Why 3 baldes?

[Ken Culbertson]

I had something happen today that has me confused.  Maybe the more technical minded can explain.  I switched from an 11.5 x 6 APC 2 bladed prop to a Master Airscrew 11 x 7 three blade.  It was electric , runs the wrong direction and I did not change rpm (9,550)  I was expecting faster laps - they were slower by .1.  I was expecting to use more battery - it used less.  What I got was vastly improved overhead tension, smoother rounds and  greatly  improved exits from corners.  What I don't understand is why?

Anybody have an explanation?

Ken

[Clint Ormosen]

I’ll let the more knowledgeable answer your technical questions, but I just wanted to let you know that the MA 3 blades are known to be underpitched. So you’re probably a using a 11/5-ish.

[Ken Culbertson]

I’ll let the more knowledgeable answer your technical questions, but I just wanted to let you know that the MA 3 blades are known to be underpitched. So you’re probably a using a 11/5-ish.

Thanks  -  That is good news since an 11-5 is the right prop for this plane!  Explains the lap and battery part.

Ken

[Brett Buck]

I had something happen today that has me confused.  Maybe the more technical minded can explain.  I switched from an 11.5 x 6 APC 2 balded prop to a Master Airscrew 11 x 7 three blade.  It was electric , runs the wrong direction and I did not change rpm (9,550)  I was expecting faster laps - they were slower by .1.  I was expecting to use more battery - it used less.  What I got was vastly improved overhead tension, smoother rounds and  greatly  improved exits from corners.  What I don't understand is why?

     I don't know enough about the rest of the system to say, but it is pretty obvious that if you are flying slower, you have less drag, so less energy is required, so the battery lasts longer. I presume that this is a bigger effect than the loss of efficiency (maybe) that caused you to lose speed.  Reducing the diameter generally makes the cornering better so that is no surprise. It may also be that it was a touch tail-heavy and that adding nose weight helped - it is certainly suggested when it made the rounds smoother.

     We don't know the actual effective pitch, it may have been more, less, or have a different pitch distribution. Usually reducing the diameter reduces efficiency, that would potentially explain why it slowed down.

    The other conditions matter, too, so I can't tell you why the overhead tension went up, maybe because your backwards-operating prop caused you to yaw outboard more than you would have otherwise in the top of the hourglass, and you needed that from a trim perspective. Sometimes, the effects of a lower-pitched prop also has the effect of increasing the airspeed stability, that can easily reduce the tension as the airplane slows down in groundspeed More pitch sometimes permits you to take advantage of the whip-up to get more groundspeed and thus more line tension.

    A lot of people have done these sorts of uncontrolled experiments and then jumped to wrong conclusions, so beware of that. This really tells you very little about 2-blade VS 3-blade per se. You changed far more than just the number of blades - you changed the diameter, the pitch, the pitch distribution, the blade shape, the airfoil, the mass, the moment of inertia, blade surface finish, and the speed. File it away as part of your trimming and setup knowledge, but it would be easy to over-interpret the results.

      Brett

   

[Brett Buck]

I’ll let the more knowledgeable answer your technical questions, but I just wanted to let you know that the MA 3 blades are known to be underpitched. So you’re probably a using a 11/5-ish.

  That is interesting. This is why you need a pitch gauge to even begin to make sense of these sorts of experiments, although there is still a lot of guesswork involved.

     Brett

[Ken Culbertson]

That MAS 3 blade is not an E prop right?

I am not sure.  It was advertised as such but it doesn't look like it.  Very stiff and narrow blades.  Not too many IC 3 blade pushers out there.

Ken

[Ken Culbertson]

https://www.masterairscrew.com/collections/3-blade/products/3-blade-11x7-propeller

I see, it's the new FB series. They are lighter and much improved from the early GF series but can still be used for glow engines.

That's the one and so far I am impressed by the prop.  It doesn't make those funny noises when you load it up and it definitely improved performance over 45.  As Brett stated though any number of things could be the real cause.  Maybe the extra weight let the motor run better.  Maybe the stiffness.  Who knows for sure but it was better, lots better.  I just wish somebody made a three blade pusher spinner that did not require credit references!

Ken

[Jim Damerell]

RSM sold similar items, I bought a 2 1/4" 2 blade from them a few years ago. Not sure if they still carry them.

[Ted Fancher]

RSM sold similar items, I bought a 2 1/4" 2 blade from them a few years ago. Not sure if they still carry them.

Hmmmm.  Wow! a 2 1/4" two blade, huh.  Must be a very high RPM set-up, huh?  Ultra low pitch as well, I expect.

Probably not a big seller so they might not be available from RSM anymore.

Ted

[Jim Damerell]

Actually, it was for a 13 X 6 APC Reverse prop running at about 7800 RPM. Just letting people know they might find the spinners at a hobby supporting location, rather than rolling your dice on the bay.

[TDM]

A lot of people have done these sorts of uncontrolled experiments and then jumped to wrong conclusions, so beware of that. This really tells you very little about 2-blade VS 3-blade per se. You changed far more than just the number of blades - you changed the diameter, the pitch, the pitch distribution, the blade shape, the airfoil, the mass, the moment of inertia, blade surface finish, and the speed.

      Brett

 

Brett we actually agree on one thing after all. Well put. Well maybe more than one thing.

I bet if he used a good quality light 2B prop he might have had similar results. Less weight is less energy demanded from the power system so less energy. Less diameter and the number of blades changes the moment of inertia and with it the power consumption. Then you have the aerodynamic load that changes everything yet again.
I think APC makes a series of props designed for electric that are more efficient and weigh less. I wonder if you used these ones or the normal I.C. props APC makes.

[Ken Culbertson]

Brett we actually agree on one thing after all. Well put. Well maybe more than one thing.

I bet if he used a good quality light 2B prop he might have had similar results. Less weight is less energy demanded from the power system so less energy. Less diameter and the number of blades changes the moment of inertia and with it the power consumption. Then you have the aerodynamic load that changes everything yet again.
I think APC makes a series of props designed for electric that are more efficient and weigh less. I wonder if you used these ones or the normal I.C. props APC makes.

It was an APC 2B electric prop that I replaced with the MA 3B.  I have never used the APC IC props.  I have used the MA IC's ever since I found out that a 10-6EW was too small for a 46.  Well I guess you could.

[Howard Rush]

Less weight is less energy demanded from the power system so less energy. Less diameter and the number of blades changes the moment of inertia and with it the power consumption.

[phil c]

A couple of things Ken- 0.1 sec/lap is a kind of iffy measurement.  You'd need at least 5 measurements on different, non-consecutive laps, or more, for any kind of solid measure.  Or you could measure 5+ consecutive laps and divide it out.

Three blade props run smoother.  The blades are always getting hit by differing angles hitting the air.  The worst case is a two bladed prop.  When one blade hits the air at a high angle as the plane moves around the circle the other blade hits the lowest angle.  This causes some vibration.  A three blade prop only lets one blade directly hit the air at high angle of attack so it's smoother.

There have been plenty of posts here that more blade area helps overhead.  I haven't measured it, but it's pretty likely the 3 blade prop has more area than the 2 blade E prop.  Since the 3 blade can handle IC engines, it also may be stiffer causing fewer losses.

[Brett Buck]

Brett we actually agree on one thing after all. Well put. Well maybe more than one thing.

   

Uh, OK.

Less diameter and the number of blades changes the moment of inertia and with it the power consumption.

     Refer to Howard's post of a disapproving Newton.

    Brett