Making a model propeller out of steel

[cfellows]

I've built several model aircraft type engines and felt that a propeller would be more suitable than a flywheel. However, I also like the way a flywheel improves the low speed running of model engines. The solution to me seemed to be a heavy propeller, one that looked suitable for an aircraft engine but was still heavy enough for the flywheel affect.

Finding such a propeller in the hobby market is near impossible for obvious reasons... model airplane builders want as light a propeller as can be found which pretty much obviates using something like steel. So, in each case, I've made my own.

On earlier models, I used cad to draw a propeller template, printed it out, and glued it to a piece of metal with scotch spray on contact cement.

This works reasonably well, but there are a couple of problems. First of all, when cutting the propeller out of steel and also when grinding it to shape later on, it gets hot. This causes the paper template to come loose. It also makes it impossible to dip the piece in water to cool it off. So, on this latest propeller, I decided to scribe the outline directly onto the metal.

Perhaps the most important part of making a multibladed propeller is symmetry. All the blades should be identical, not only for appearances sake, but also for balance. So, I made a template of one side of one blade of a propeller.

It's made from steel stock about .1" thick, an inch wide and 4 or 5 inches long. It has a scribed line, on both sides, that define the centerline of the propeller blade. The curved part of the template should be a smooth curve, and on this propeller, not much metal needed to be removed. The curve only sweeps in about 1/8". The overall width of the propeller blade at it's wides point is 1/2". I then drilled a hole at what will be the center of the finished propeller.

Okay, I drilled two holes, but the bottom one was a mistake!
I used a carpenter square to layout the two perpendicular axes of the propeller, which, by they way, will have 4 blades
.

I drill a hole at the intersection of the two perpendicular axes so I can use a screw to line up the template with the propeller blank.

The template is first used to draw one side of all propeller blades. Then the template is flipped over and the other side is drawn for all 4 blades.

After all the lines are scribed, I used my bandsaw to cut the propeller out then used my belt grinder to grind up to the scribed lines. The next task, and I don't have any pictures of this part, is to mill out the waist at the bottom of each blade above the hub. This was pretty simple process, using a rotary table to index the blade and mill out the waist at the bottom of each blade.

Next, I clamped each blade in turn into my bench vise and used a crescent wrench to put about 15 degees of twist in each blade.

The last step is grinding the leading and trailing edges of each blade. Using a belt grinder, this is not a particularly difficult task since not a lot of metal needs to be removed. Here is an end shot of one of the blades toget an idea of the blade profile. Again, you don't need to remove a lot metal. It just needs to look like a propeller, not behave like one.

Here is a picture of my latest, finished propeller.

Chuck

 

[vcutajar]

Thanks Chuck for the writeup. It was always a mystery how people do these things and you have shed some light (actually a lot) on the subject.

Vince

 

[nemoc]

Nice job Chuck! Thanks for showing us how you made it. When will we see it on an engine?

Craig

 

[Brian Rupnow]

Chuck--Great work, as usual---but I must ask--What are the advantages of a propellor over a solid flywheel. Does the fact that it is moving some air put a bit of 'load" on the engine to make it run smoother? It just seems to me that adding a propellor increases the risk of removing a digit, which would be far out of proportion to any benefit derived over a solid flywheel.----Brian

 

[Bluecat]

Simply fantastic. Now all you have to do is paint the blade tips yellow.

 

[ddmckee54]

Yup - paint the tips yellow for safety, don't forget the anti-corrosion paint for the blades and you'll definitely need to add the de-icing boots.

Don

 

[ironman]

Chuck, that is a neat propellor. Be glad when it is painted and on an engine. Then we can see the *beer* video. Is it going on that V8?

Ray

 

[cfellows]

Chuck--Great work, as usual---but I must ask--What are the advantages of a propellor over a solid flywheel. Does the fact that it is moving some air put a bit of 'load" on the engine to make it run smoother? It just seems to me that adding a propellor increases the risk of removing a digit, which would be far out of proportion to any benefit derived over a solid flywheel.----Brian

Hey Brian, thanks for the kind words. The propeller is purely visual. Since it is supposed to be an aircraft engine, a flywheel just seemed out of place. And yes, it is more dangerous, but all the edges and tips have been ground and sanded smooth, so hopefully, if my fingers do get in the way, they'll just get a good thumping and nothing that draws blood!

Chuck

 

[blighty]

but all the edges and tips have been ground and sanded smooth

and so are the rotor blades on a rc helicopter and they will happily take your head if given half a chance.......... please be careful.


forgot to add....

in the UK metal prop's where band by the BMFA in the 50's and the AMA (U.S) does not allow them........ it's not because the weigh to much.

 

[nel2lar]

"OPPERATOR WARNING"

FINGERS BEWARE


Nelson Collar





May the Lord Direct Your Steps
"When injustice becomes law - resistance becomes duty."

 

[cfellows]

Thanks all for the kind words and the advice.

This is my third aircraft engine with a steel prop. I mostly I just run the engines at an idle and I never display the engines at shows or run them around children. I'm well aware of the dangers and treat the engines with the respect they deserve.

Chuck

 

[kf2qd]

I would not even attempt to try that prop.
#1 - What is the grain structure of that steel and how will it react to the constant impact when the engine fires. When it fatigues it WILL FAIL and you WILL HAVE 1 or more knife blades flying through the air.

#2 - a wood, plasic or fiberglass prop is flexible and thus the ignition pulses are able to be absorbed without fatiguing the prop. Even then one does on occasion fail in flight.

#3 - Metal aircraft props are made from forgings. Properly designed forgings have a known predictable grain structure. You blade from plate? I don't think so.

To add mass close to center of the shaft, make a larger prop hub which would also add some flywheel effect.

DON'T USE THAT PROP. It is as unsafe as anyhting we modelers could possibly create.

 

[archer3d]

Chuck you did a beautiful job with that the polish and finish is superb! as for safety other than the obvious which you are well aware of having it fail due to fatigue would be very unlikely with our size engines, they do not run hard enough or long enough for that ever to be a concern.

great work I think it adds beautifully to an aircraft engine as opposed to a plain round disk

tom

 

[rhitee93]

I have to agree with Tom on this one. I think some of us have forgotten what Chuck plans to put this prop on. Sure, any model IC engine that could turn a prop that heavy fast enough to get airborn would be dangerous. However, that is not the situation here. Chuck's engine is far less powerful than any aircraft engine that could effectively swing that prop. It won't be turning at flight speeds, nor will it be experiencing explosive ignition pulses.

 

[Fuelrush]

I like it. Its a good idea. I enjoy seeing engines run at idle. Treat it like a gas turbine, don't stand in plane with the rotation of the prop and you'll be fine. You could also turn a spinner (which can be quite large on some aircraft) out of brass and gain a lot of rotating weight if you wanted.