Prop Pitch

[Motorman]

Airplane flys 30 mph and engine turns 4K, what pitch should the prop be for straight and level flight?

Thanks,
MM

[Tim Wescott]

That depends.  "Pitch" is a nominal measure, and there's this thing called "slip" to complicate things.

For a platonic perfect propellor (Howard Rush's "knife through cheese") slip = 1.  For a stopped airplane and a working motor, slip = infinity.  Reality is somewhere in between.  And, pitch is a nominal measure.

slip = (airspeed)/(prop rotational rate * pitch),

pitch = (airspeed)/(prop rotational rate * slip)

30mph/(4k RPM) = 7.92 inches.

So -- eight to sixteen inches.

Whachu doing with something so slow and with a slow shaft speed?  Going to break your own endurance record?

[Howard Rush]

There is no simple answer.  Just put a prop on and see how fast it goes.

[Al Ferraro]

 I would start with 8

[Tim Wescott]

I would start with 8

Oh right.  I left out the simple part -- me too.

[Motorman]

I've got to try one of these but I think they're more for hovering? They say it's good up to 7200 rpm.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHAP2&P=7


MM

[Kim Doherty]

I've got to try one of these but I think they're more for hovering? They say it's good up to 7200 rpm.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHAP2&P=7
MM

These props are for electric power only.

Kim

[david smith]

I could definitely be wrong but my little bit of knowledge tells me that I don't think that prop from tower will get you 30mph at 4000 rpm I think you will need closer to a 10 pitch unless the plane is really really clean and air is perfect you might get close. By my figuring for an "average plane" with that 7.5 pitch tower prop you will probably be around 24mph.

[Motorman]

These props are for electric power only.

Kim

What if your engine turns slower than an electric?

I found a nice Grish Tornado 9-8 in my stash that looks similar in blade area to a slo-flier prop except it has a thicker root. I'm going to do some rpm testing today with the new induction system.


MM

[Mark Scarborough]

I've got to try one of these but I think they're more for hovering? They say it's good up to 7200 rpm.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHAP2&P=7


MM

if this is intended for a glow application or any IC application just do NOT use this, just DONT

[Mark Scarborough]

What if your engine turns slower than an electric?

I found a nice Grish Tornado 9-8 in my stash that looks similar in blade area to a slo-flier prop except it has a thicker root. I'm going to do some rpm testing today with the new induction system.


MM

an electric will turn from 0 rpm to the rpm limit, so not sure there can be anything slower than electric.
the problem with that prop is the hub design is not engineered to take the pulse loading of a combustion engine. EVERY power stroke of the glow, or combustion engine, accelerates the prop radically and then decelerates it on the off power stroke,
simpley put, do NOT use this prop on anything but an electric period end of story

[Brett Buck]

I've got to try one of these but I think they're more for hovering? They say it's good up to 7200 rpm.

  DO NOT run this prop on an internal combustion engine at any RPM. It will take a tiny average torque from the engine to turn it at 4000 rpm (maybe 10 in-oz) but the torque impulses as it fires and then coasts down between strokes might be 1500 in-oz, 66 times a second. This will lead to in-plane distortion of the prop as the hub gets first ahead of the blades, then behind, so you get a rapid and very large bend in the blades, which will very rapidly fatigue the plastic. An electric motor, which is what this prop was designed for,  will still have some cogging-type torque but much less at each impulse.

     Brett

[Motorman]

I was more interested in the design than the material. The material can always be changed. They say the GWS is better than the APC and it looks beefier in the hub area. Can't find the size I need though (9-8). This is a 10-9 that could be reduced to 9" dia and so would have a thicker root than one made as a 9".

http://www.hobbyking.com/hobbyking/store/%5F%5F27340%5F%5FGWS%5FEP%5FPropeller%5FRD%5F1080%5F254x203mm%5Fblack%5F6pcs%5Fbag.html

MM

[Dan McEntee]

   You just gotta be crazy to even consider even a copy of the electric prop even if it is carbon fibre, kevlar or whatever. If something like that would work or have any kind of advantage, it would be on the market and IC props wouldn't all look like they do. Especially APC brand. Just not strong enough at the hub.
    Type at you later,
   Dan McEntee

[Motorman]

That's ok Dan, try to imagine a Cox .020 with a 9" prop and you'll get a better picture of what I'm doing.

MM

[Dan McEntee]

   As 1/2A R/C Texaco progressed, guys resorted to running 10-3 props on .049s to stretch out the fuel run. For those who don't know, 1/2A Texaco is an R/C discipline where one flies replica old time free flight models down sized for .049 power, guided by two channel R/C, and timed to a 15 minute max. The 10-3 trick stretched the fuel run out immensely, so one would have to rely less on thermal catching ability. It was a very slow run and took almost the whole 15 minutes to get any appreciable altitude. The practice was later banned and the fuel capacity was cut to a normal size baby Bee tank and prop size limited to 8 inches, I believe. I don't think you will ever get a .020 to swing a 9" prop, even with low or no pitch. At least not well enough to make any kind of useful thrust. to test a theory, just bush out any wood IC  9-4 or 9-3 prop, if you can find any, bolt it on and see what happens. Probably a lot of popping and not much else.
   Type at you later,
    Dan McEntee

[Motorman]

I'm not using a Cox .020 I'm saying very low power swinging a 9-8 at 4,000 rpm or hopefully less. A CF prop that would break would have to be paper thin.

MM

[Pat Chewning]

I'm going to make a request, identical to the requests I got when I proposed to fly my C/L model with one handle on each line:

Please get a video if you try this.

[Pat Chewning]

I don't see why a cox .020 couldn't be able to spin the prop at a lower RPM, if you attach a gear reduction to it.

Also, if you want to reduce the torsional vibration in order to use an electric prop, you could add a pendulum damper to the output shaft like this:

http://www.epi-eng.com/piston_engine_technology/crankshaft_torsional_absorbers.htm

[Dan McEntee]

I'm not using a Cox .020 I'm saying very low power swinging a 9-8 at 4,000 rpm or hopefully less. A CF prop that would break would have to be paper thin.

MM
[/quote

   But that's what you aid to "imagine."  That's small displacement and not much power. If it's something in that power range, the 8 inch pitch would kill it. To do what you are talking about, you need something that is putting out at least 80 to 90% of it's power at 4000 RPM to handle that much pitch, and that pretty much eliminates anything direct drive I think. Sounds like you have too many things working against each other here. One of them will be line drag.
  Type at you later,
   Dan McEntee

[Tim Wescott]

Dan, he holds the record in CL endurance, unless it's been broken recently.  I think he should know how to modify an engine to run slow and steady with a big prop.

[Dan McEntee]

Dan, he holds the record in CL endurance, unless it's been broken recently.  I think he should know how to modify an engine to run slow and steady with a big prop.

     Then why did he ask the question? Kind of talking in riddles. First he says Cox .020, then says it isn't! Why not just say, "Hey, I'm working on an endurance engine. It's this size displacement and I want to try to do this with it." It removes all the mystery. He obviously scared a bunch of guys when considering a slow slyer R/C prop, even for his intended purpose, which he didn't disclose. Even at just 4000 RPM a carbon fiber prop blade can do a lot of damage if it's thrown off. A little more information would have gone a long way. It's like going to the doctor and saying, "My arm hurts, what's wrong with it?"
   Type at you later,
    Dan McEntee

[Motorman]

Well, the engine is a P.A.W. Texaco 1.49cc diesel.

I ran it today on some junk fuel from years ago basically to see if the new reed valve induction would work. My thinking was the reed would match the rpm where drum induction is fixed. Can also use pusher props.

It worked well and was able to get it down to 2200 rpm with my smallest pin hole venturi on the tornado 9-8. Fuel consumption was 1cc in 1 min. 30 sec.

There was almost no thrust and it wouldn't hold the setting. I believe because there's not enough heat at that power level to support combustion so you have to run rich and over compressed to keep it lit and it didn't like it. If I went too hard on the compression it would get a knock like some kind of hydraulic lock. Not enough airflow to atomize the fuel?

I put on a big venturi (.0625") and everything went back to normal, ran steady and responded well to settings. Engine turned 5,000 rpm and used 1cc in 1 min. 10 sec. and it put out what seems like enough thrust to fly.

That Tornado weights a ton, I think I need some kind of light weight 10x8.

MM  

  

[Tim Wescott]

Does more exhaust restriction help?  I would think that to really get one to be efficient and slow you'd want to re-time it for less duration on exhaust and transfer -- but, I'm just letting wild theories whirl out of my head.

[Motorman]

The pressure release is being controlled. One of the problems of holding the engine back with exhaust restrictions and a big prop is reversion. The whole lower crankcase area is rather sooty.

MM

[Randy Cuberly]

I don't see why a cox .020 couldn't be able to spin the prop at a lower RPM, if you attach a gear reduction to it.

Also, if you want to reduce the torsional vibration in order to use an electric prop, you could add a pendulum damper to the output shaft like this:

http://www.epi-eng.com/piston_engine_technology/crankshaft_torsional_absorbers.htm

Pat,
That's a very interesting article.  I'm not sure if it's applicable to what Motorman is doing but I enjoyed the article.

Thanks.

Got me interested in doing a little analysis.

Randy Cuberly