because I thought it was a good question, one I surely can't answer. I do know I get really good performance with APC narrow blade props though, I just have no idea why. This is not my question, but I'm interested in the answer.

Quote:

"Recently I started using some of the Top Flite Power Point props, they work pretty well,(and they are available at the local hobby shop), they have narrow bldes as compared to the old Rev-ups or even the Zinger props. Does anyone have an opinion what might be better, wider blades, or the thinner type such as the Top Flite variety? Any reasons why one might be better than the other?"

Since nobody else wil try, here are a few thoughts.
The shape of the propeller airfoil is important.  Wood props are nice because they are light, but most of them don't have very good or very well-made airfoils.  They generally are cut with two rotary cutters, one on the front and one on the back(usually flat).  So they often have a flat on the front instead of a rounded airfoil point.  This varies from prop to prop and manufacturer to manufacturer.  Look for ones with a smooth, rounded leading edge and a fairly thin airfoil.  You can make big improvements in a wood prop by carving decent airfoils into it.

Wider blades move more air, absorb more power, and are a bit less efficient.  That lets you trim how the engine runs, and to some extent the pulling power when the plane is going uphill.

APC uses a very effective design program that calculates the airflow over the prop at every diameter, so the airfoil changes about every .005 inches along the blade to get the maximum lift and minimum drag.  If you can find one that works well on your plane/engine you can get very good performance.


The weight of wood versus plastic has a big effect on prop precession(the torque the turns the nose left on a hard outside corner).  Diameter has a big effect on precession too.  So big heavy props can cause problems, the reason Al Rabe invented the moving rudder.

As shown by the great success of using a bigger motor in a relatively smaller plane, having a engine that can loaf along and use pretty much any prop you put on it, it is easier to trim.  The trick is getting the prop to run in the engine's sweet spot( somewhere between the peak torque and the peak rpm) and also matching it to the pulling power the plane needs.  As posted above, that puts you back in the swap-a-prop mode.  Mark's advice about starting with props other folks are using for a similar setup is the best way to quickly get into the ballpark.

[Ted Fancher]

Here's another quick thought that I haven't really fleshed out in my own mind.

I believe it is entirely possible that much of the difference we think comes from prop shapes etc. might have more to do with the load they put on the engine and where on the torque curve the engine prop/combination then gives us the lap times we think we're after...lap times, almost by default, having become the gauge we use to "define" performance for no better reason than it can be recorded reliably.  It seems logical (I'm no aerodynamic scientist) that props of different shapes and areas will put different loads on the engine and those different loads will have an effect on where on the torque curve of the engine the load will fall when our ship is unloaded (essentially level flight).  The response we then experience when doing our tricks will be largely influenced by where the needle of the torque curve of the engine goes as we add load.  The less "draggy" (for want of a better word) a prop is it will deliver unloaded performance (airspeed) below peak torque so that when we add load the torque increases, helping to maintain RPM and preventing loss of airspeed, etc.

I know that I've never been a slave to lap times instead seeking the in flight performance I'm looking for and then, assuming nothing radical, accepting the lap times that result as what is necessary for flying the airplane as well as I can.

Just a thought.

Ted

[George]

I believe GMA was doing some prop research just before he got sick. I remember he had one prop for a Fox .35 running in a 4-2-4 that he referred to as a "canoe paddle". This would tend to go along with Ted's comments (if I understand them correctly) that you may want to go with a prop shape that fits how you intend to run your engine.

I think most props, like APC, are designed for high RPM use. Perhaps this will inspire folks to experiment with prop shapes.

George

[Jim Thomerson]

I have two Supercool GMA Paddle Blade 10 x 5 props.  I've flown them  a couple of times and they are back in my prop box.  Tried them on Tower 40 and Fox stunt 35.  No doubt they will be just the thing for some airplane/engine down the road. 

[Randy Ryan]

I have two Supercool GMA Paddle Blade 10 x 5 props.  I've flown them  a couple of times and they are back in my prop box.  Tried them on Tower 40 and Fox stunt 35.  No doubt they will be just the thing for some airplane/engine down the road. 

I also have one of Greorge's props, tried on a Fox .35 in a Kenhi Panther, not impressive in that combination.

[Bob Reeves]

Here's another quick thought that I haven't really fleshed out in my own mind.

I believe it is entirely possible that much of the difference we think comes from prop shapes etc. might have more to do with the load they put on the engine and where on the torque curve the engine prop/combination then gives us the lap times we think we're after...lap times, almost by default, having become the gauge we use to "define" performance for no better reason than it can be recorded reliably.  It seems logical (I'm no aerodynamic scientist) that props of different shapes and areas will put different loads on the engine and those different loads will have an effect on where on the torque curve of the engine the load will fall when our ship is unloaded (essentially level flight).  The response we then experience when doing our tricks will be largely influenced by where the needle of the torque curve of the engine goes as we add load.  The less "draggy" (for want of a better word) a prop is it will deliver unloaded performance (airspeed) below peak torque so that when we add load the torque increases, helping to maintain RPM and preventing loss of airspeed, etc.

I know that I've never been a slave to lap times instead seeking the in flight performance I'm looking for and then, assuming nothing radical, accepting the lap times that result as what is necessary for flying the airplane as well as I can.

Just a thought.

Ted

I think Ted has hit the nail, at least this is what I've concluded searching for four stroke props and why I use such high pitches compaired to two strokes. I'm running more in the torque curve not up in the HP curve and believe this is one reason my four strokes seem to fly the airplane at one speed.

[RandySmith]

Here's another quick thought that I haven't really fleshed out in my own mind.

I believe it is entirely possible that much of the difference we think comes from prop shapes etc. might have more to do with the load they put on the engine and where on the torque curve the engine prop/combination then gives us the lap times we think we're after...lap times, almost by default, having become the gauge we use to "define" performance for no better reason than it can be recorded reliably.  It seems logical (I'm no aerodynamic scientist) that props of different shapes and areas will put different loads on the engine and those different loads will have an effect on where on the torque curve of the engine the load will fall when our ship is unloaded (essentially level flight).  The response we then experience when doing our tricks will be largely influenced by where the needle of the torque curve of the engine goes as we add load.  The less "draggy" (for want of a better word) a prop is it will deliver unloaded performance (airspeed) below peak torque so that when we add load the torque increases, helping to maintain RPM and preventing loss of airspeed, etc.

I know that I've never been a slave to lap times instead seeking the in flight performance I'm looking for and then, assuming nothing radical, accepting the lap times that result as what is necessary for flying the airplane as well as I can.

Just a thought.

Ted

Hi Ted

You are correct, and hit the nail dead on the head, which is why I wrote this in a PAMPA article years  back:
 
"Correct Props: Props are also critical for achieving good engine runs; a prop needs to “load” the motor correctly without over or under loading the motor. It also needs to be the correct size and pitch to pull the airplane. This is an over simplication, but generally the diameter will be what you use to “load” the engine and the pitch will be what you pick to set the plane speed
Some of the things you will run into when over propping an engine are; hard to set the needle on the ground; the engine will unload a lot and go rich in the air; the engine will run hot, or not cycle very rapidly.
When under propping generally you will notice the engine will also not cycle correctly. They at times will just 4 stroke thru everything, sometimes going into a 2 cycle at weird times.
A properly loaded engine will use a prop that if running a 4-2 break, will come onto a 2 stroke a 10 O’clock and back to a 4 at 2 O’clock. There are variations on this; You can have a strong motor just beep 2 stroke at the tops of maneuvers, but you shouldn’t have one that 4 strokes uphill and switches to a 2 stroke on the downhill parts of maneuvers."

This also explains why some people can change for a wide 10.5 x 4.9 2 wide blade prop ,to a 12 x 4 N arrow 2 blade , then to a 11 x 4 3 blade N  with the same run and same load on the motor, I have seen this done many times with a Classic sized ship with an Aero Tiger.
I have seen the exact same with big motors on Modern piped ships, You can go from the ultra wide CF 13 x 4 2 blade  to a  13 x 4.5 3 blade with out even changing the needle setting most times and the lap times are the same with the same performance in the tricks..

I have 3 totally differant prop setups for my KATANA with 2 and 3 blades that both work the same because they load the motor identical. Motor load is very important in trying to get a Great engine run.

The entire article is still posted in the PAMPA archives and also here in the Engine section

Regards
Randy

[Garf]

Props are the most variable item in the equation. They even vary from prop to prop in the same brand and size. I used a carbon fiber copy of the Rev Up 10-6EW, and it was a lot different from the original. Back to playing musical props.

[Ted Fancher]

Hi Ted

You are correct, and hit the nail dead on the head, which is why I wrote this in a PAMPA article years  back:
 
"Correct Props: Props are also critical for achieving good engine runs; a prop needs to “load” the motor correctly without over or under loading the motor. It also needs to be the correct size and pitch to pull the airplane. This is an over simplication, but generally the diameter will be what you use to “load” the engine and the pitch will be what you pick to set the plane speed
Some of the things you will run into when over propping an engine are; hard to set the needle on the ground; the engine will unload a lot and go rich in the air; the engine will run hot, or not cycle very rapidly.
When under propping generally you will notice the engine will also not cycle correctly. They at times will just 4 stroke thru everything, sometimes going into a 2 cycle at weird times.
A properly loaded engine will use a prop that if running a 4-2 break, will come onto a 2 stroke a 10 O’clock and back to a 4 at 2 O’clock. There are variations on this; You can have a strong motor just beep 2 stroke at the tops of maneuvers, but you shouldn’t have one that 4 strokes uphill and switches to a 2 stroke on the downhill parts of maneuvers."

This also explains why some people can change for a wide 10.5 x 4.9 2 wide blade prop ,to a 12 x 4 N arrow 2 blade , then to a 11 x 4 3 blade N  with the same run and same load on the motor, I have seen this done many times with a Classic sized ship with an Aero Tiger.
I have seen the exact same with big motors on Modern piped ships, You can go from the ultra wide CF 13 x 4 2 blade  to a  13 x 4.5 3 blade with out even changing the needle setting most times and the lap times are the same with the same performance in the tricks..

I have 3 totally differant prop setups for my KATANA with 2 and 3 blades that both work the same because they load the motor identical. Motor load is very important in trying to get a Great engine run.

The entire article is still posted in the PAMPA archives and also here in the Engine section

Regards
Randy

Hi Randy,

Really good stuff that I had never read before.  We are clearly on the same page in this regard.

This is another good argument for somebody (of course I'm not volunteering) to develop a  PAMPA produced "Stunt Bible" made up of the really really good information that has been published over the many years people like us have been doing this stuff.  The magazines and internet are great but require that wheat and chaff be regularly separated from one another because not everything that is printed in either resource is something that should be considered gospel.  A well researched and edited publication that gathers the really good stuff into a single resource would be a monstrous blessing to the event/sport.

Ted

[TDM]

Randy you are right on.
I have experienced the same if the prop was too small the motor would go in the 2 cycle mode and stay there. If the prop was too big it would stay in the 4 cycle all the time.
As for efficiency I believe a high aspect ratio, a good airfoil and light prop will win the war all the time. Load the motor aerodynamically not mechanical and you are ahead.

[Wayne Collier]

Props are the most variable item in the equation. They even vary from prop to prop in the same brand and size. I used a carbon fiber copy of the Rev Up 10-6EW, and it was a lot different from the original. Back to playing musical props.

Don't overlook twisting of the blades as part of the equation.  As the prop pushes (or pulls) air, the force of moving the air will attempt to flatten the pitch.  This results in some amount of blade twist.  Two props of identical dimensions made of different materials will likely have different stiffness.  One may twist more under load than the other.  Even though they are dimensionally the same at rest, because one has more flex underload, they will not behave the same in use.

[TDM]

One last thought if your chosen motor likes low RPM then a wider blade will work better than a high aspect ratio blade. The oposite is true for high RPM.

[Howard Rush]

A properly loaded engine will use a prop that if running a 4-2 break, will come onto a 2 stroke a 10 O’clock and back to a 4 at 2 O’clock.

I have an engine like that.  My noon flight will be at 5.0-second laps,  and my 3:00 flight will be at 5.5.

[Pinecone]

Hmm, I wonder if looking at props like the electric size does might not get into the ballpark faster?

In electric, you tend have a given RPM to run based on battery voltage (sweet spot on a glow).  So you pick the pitch based on the speed you want to fly at that RPM (lapt time).  Then change the diameter (and blade width) to get the load proper to hold the desired RPM.

Basically what Randy said, in a different way.

[Tim Wescott]

I took this off SSW because I thought it was a good question, one I surely can't answer. I do know I get really good performance with APC narrow blade props though, I just have no idea why. This is not my question, but I'm interested in the answer.

Quote:

"Recently I started using some of the Top Flite Power Point props, they work pretty well,(and they are available at the local hobby shop), they have narrow bldes as compared to the old Rev-ups or even the Zinger props. Does anyone have an opinion what might be better, wider blades, or the thinner type such as the Top Flite variety? Any reasons why one might be better than the other?"

I've looked for this sort of information on the web, and boy is it scanty!  Wikipedia does have a good entry, if you look for their article on airplane propellers: http://en.wikipedia.org/wiki/Propeller_%28aircraft%29.  It's scanty, but it seems to be on target with what's there.

I suspect that this is because the question "what makes a prop do what a prop does" doesn't have a really simple answer.  From what I can gather, from the general aerodynamics books that I have and from readings on the web, this is something that could consume years of your time in a University course of study; anything that we do as modelers is going to be vastly oversimplified.  One of these days I want to find a book that goes into it, but I expect that if I do I'm going to regret the fact that I'm an electrical engineer, not a fluid dynamicist.

Here's what I know, and it's all vague and confusing:

A prop is just like a wing, only it's spinning. 

A prop is just like a wing:

This means that anything rule that you can apply to a wing -- area, aspect ratio, angle of attack, etc., all apply to props.  I've even seen props on full-size airplanes with sorta-Horner 'wing' tips (on a Piper Cheyenne III owned by the US Customs Service).  This also means that airfoil choice is important, as is surface finish and the lift distribution over the diameter.

Just from momentum vs. energy considerations, a big slow prop is going to use less power to generate a certain amount of static thrust than a little fast prop.  Ditto big & two-blade vs. little & three blade, or big & low pitch vs. little & high pitch.  It's just like a glider vs. a fighter jet.  Consider a helicopter main rotor -- in a way, it's just a propeller.  But it needs to generate lots of lift at really low airspeeds (think "hover") -- so compared to a propeller that you might use on a same-sized airplane, it's got really skinny blades, it's really big, and it's really slow.  On the other hand, take the motor out of the helicopter and put it on a racing plane, and all of a sudden you want a little, high-pitch prop.

There are obvious issues with really skinny blades -- how in heck are they going to hold together?  Also, as speed goes up the induced drag (that which comes from diameter vs. thrust) claims a smaller and smaller proportion of the total drag than parasitic drag (that which comes from pushing all that material through the air).

Only it's spinning:

Because the prop is spinning, its potential for generating lift changes from the root to the tip.  Worse, even at a constant RPM the distribution of lift from root to tip will change with airspeed -- and an efficient prop at low airspeeds isn't efficient at high airspeeds, and visa-versa.  Worse yet, the "pitch" (the amount of lead built into it, as if it were a bit of screw thread) can and should vary over the diameter of the blade -- and each manufacturer is going to have a different idea of what this variation should be.

None of this helps to answer your question -- if I've done anything good with this babbling, it will only serve to give you more specific questions to ask.  There are so many variables to this that I don't think a universal ideal will ever be found.  I think the "ideal" stunt propeller is going to be one that is matched not just to the airframe and engine, but to the flying style of the pilot.  I think that the elements that go into making the prop 'best' are going to go beyond mere aerodynamics -- at the very least they're going to extend to structural integrity, weight (and gyroscopic properties), and maybe even aesthetics (what if the prop that flies the best has a butt-ugly shape -- would you use it?)

So, should someone out there start working up a report on propellers, I think that these are the points you should think about:

* What is its aspect ratio (i.e. blade area vs. diameter)?
* What is its pitch distribution (i.e. blade angle vs. diameter)?
* What is the area distribution (i.e. blade width vs. diameter)?
* What is the airfoil?  Does it change with diameter?
* What is its surface finish?
* Does it curve (i.e. is it a scimitar prop)?
* How does it fly a big light slow plane?
* How does it fly a little heavy fast plane?
* How do a half a dozen of your buddies like it, in each of the above planes?  Why?
* How does it pull on the flat?
* How does it pull up a vertical?
* How does it behave down a vertical?
* How does it seem to affect inside corners?
* How does it seem to affect outside corners?
* How does it accelerate out of a corner?

When you're all done, things will still be incredibly vague -- but if we're all lucky we'll at least have some general guidelines!

I would do this report, but for two things:  (1), I wouldn't get it done until I'm 102 years old, and (2), the most likely report for any props I test would be "broke on flight number (1 through 4) when I crashed".

[john e. holliday]

Now I know why I try not to get serious about this hobby/sport.  The preceding post is very deep.

[Tim Wescott]

Now I know why I try not to get serious about this hobby/sport.  The preceding post is very deep.

Too deep for me, and I wrote it!

[Andrew Borgogna]

I had quite a bit of experience with Top Flite power props during the eighties and me experience follows what
Ted said about loading.  At the time I was running a piped 40 that would not unload during roll out unless I used a very light prop loading.  The power props fit the bill, I used a 9x5 power prop that would allow the engine to get on the pipe about half way down the runway.  If it didn't get on the pipe it would not take off, on the pipe it went like a rocket ship.  I tried many props before I settled on the TF power prop, I question whether this would make a good control line prop since it was designed for high RPM use. 

Andy

[RandySmith]

I have an engine like that.  My noon flight will be at 5.0-second laps,  and my 3:00 flight will be at 5.5.

Hmmmmm  mine  do it at 5.3 at noon  and  5.3 at 3:00 PM, I am glad i don;t have a serious engine problem like that  :-)

But serious  I have never had a 1/2 second change in the needle in a 3 hour timeframe. I can get the almost identical 2/4 break, or I can set it up to run all 4/4 with no problems between noon and 3

Randy

[john e. holliday]

Tells me that the engine is not broke in completely yet.  Seems some engines take forever and others are almost ready to go out of the box.