Bad News & Good:

* APC is not willing to make an electric 3-blade.  This is unchanged from what they told me in the past, only they were a little more adamant about it this time!

* APC suggested what could be a very good alternative.  They apparently developed a special prop for R/C Pattern Flyer Chip Hyde.  Chip was interested in good static thrust and good braking.  In his case they DID try 3-blades but they did not work well.  What did work was a new wide blade wide blade design.  Chip's using a 24" diameter, however APC proposed to applying the same design philosophy scaled to a 13" dia prop.

* This sounded like a viable alternative and I gave them the go-ahead for prototypes of a new 13x5.5E Pusher Wide Blade.  The new prop will be based on the 13x4.5 EP, with the blade profile widened - it is not a linear scale-up.  The pitch will be increased to 5.5".  That should make it a low 9000's RPM prop.

While this is not the 3-blade people were asking for it should have design elements that work in our favor:
* The 2-blade design will be more efficient than 3 blades.

* The wider blade fills up the disk area - mimicking what we achieve with a 3-blade design.

* 2-blade will be lighter than the 3-blade would have been.

* The new design taps into APC's design experience on the wide blade design for pattern props.  From checking their web site, APC makes a wide blade electric prop in 17" - 22"; the 24" prop they mentioned to me must be a new one.  I am hoping our RC Pattern & IMAC voices can check in on this point and affirm (or counter) whether the wide blade props have been a success for those folks.

* Finally, from a purely pragmatic point of view, APC is willing to make the 2-blade; they are not willing to make the 3-blade!

* The lead time on the prop will likely be in the fall, the prove-out samples later than that.  NOT what we wanted to hear but we are waiting in line behind several other APC projects.


Back in the IC engine days (!!!) lots of empirical data - and opinions - were generated on wide blade versus narrow blade props.  I suspect a lot of those rules will apply EXCEPT with a governed electric the braking effect for wide blades should be even more powerful.

Dennis, I do not know what exactly they call "efficiency" but if they say that the wider blade has better efficiency, then I am on doubt 

3 blade prop has 3 blades making drag, but also 3 blades making thrust, so the efficiency is very similar to 2 blade of the same blades. However 3 blade prop can be replaced 1" larger 2 blade prop of the blades which are longer, but has aproximately equivalent width, and in that case we WILL come to better efficiency, because drag of blades when prop makes lot of thrust is dermined by induced drag on its tip and that is determined by aspect ratio of the blade. That is reason why multiblade props are typically less efficient, but if we use prop of the same aspect ratio, we will have similar efficiency, and if they say the wider blade will reach better efficiency, then it is hard to believe ... may be only some kind of troubles with RE number can make 2 blader better, but I think we have those props large enough and rotating somewhere at 10 000 what is much higher than r/c flyers use to do with those plastic APC, so I do not think it is really a problem.

I use apc wide two blade wide props on glow 3D planes and they work extremely well. I am usually working with full rpm in the 9000 rpm range the way I prop glow motors for this, using large diameter and low pitch. They give dramatic thrust and excellent braking. My guess is the pattern profile is a bit different, but maybe not. Dennis, can you get a model number on the pattern prop so I can compare?

There are lots of ways to define efficiency as Igor suggests. Several years back during the period of time we did not have a timer that talked to the CC esc's Rick Sawicki and I did quite a bit of prop testing of basically anything we could get a hold of to try to find props that would work in the lap time ranges we needed for the axi 2826/10 and /12 motors we were using. If you look at thrust vs amp draw, the apc E design is very difficult to beat. It had dramically lower amp draw than glow props that generated the same lap times.

bob branch

There are lots of ways to define efficiency as Igor suggests.

I thing Igor is using the standard engineering definition of prop efficiency.