Certain undercamber shapes can cause tremendous drag at certain speeds. I would try slightly more diameter.

  Sort of - its more that they have greatly reduced drag with higher AoA. It's called the "Drag bucket". You can use this effect to control the speed and engine response to cause the engine to run at very low AoA in level flight, with relatively high drag, and have the drag go down drastically when the airplane slows down and AoA rises. In fact, it can be far too effective in some cases.

     Brett

I've been using the RSM props for a while, very similar feel to a REV-UP EW but I notice the RSM accelerates quicker on takeoff, maybe the motor breaks a bit harder. Lap times are essentially the same. I use lot's of LA's and 35S motors and the RSM worked well on a FOX too. For fun knock around or practice planes I'd say they're great props. Not a Bolly or Eather but a great sport prop at a decent price.

     That appears to be a function of the increased effective pitch (which is the next step in the "speed control" thread...) more than anything interesting about undercamber per se. In almost all cases "undercambered" and "more camber" are synonymous, and the only important effect. When you measure a Bolly (with a semi-symmetrical airfoil) and an U/C Eather, you might get the same measured pitch. But the Eather will have to turn much slower to get the same speed, or pull the airplane faster same RPM. That's because the U/C increases the camber compared to what you have measured, so depending on how you look at it, either has more lift at a given angle of attack, or, easier to visualize, requires less AoA for a given lift - which is exactly the same as increasing the pitch. Stuart Spurlock calls it "experimental pitch", I called it "effective pitch" in the SSW threads on the same topic. A "4 inch" Bolly will have to spin *much* faster than a "4 inch" Eather UC to get the same RPM. You could accomplish that effect just by making the airfoil thicker to increase the camber.

   For all intents and purposes, the camber alters the pitch from what you measure, and it also alters the effective pitch distribution, because 5% camber changes the effective AoA by the same absolute angle, but the prop blade angle needs to vary from root to tip - so more camber causes the effective pitch to be increased much more a the tips than the root.

    The only really tricky part about undercambered props is the "drag bucket" that Igor realized was also happening at the same time, where you actually have less drag on the prop at some positive Cls than you do at a Cl of 0. This drove me crazy when I was trying certain combinations of prop and engine, and I think it affected Paul Walkers experiments with the same combination fo engine and prop. The drag falling into the bucket had very unexpected effects on the engine response, and engines that respond to load very aggressively (like the the 136-degree exhaust timing engines like the PA, the PA40 in prticular) did all sorts of odd things that we had not found with engines with weak response to load (like 144-5 degree engines like the VF) when coupled with a UC prop.

     Until Igor realized what was going on and posted it on SSW, I had no real understanding as to why certain combinations of prop and engine setup were doing what they were doing. The Beringers used a different effect (load-variable pitch) to create a similar situation with engines that have nearly no response to load, 4-strokes, and people are still doing it with governor-style electrics. People have known about "drag buckets" since the 30s, but I had never realized that this was the underlying problem I was having trying to get my PA40 to work like a 40VF.

   Brett

Brett,
Where is the "speed control" thread? I am interested in following it as I like fooling with props.

BTW, I took one of my Brodak 10x5's and sanded it rear undercamber centered at the back 20% of the blade width. On my electric thrust test stand compared to stock at 9500 rpm it picked up 5% additional thrust with about the same increase in amp draw. I need to fly it to see how it pulls through the vertical eight to see if it has any advantage of being able to fly at a slower lap time while still penetrating wind.

   That is probably almost entirely due to increasing the effective pitch, since you increased the camber. Obviously, it didn't fall into the drag bucket. That is likely to happen in flight, when the AoA goes down (see this thread, where I am laboriously and tediously laying out basic prop effects: https://stunthanger.com/smf/engineering-board/effect-of-wind-on-maneuvers/ ) , which will almost certainly be beneficial if you have a governor-only system, and may have more complex effects if you have an Igor gadget.

     Note in the case of the governor-only system, suddenly reducing the drag helps only because the governor has a finite bandwidth, so the rpm will temporarily speed up when the drag is suddenly reduced, like in a corner, before the governor can respond. Sound familiar? George Aldrich liked it in 1951...

   Brett