Props with different pitch on each blade.

After reading Doug Moon's comments on props with descending pitch, I decided to bring up a related topic -- props witn different pitch on each blade. For years Windy was telling me that what I wanted was a prop with different pitch on each blade. I thought he was kidding me and I never went there. One summer I found a prop that I really liked, a cut down 14-5 Rev-Up, and wound up flying it the whole summer. I hadn't checked the pitch before using it, but when I did I was amazed. One blade was 5 pitch, and the other was 4.5 pitch.

Since this epiphany I've used only props with different pitch on each blade. I'm currently flying my muffled PA65RE with a 14-5 Rev-Up cut to 12.75 inches. One blade is 4 pitch and the other is 4.5 pitch. I run it in a 4-2 break mode, launching at about 9000 RPM. I'm very happy with my setup. Windy has flown my plane and tells me that I'd be crazy to change anything (even though he sells RO-Jett engines).

I believe that the props I use do a better job of holding a constant speed, and don't wind up in the wind. I have talked to other fliers with similar experience, but no one has a good explanation of why this works. I'd be interested to hear if anyone out there can explain the effects of using props with different pitch on each blade.

Noel Drindak

Just from a physics perspective, I would assume that you would get similar results from a prop which had an overall pitch equal to the average of your two blades. Of course I am assuming that you aren't pulling our leg!

And I am not suggesting changing anything. If it works, leave it!

Didn't it vibrate badly?

Noel,

I suspect this is always the case to at least some extent. I doubt there is a single prop out there, wood or carbon that both (or all 3...or 4) blades are exactly alike. Close, but not exact. We spend a lot of time talking about repitching carbon props and I suspect many are led to believe that it is an exact science. Go to station whatever and repitch to this. But in reality, it just isn't that clean. I think we can build in tendencies, like wash in or wash out at the tips and get close in pitch the entire length of the prop, but no matter how much effort you make, both blades are not going to be identical. Even if you could do it, it wouldn't stay that way. Close, hopefully, but not perfect.

That being said, I suspect that there is some benefit to having some difference in the blades. It may be that one blade digs a bit more, the other blade retarding that effect. Could be you can come up with a sort of harmonic balance that aids in preventing the engine from spinning up as much. Hmm, I can't imagine what the formula, if any, would be, but I think there is a sort of balance and compromise to this stuff.

I think props are one area of this event that has pretty equal parts of Art and Science. There's a LOT of science in there, but carving, re-pitching, shaping and generally fooling with props is very much an Art. You get one that works in a particular plane, it's gold. Eariler this year I lost a plane. While I was down about the loss (one of the prettiest planes I'd ever built), I was much more upset about the loss of the prop. That particular prop was an absolute killer on that plane with that engine. I've got 3 or 4 others like it, but something about that particular prop just worked better than the others.

We had one that was off once. It was a goof and it was a disaster. If you truly have 4 inch pitch from hub to tip on one blade and 4.5 pitch on the other, have you tried one with 4.25 pitch hub to tip as a comparison? Although you may have found a "happy" prop, the only benefit I can see with the set up would be the side wash from each blade hitting the sides of the plane at different locations. But I can't see it as having greater thrust (lesser, yes) or having any benefit in preventing wind up over a properly pitched (for your system) prop with each blade equal.

Now, the sidewash hitting the side of the plane...there might be something to that, but how beneficial that might be (ir any) I don't know.

Hmmmm... Wouldn't one blade be pulling harder than the other blade, producing side forces on the crankshaft? The center of lift of the prop will be rotating around the crankshaft, rather than being centered on the middle. Weird.

Some interesting comments here. I do not have any known exerience with flying with props that do not have the same pitch on both blades. I cannot think of any reason why there might be some advantage in doing so. I believe that there would be an unequal load on the crankshaft and the bearings. I would not think that could be good for the engine.

My following comments are a bit off topic, but I think appropriate to mention here.

In other forums and other threads, I have read where individuals have have found it necessary to reshape both blades so that both would have the same pitch. I think that most machines and production processes will result in both blades being very close to the same shape and pitch. What can happen is that the back face of the propeller hub might not be true. The hub back face would not have to be slanted very much to make a 0.5 to a full one inch difference in the measured pitch from one blade to the other. Instead of reshaping the blades, all that would be necessary is to "true-up" the hub back face (and front face) so that the measured pitch is the same from one blade to the other. It might also be necessary to re-drill the hole to be perpendicular to the now trued-up rear and front faces of the prop.

It is a lot easier to true up the faces of the prop hub than it is to reshape both blades. Something to think about.

>Some interesting comments here. I do not have any known
>exerience with flying with props that do not have the same
>pitch on both blades. I cannot think of any reason why
>there might be some advantage in doing so. I believe that
>there would be an unequal load on the crankshaft and the
>bearings. I would not think that could be good for the
>engine.
>In other forums and other threads, I have read where
>>It is a lot easier to true up the faces of the prop hub than
>it is to reshape both blades. Something to think about.

Hi Keith and everyone; I have some more food for thought. I have flown 1/2A Proto, 1/2A speed and Mouse Cl 1 using single blade props
that exibited absolutely no vibration...the cranks showed no sign of Galling either. The TD's were turning 26k+ in the air. I only did a static balance for the single blade. Tony

Single-blade props are extensively used in CL speed events, occasionaly in FAI FF. I've wondered how a single-blade prop would work on a stunt airplane. I used to balance the nylon props I ran on sport racers and early slow rats by cutting off the tip of the heavy blade. Seemed to work fine. I did once get some 11 x 5 APC's which were seriously out of balance. One blade was visibly thicker than the other. Usually APC's are pretty close.

>Some interesting comments here. I do not have any known
>exerience with flying with props that do not have the same
>pitch on both blades. I cannot think of any reason why
>there might be some advantage in doing so. I believe that
>there would be an unequal load on the crankshaft and the
>bearings. I would not think that could be good for the
>engine.
>
>My following comments are a bit off topic, but I think
>appropriate to mention here.
>
>In other forums and other threads, I have read where
>individuals have have found it necessary to reshape both
>blades so that both would have the same pitch. I think that
>most machines and production processes will result in both
>blades being very close to the same shape and pitch. What
>can happen is that the back face of the propeller hub might
>not be true. The hub back face would not have to be slanted
>very much to make a 0.5 to a full one inch difference in the
>measured pitch from one blade to the other. Instead of
>reshaping the blades, all that would be necessary is to
>"true-up" the hub back face (and front face) so that the
>measured pitch is the same from one blade to the other. It
>might also be necessary to re-drill the hole to be
>perpendicular to the now trued-up rear and front faces of
>the prop.

This is always the first thing to check. A tilted hub adds a constant angle to one blade and subtracts a constant angle from the other blade. The net result is that the measured pitch diverges towards the tips. 1/2 a degree of hub tilt means a bigger and bigger "error" in pitch as you get further from the hub.

If it *doesn't" diverge, and the blades are reasonably the same measured amount off on either side (like 5" all the way on one side, and 5.2 on the other) then it's not the hub.

Back in the days of old, when I was semi serious about F2D, I got some single blade props from Billy Hughes. These were based on some Free Flight props that he was making and were of combat happy pitches. I played with one quite a bit and had some very nice results. Finally lost it when I had a leadout break. I felt it was as fast (stopwatch) and it was faster through a number of lazy eighths (stopwatch) again. I felt there was an advantage there but was not really in a compete mode any longer so never followed up. These were statically balanced only.

Back in the days of old, when I was semi serious about F2D, I got some single blade props from Billy Hughes. These were based on some Free Flight props that he was making and were of combat happy pitches. I played with one quite a bit and had some very nice results. Finally lost it when I had a leadout break. I felt it was as fast (stopwatch) and it was faster through a number of lazy eighths (stopwatch) again. I felt there was an advantage there but was not really in a compete mode any longer so never followed up. These were statically balanced only.
Had me as an unregestered user:'(

Just last week while trying different props, I found one that was hard to needle and vibrated a bit. The 3.7 ran great, but the 3.8 gave me problems. On the way back from the field, I called Randy Smith to get his feelings on the symptoms. He suggested I check the pitch on both blades. Sure enough, one blade was 4.0 with the other at 3.7. I repitched the prop that evening to a 3.8 on both blades and it worked perfectly, and predictably.

Everytime I have found a prop that did not work, yet should have, it has always been that the blades were not pitched the same.

>Everytime I have found a prop that did not work, yet should
>have, it has always been that the blades were not pitched
>the same.

Agreed. The closer the pitch of all the blades, the smoother the prop will typically run both vibration and power wise.

LAST EDITED ON Aug-24-03 AT 03:27 AM (CDT)
 
>Back in the days of old, when I was semi serious about F2D,
>I got some single blade props from Billy Hughes. These were
>based on some Free Flight props that he was making and were
>of combat happy pitches. I played with one quite a bit and
>had some very nice results. Finally lost it when I had a
>leadout break. I felt it was as fast (stopwatch) and it was
>faster through a number of lazy eighths (stopwatch) again. I
>felt there was an advantage there but was not really in a
>compete mode any longer so never followed up. These were
>statically balanced only.
>Had me as an unregestered user:'(

Hi Steve...I had almost the same experience. I reasoned that that what was good for 1/2A Proto would be good for 1/2A Combat..and it was.The single blade advantage should be obvious..you get a bigger disk...greater thrust in turns...same pitch bigger diameter....However, I was using Dale Kirn design props, which has different pitch at different stations..I have the numbers somewhere, but basically, higher pitch near the hub and an optimum pitch 50-75%..then a lower pitch than optimum at the tip.. which gave a prop that allowed ferocious acceleration at model release and within a third of a lap was almost at full speed ..Dale had worked out the surface areas ahead and behind centerline that actually allowed the prop tip to flare and match the optimum pitch for the speed desired. This prop was designed for his specdially cast "Orange" left-hand two blade he used to sell. So I carved some maple single blades for 1/2A Combat with appropriate pitch values....I never got it right; And there's the Rub, It was too darned much work. I am still convinced that the large disk single blade can give superior acceleration and terrific thrust in turns as well as a chosen top speed. I have to say that my Tornado Black 5x3's, reshaped and sized yielded 71 to 74 mph when used on three different TD .049s.
By the way, how does a 315-320 sq" Sickle with a 12% thk airfoil ,powered by a .049 Fora sound to you? I'm thinking along those lines these days. Tony G.

I can see a single blade prop working better at speed. Here you want the minimum diameter needed to develop the needed thrust and the maximum pitch to develop the highest speed.

Reversing that, this may be effective in stunt and combat as well. Here More diameter produces more thrust while maintaining the speed that you want.

For pure stunt, you could get a greater diameter with the same pitch and rpm on the engine.

And for combat, well there are two thoughts, a little more pitch for a little more speed, using the same diameter, or a little more diameter with the same speed for a little more thrust in the turns. There would be an added side benefit which I have always wondered about: Greater diameter in combat gives a bigger "gun" with which to shoot at the other guy's streamer.

>may be effective in stunt and combat as
>well. Here More diameter produces more thrust while
>maintaining the speed that you want.

>And for combat, well there are two thoughts, a little more
>pitch for a little more speed, using the same diameter, or a
>little more diameter with the same speed for a little more
>thrust in the turns. There would be an added side benefit
>which I have always wondered about: Greater diameter in
>combat gives a bigger "gun" with which to shoot at the other
>guy's streamer.

I like your last point.
In combat, we also want to run the engine at an RPM giving peak
horsepower, at perhaps a bit greater. But we don't want the prop
tips to get too close to Mach 1. So larger engines are pushed
to 2-bladed props.

I'd like to see a geared 15 (adapted from free flight) for F2D.
I wonder if it could last an entire match?

> By the way, how does a 315-320 sq" Sickle with a 12% thk
>airfoil ,powered by a .049 Fora sound to you? I'm thinking
>along those lines these days. Tony G.

That sounds about right based on what I have seen for sizes for these engines. I would just do a Xerox reduction to about a 34 /36 inch wingspan. I just went out to the garage to look at one of the remaining 1/2A sickle and none remain! I think they were about 30" for a good Tee-Dee. I think I even used the same leadout position, an indicator that I had too much for the F2D version, but we have constant winds here in Dallas. I had a Gean Hempel reworked mark 3 fox on one and it did not blow up, an indication of to much structure for F2D, but it did fly good. Post how it goes

The two blades of different pitch are most likely acting against each other to help limit RPM. If it works stick with it.

Shortening the heavy blade to balance the prop work in the full scale world too. A friend of mine (now deceased) told a story about coyote hunting from a J-3, aiming a little to far forward, his gunner shot the tip off of one blade of the prop. This put the prop well out of balance. landing in the pasture and shutting down the engine they figured to balance the prop by shooting off the other tip. it took several tries , but it balanced well enough to fly it home after a very long takeoff roll.
Andrew Raney

I guess with modern engines the internal balance is about as good as it can get, and we get good results by carefully balancing our props. This is not always the case. With some older engines, particularly those with heavy pistons and unbalanced cranks, engine vibration can be quite a problem. A particular example in England was our BB Amco 3.5cc. (Arne Hende makes a good repro). This was a real heavy vibrator, but could be turned into a real sweet running engine by unbalancing the prop. I used to use Frog nylon 9 x 6 props, the trick was to crop 3/8" off one blade. With the engine upright, and setting the piston at bdc and the long blade vertically upwards, an amazingly sweet running engine resulted.

On purpose, Noel?

[photo not recovered: 3e69bb3870f12ad5.jpg]

"Could you please step away from the bar? You're melting all the ice."

IF, and that is still a big "if", different pitches on the two blades work, I would suspect that it is broadining out the region around stall. When one blade is going down the backside of the lift curve, the other one is still climbing up to the peak, and you get a long reigon of relatively stable thrust.

Just guessing.

Yes, Larry, on purpose. I buy props by the dozen, pick out the ones with different pitched blades, and send the rest back.

To address some of the other responses:

1) Its definitely not the same as a prop with pitch equal to the average of the two blades.

2) It dosen't vibrate badly.

3) I've worn out pistons and cylinders on my PA's, but never a bearing.

4) I will keep using it because it works for me.

5) To reiterate, I learned this trick from Windy. I didn't try it originally because Windy didn't have an explanation that satisfied my engineer's brain (as is frequently the case). However, over the years I've learned that while Windy may not always have an explanation that makes sense to me, he's almost always got the right answer.

Thanks guys,

LAST EDITED ON Aug-25-03 AT 06:45 PM (CDT)
 
Noel,

This is great! Would you please send to my home address all those substandard props. You know, the ones with the same pitch on each blade. I will of course be paying a relatively low price for the props. After all, they are rejects...

Would you mind including rejected maple motor mount blanks, those "heavy" chunks you won't use? (Yes, people, Noel weighs his maple mounts!)

Further, should you have any plugs which are missing the idle bar, send those along as well.

In advance, many thanks.

Dan Rutherford

(Late edit because I stupidly left out a whole word!)

>So I carved some maple single blades
>for 1/2A Combat with appropriate pitch values....I never got
>it right; And there's the Rub, It was too darned much work.

On the other hand, a case might be made that it might be less work to MOLD a one bladed composite prop than a two bladed one!

>>So I carved some maple single blades
>>for 1/2A Combat with appropriate pitch values....I never got
>>it right; And there's the Rub, It was too darned much work.
>
>On the other hand, a case might be made that it might be
>less work to MOLD a one bladed composite prop than a two
>bladed one!

Yep Isky! ...That was the plan, after I develpoed a workable prop I was going to have Dave Braun make several for me. Now that I'm gearing up for 1/2A combat, I may give it a try again, starting with a reworked Tornado black 6x3 single blade...easier to rework than carving a maple blank. The development stage is a difficult path though. each engine will require a specific dia/pitch for optimum performance. I refer to acceleration through turns and severe direction changes. I can pitch the prop to the highest speed that I can fly. On 35' lines that used to be 74mph(on a good day).
Tony

>>On the other hand, a case might be made that it might be
>>less work to MOLD a one bladed composite prop than a two
>>bladed one!
>
>Yep Isky! ...That was the plan, after I develpoed a workable
>prop I was going to have Dave Braun make several for me. Now
>that I'm gearing up for 1/2A combat, I may give it a try
>again, starting with a reworked Tornado black 6x3 single
>blade...easier to rework than carving a maple blank. The
>development stage is a difficult path though. each engine
>will require a specific dia/pitch for optimum performance. I
>refer to acceleration through turns and severe direction
>changes. I can pitch the prop to the highest speed that I
>can fly. On 35' lines that used to be 74mph(on a good day).
>Tony

As a follow-up to this thought, why not start with a carbon fiber prop with one blade cut off from the get-go and simply repitch it? You could do several (OK, it costs more money, but this is development work) and try them out one after the other. The best becomes "square one" and the other (one or two) get repitched in the appropriate direction to use as a comparison. The best one is kept again and the others repitched until you find the best. You can use the others to try slightly shorter blades and go at it again. Once you find the solution, it becomes the mold for reproduceable props. And these can always be "tweaked" to suit yours and others needs, perhaps with different engines or different planes.

>
>As a follow-up to this thought, why not start with a carbon
>fiber prop with one blade cut off from the get-go and simply
>repitch it? You could do several (OK, it costs more money,
>but this is development work) and try them out one after the
>other. The best becomes "square one" and the other (one or
>two) get repitched in the appropriate direction to use as a
>comparison. The best one is kept again and the others
>repitched until you find the best. You can use the others
>to try slightly shorter blades and go at it again. Once you
>find the solution, it becomes the mold for reproduceable
>props. And these can always be "tweaked" to suit yours and
>others needs, perhaps with different engines or different
>planes.

Yep Leonard, That's a good suggestion. if I can find a six" carbon prop in a workable pitch, I will.
The only carbon props in that diameter range I've found are Team Race props, but the pitch is all wrong ...and carbon props are very difficult to repitch compared to fiber filled such as Grish or Cox...but I'll give it a try if I can..Thanks
Tony

There's a variety of glass props made by the Russians for 1/2-A Combat. Driskill and Cleveland sell them. Might be a good basis for this project (see Winship's web page for a way to add extra diameter..)

How do you propose to balance one of these things?

See Par. 14. Propellers., Control Line, General, in the AMA rulebook. Construction is shown on the next page in Figure 4.