So, how much direct drive thrust should we expect from the 810cc engine?
Disclaimer: I’m not a propeller expert. The “method” below may or may not be valid.
As the point of all this tinkering with the engine is to make thrust for an airplane, it’s reasonable to ask how much thrust we can expect. For the purposes here, I’ll assume we want to know how much thrust we’ll get at 2000’ MSL from an engine rated at 30 HP at SL, and driving a fixed-pitch wooden two-blade prop at 3400 RPM. I know there’s reason to believe that the engine will produce more HP than that at higher RPM, but this may be a conservative appraisal of what the engine can make on a continuous basis.
Methodology: The table contains estimates for several propeller lengths and design speeds. In each case I used Jan Carlsson’s propeller design program to give the expected efficiency of the propeller at its design speed. I was not able to make the program give me reliable projections for a propellers at airspeeds other than the design speed, so I estimated “off design speed” propeller efficiency using a graph from Raymer’s Aircraft Design: A Conceptual Approach (Ed 1, Fig 13.10). That lookup table, made from the published graph, is in the spreadsheet attached.
In general, relative to its design airspeed, a propeller loses efficiency fairly slowly at airspeeds below its design speed, but loses efficiency rapidly at airspeeds above the design speed (at airspeeds above its design speed the propeller blades are at low AoAs). For example, if a particular prop has a design speed of 100 MPH, it might be 75% efficient at that airspeed. At 30 MPH below its design speed, 70 MPH, its efficiency is reduced to 0.82 of its 100 MPH design efficiency, so its new efficiency is 0.75 x 0.82 = 62%. Now, if we go the same 30 MPH above its design speed, to 130 MPH, the prop is just 50% as efficient as it was at its design speed (so, 0.75 x 0.50 = 37% propeller efficiency).
After finding/estimating the efficiency for the prop at various airspeeds, I found the thrust by applying the following equation:
Thrust (in lbs) = Engine power (HP) x propeller efficiency x 375 (a constant) / airspeed (MPH)
The tyrannical term in that equation is the divsion by the airspeed at the end. Because of that, thrust declines with increasing airspeed even for propellers designed for higher speeds.
I’ve attached an excel spreadsheet in case you’d like to sort things differently (don’t blame me if it falls apart!) or catch my spreadsheet errors. The PDF version may be useful if you just want to look at the numbers. The process I used has a lot of manual steps, which means it is both slow and prone to error.
Observations from the estimates:
- Though these 42-28" props are short compared to "regular" GA aircraft props, because of the modest HP we are using, they have very reasonable propeller disk loadings (lower than a C-152, etc). Even at modest climb speeds, at these RPMs there appears to be little gain from going with long propellers. If these numbers are right, it looks like most people choosing these engines and running direct drive to climb and cruise a small sportplane at 60-120 MPH will probably choose a 44"to 46" propeller.
- As noted above, prop efficiency (and, especially, thrust) numbers fall off relatively rapidly above a propeller's design speed, but the efficiency (and esp the thrust) hold up somewhat better at airspeeds below the propeller's design speed. For folks looking to balance a desire for higher cruise speed and good climb rate, this would argue for choosing a propeller optimized for speeds closer to the cruise speed than the climb speed. OTOH, from a safety standpoint, acceptable climb rate almost always trumps cruise speed. So, more compromises . . .
If anyone has prop design/thrust information for this engine (or other small engines) in direct drive that we could use for comparison, that would be great. I believe MiniSport uses a Helix 48” composite prop on their SE-33 engine, but I’m not sure of the pitch or of the thrust they are getting in flight. M. Colomban reportedly went through quite a development effort to design the Arplast propeller used on the MC-30, but I’m not sure of the specs on it, performance, etc. The data here is “open loop”--based on Jan’s propeller calculator and some extrapolation for off-design speeds. It would be great to get some real world numbers.
Notes:
– The prop planform is the “Jan Carlsson Standard”
– The prop pitch optimization I chose in Jan’s program is “standard”, so, it is coarser than a pure climb prop but finer than a pure cruise prop .
– The tables show the blade width at 75% span because the propeller blades designed by Jan’s program get quite narrow (and thin) at longer prop lengths and these relatively small HP levels. Some people may choose to give up a little bit of efficiency to get a propeller that is more robust.
- The estimates are for a 30HP (at sea level) engine taken to 2000’ MSL (so, producing 27.9HP) at 3400 RPM. The propellers are wooden, 2 blade and (obviously) fixed pitch.
- Jan’s calculator does not provide thrust values below 40 MPH, and traditional methods don’t do well at those low speeds. So, for takeoff roll, etc we’ll have to use other means to get thrust estimates. Even a completely stalled blade produces thrust (though inefficiently).
- For many small airplanes, rate of climb is very sensitive to the available thrust. Even a few pounds difference in thrust levels can significantly change the climb rate. My data has some obvious glitches (e.g. I can't explain why the props designed for 70 MPH are shown as being more efficient at 60 MPH than the props designed for 60 MPH. The method I used probably also overstates the 60 MPH capabilities of the higher-pitched props at 42" diameter--surely all of the 42" diameter props have efficiencies of less than 35% at 60 MPH). The estimates here (and elsewhere) may be useful for planning, but getting real numbers in flight will be critical.
