Is a one-bladed propeller 10% to 15% more efficient?

Based on stuff I've read in the past and can vaguely remember today, a one-bladed prop will give me an additional 10 to 15 percent efficiency. Is this optimistic?

From : Don Stackhouse

Probably VERY optimistic. I'd say the 2 or 3 percent numbers Scott Black posted on this subject are probably in the ballpark, although you might do a little better.

There is an optimum number of blades for any given airframe/engine/flight condition, and it isn't always one, or two, or six, or whatever. In general, if you are limited in diameter and high in power, more blades tends to be better. I think this particular case falls more towards the opposite end of that spectrum, so one blade might be appropriate.

I was thinking that I could maybe spin a 6.5 inch pitch prop at 20,000 RPM or so if I can find a really thin folder and just use one blade. The only problem is that I don't have any information on single bladed props.

At that rpm and the airspeeds typical of these models, you're looking at helical tip speeds around 400 mph for that example. This is not very high at all as propellers go, so you have no nearby mach limits on diameter to worry about. This gives you three options: more diameter (as Scott's post suggested), or more blade chord, or some of both.

More diameter usually helps efficiency, but not always. In particular, aircraft that fly at relatively high speeds usually benefit less from increased diameter than do slower aircraft. In any case, since power is related to the 5th power of diameter, you won't be able to change diameter very much.

The other big factor here is Reynolds number ("Re"). Yes, that same troublemaker that robs the performance from model wings (in comparison to full-scale) steals performance from model propellers too.

Consider the Graupner CAM Speed Prop. At 20,000 rpm, the helical speed at 2.25" from the shaft will be around 270 to 300 mph. The chord at that point is about 11/32", so the Re at sea level is about 80,000. This just happens to be about the same Reynolds number as the wing of a typical 1.5 meter R/C hand-launched glider in a thermal turn, something I just happen to have a little experience with! ;-)

For one of the popular published airfoils used on some R/C HLG's (none of mine, BTW, I design my own), going from a Re of 120K down to 80K doubles the drag. It doubles again going from 80K down to 60K. While other airfoils may do better, the point is that airfoils in this regime are very sensitive to small changes in Reynolds number.

This is also one reason why V-tails are so effective in small models; even though a properly designed V-tail has about the same total area as a conventional or T-tail of the same effectiveness, its area is spread over two panels instead of three. The resulting higher area per panel can be used to give the panels a larger chord and improved Reynolds numbers.

This same principle can be applied to propellers. Using a one-bladed prop concentrates the total blade area (which is relatively constant for a given power and rpm) into a single blade, which allows for an increase in blade chord and a corresponding improvement in Reynolds numbers. The improvement in performance from this may be better than you can get from a diameter increase. My guess is that a little of both will most likely be the best approach.

Sooooo, is there anyone who can supply some info on how to make one-bladed props (for speed - not free flight)? Or maybe you can direct me to a magazine article or a book? I know the control line guys have been using them in their speed events for some time.

There was an article in Model Aviation back in the late 70's I believe, that discussed the techniques used in 1/2A proto speed, where the records were all held by 1-bladed props. Typical rpm's and airspeeds for these models were in the same general region as our Speed 400 pylon ships today. The article went into quite a bit of detail on 1-bladed prop construction. I have that issue in my collection out in the barn, and I'll try to find it, but it might take me a LONG time. (If you saw my collection, just recently unpacked, but not yet sorted, you'd see why. I get about a dozen aviation magazines per month, ever since the 70's, and I don't throw ANY of them away!) If someone else can find the date of that issue, please post it!

The props used a maple blade with a brass counterweight on the other side. The counterweight had a slot in the side of it (like a piece of brass channel with a thick base), so that the root of the blade was epoxied into the slot with the brass webs on either side of the slot in front of and behind the blade root. The prop mounting screw went through both brass webs, plus the wooden blade root in the middle. They were about 1/4" bigger in diameter than the equivalent 2-blade props.

On a 1-bladed prop, the C/G of the counterweight has to be shifted aft (towards the motor on a tractor prop) to counterbalance the bending moment from the thrust on the blade. It also has to be shifted slightly towards the leading edge of the blade to counteract the drag of the blade (i.e.: the "timing" of the counterweight C/G has to be retarded slightly, in the direction opposite the rotation of the prop). Even so, it will not be possible to exactly balance the loads on the single blade, and you can only come close at one specific combination of rpm, airspeed and power. You WILL have some vibration to deal with, so you may need to use airframe construction techniques more like the ones used on recip-engined models.

Also I need to know how many watts will be absorbed by a prop with one blade. My 6.5 inch pitch example above is probably bad science on my part. Given that I can currently get my 4.7x4.7 to spin at about 19,500 what would be the most efficient one-blader that would absorb the same number of watts?

Can't answer that without considering the entire model. Even the shape of the nose has an influence on that. The Hamilton Standard propeller performance charts may help get you a reasonable starting point. They only go down to two blades, so you'll have to do some extrapolating. They're also based on full-scale propeller performance, so you'll have to throw in some allowances for Reynolds numbers.

As with other classes of powered pylon racing, the prop that does the best on the turns will probably not be the same as the best one on the straights. Your personal flying style also enters the equation at that point. Do you like to pull tighter on the turns? You probably need a prop optimized more for acceleration. Fly a wider, more elliptical course with a more constant speed? A prop with less acceleration but more top speed could be better.

Oh, well, at this point we've thrown in so many factors that the numbers are of questionable usefulness, and the Ham. Std. charts are almost impossible to get these days anyway (if you don't already have a set). At this point I suspect your best solution will inevitably involve a lot of carving and flight testing. Of course that means you now have a great excuse to do more flying!

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/