stunthanger.com
General control line discussion => Open Forum => Topic started by: Mike Alimov on October 27, 2008, 01:16:36 PM
-
It seems that the new series of composite props labeled for electric-only use have nice, thin, efficient blades. I've had a chance to closely examine my friend's 'electric' props made by APC and Graupner, and liked the shapes very much. The recent announcement by Cermark of their new scimitar ultra-efficient composite series props made me wonder: would it be safe to try those props on our stunt engines? After all, this is as close as you can get for $5/prop to the carbon-prop performance level.
I specifically have 4-strokes in mind, so I don't see RPM much exceeding 9000. Which, incidentally, about the RPM that electric motors run anyway. Centrifugal forces on the blades therefore should be about the same.
Anyone tried this yet?
-
The one factor you are not taking into account is the surge loading. There is a thread on SSW now about props and "building your own 4 blade" ( NOT a good idea by the way) There were numbers listed and a graph,that the power pulse is signifficant! and that is my personal concern with the electric props on a wet motor,,
-
These props are made to be lighter than IC props. I don't believe the hub area is as beefy, and when you drill it out to fit an IC crankshaft, there wont be enough meat left holding the blades safely.IMHO, I wouldn't do it.
-
I agree with Mark.
An electric motor delivers the torque almost continuously---whereas a 2 stroke glow engine delivers it only during a part of the power stroke, and when it is running in a 4 stroke, less than 1/4 of the time. Same for a 4 stroke glow engine. That's why we can get away with a much weaker front end than a glow setup.
The electric props are really wimpy around the hub. As you obviously see the difference between the APC electic and glow props, there must be a reason for APC to make them that way.
Of course you could join the future and use electric--that way you can use those E-props with a clear conscious! y1
-
I do not know how they would work for C/L, but the Sam old timers have been using them on 60 size sparkers for Texaco competition. They do run the engines way slower than full power to try to conserve fuel though. I would think the starting loads would be the biggest factor if you hand prop them. Using a starter would probably work OK. Having said that, I don't think I would use one.
-
The manufactures strongleylrecommend against it. Ever seen a blade blow off a running motor? I'm not interested in that experience, especially since I spend so much time in the target zone.
bob branch
-
I don't want to spill the beans as I have been waiting on an individual to come out in the open about electric props on IC engines. This person states that he as well as several others have been using them for almost a year. He also stated the props in use are rated at 18,000 rpm max. He also stated his engines have not exceded 11,000 rpm. But, because of an individual that does not fly stunt, called the AMA technical director for a ruling. It seems according to him they are not covered by AMA insurance if they were used on IC engines. I myself have not tried them yet. So far being a licensed CD and on the rules board have not received any info from the AMA. Later, DOC Holliday
-
I think most of us see max rpm loads and then leave our brains there.
RPM only considers the "centifugal" force load plus any steady state twisting that the prop may see. With an electric motor, that about covers it. An electric motor is providing it's torque pretty smoothly for 360 degrees of its rotation. A glow engine is at best providing a pulse of torque only during the power stroke. Of course the flywheel effect will keep the prop spinning, but it is beginning to slow down after the exhaust opens and especially during the compression. The power stroke re-accelerates the prop.
If you go to the SSW thread on the 4 blade prop, Brett Buck posted some plots of what those power strokes look like.
To be honest, these "idiots" who have probably been warned may rank right up there with the guy who flew his CL plane into the High tension lines and wiped out the CL handle company. I don't care if they would be the ones to suffer in case of an accident, but as you know it will be some bystander and us---when said bystander sues AMA and APC for this knucklehead's actions. I think the AMA should kick out people who commit such egregious acts of idiocy. May save us and the hobby.
Well that's my 2 cents! ~^
-
Hey Alan, tell us how you really feel,, lol y1
I agree totally,, I do NOT think that an electric prop, at least the ones I have seen , have NO PLACE on an IC engine. If you doubt this, look at the graphs posted by Brett, they tell the story.
as a teaster, here is a quote
>the instantaneous torque was a whopping 1600 in-oz, or 8
>foot-lbs. And this is a little wimpy ST46
so take that little E prop, hang a bar a foot long on it and loaad it with an instantanious surge of over 8 lbs. watch what happens to the blades.
-
What Allan said! y1
Guys, the people at APC are real smart, they know what they are doing. Like others here have said: "DON'T DO IT!!!" mw~
Using an Electric APC prop on a 4 stroke is about as irresponsible as letting an 8 year old use a machine gun! HB~>
-
Hey Alan, tell us how you really feel,, lol y1
I agree totally,, I do NOT think that an electric prop, at least the ones I have seen , have NO PLACE on an IC engine. If you doubt this, look at the graphs posted by Brett, they tell the story.
as a teaster, here is a quote
>the instantaneous torque was a whopping 1600 in-oz, or 8
>foot-lbs. And this is a little wimpy ST46
so take that little E prop, hang a bar a foot long on it and loaad it with an instantanious surge of over 8 lbs. watch what happens to the blades.
Or even worse in the case of the many more powerful motors available! The only chance you would have is counting on the likelihood of the props being greatly over-engineered and working into the safety margin. People have run electric props on regular engines and gotten away with it, but I would really think twice about it.
Brett
-
APC props for glow engines are designed to shed the tips first. I have never seen, and never heard of one breaking more than 1/3 of the blade in during a run. Usually it's just one or both tips, ~ 1/4 in., come off from too much supersonic operation. Once part of a tip comes off the motor slows quite a bit. The inner parts of the prop are way oversize to take the vibration and support the swept shape, with it's kink near the root.
Many of the electric props are reminiscent of the Czech(?) balsa/carbon laminate props coming from Belko(I think). They look well-made, but are very thin and fragile looking. From considerable testing(not in stunt) RPM is the biggest killer of props. Higher rpms will always blow a blade.
-
I guess the real question is, WHY there are plenty of good props out there that are designed for glow operation, why would you want to take this chance, they are decent props, but decent on an electric.