Discussion in 'Firewall Forward / Props / Fuel system' started by Hephaestus, May 12, 2019.
I am curious if the airspeeds are indicated or true? This could effect design choices quite a bit.
What are the parameters of a flexible propeller, 3600RPM flattest pitch @ what RPM is coarsest pitch and what would the pitches be? How do you design a flexible propeller that has predictable pitch variation?
Also putting the "P tip" on a propeller is supposed to effect its radius about 2" so a 42" propeller with the "P tip" would have the performance of a 44" propeller. Does anybody understand how to incorporate a "p tip" onto a blade that works?
Short answer: It is a little ambiguous, but I think the thrust numbers will correlate most closely to TAS.
Long answer: Per the (too long) text in my earlier post, I started by getting an "on design speed" calculation of propeller efficiency from Jan's calculator. This required that I put in an airspeed for which the prop would be optimised, but Jan's program doesn't specify if this input is supposed to be TAS or IAS. The output thrust graph his program provides is labelled "TAS," so (to me) that's an indication that TAS is expected in the entry form.
After I got the prop efficiency, I used a formula and a graph of off-design prop efficiency corrections to get the off-design expected thrust. The design->actual conversion is definitely a function of TAS.
As a note: The relationship of on-design propeller efficiencies to off-design propeller efficiency is a function of "advance ratio" ("J").
J = aircraft velocity / (RPM * prop diameter). As "J" changes from the "J" that the prop was designed for, efficiency declines (this is just the compromise we make with a fixed-pitch prop). Notice, though, that if our velocity changes by 10% (say, from 100 KTAS to 110 KTAS) but we also increase our RPM by the same amount (say, from 3200 to 3520), then "J" remains unchanged, and our prop is still the "ideal" prop for this new airspeed.
The prop efficiency conversions that I did in that previous post accounted >only< for a change in aircraft velocity. In this respect, they may be overly conservative since when we are flying faster it's likely we'll also be at higher RPM (and the converse when going slower). So, we may get more thrust at off-design airspeeds than I estimated.
This is all a bit like using a micrometer to measure a marshmallow. We can get more refined when we know max torque numbers at various RPMs.
While I can't help much in answering your question, I think Lonnie Prince (owner of Prince Aircraft Company, manufacturer of the P-Tip props) can provide useful assistance when we have enough baseline info for him to work from. I suspect the "coning" and pitch change functions of his props is fairly subtle. The PAC web site claims that the pitch varies approx 4" from takeoff to cruise, but I would think this might also be different for different diameter props. For perspective, according to Jan's program the ideal pitch for a 42-47" diameter prop at 3600 RPM and 60 MPH (climb?) is 19", and at 100 MPH (cruise?) it is 31". So, a 4" change by the P-Tip will be a help, but nobody should expect that it'll be perfect.
Lonnie Prince has experience with small props (his company makes props for small UAVs, too). I know that he's been diligent in helping folks who were installing the Aerovee Turbo package and needed a lot of adjustments/pitch changes. I'm not advertising for him, but he does have a good reputation in the Sonex community.
When M. Colomban was working on the Luciolle I know he spent a lot of time working to get the prop right, and it seems likely that MiniSport (the SD-1 folks) have done development work on whatever prop they are fitting to their B&S derived engines, too.
When I think about a flex prop I think max hp and max RPM = max flex (finest pitch) and when you back off the power you eventually get coarsest pitch at whatever power and RPM that is. It seems logical that a short thick prop will flex less than a long thin one as well. So for these engines we may get less than 4 inches of pitch change but anything will be better than none considering the RPM band.
For these engines to work well the propeller is going to be as critical as the cooling.
I have pondered a twisting prop, but have moved it to my after-it's-flying list...
There may be different types of flex props. From the description on the PAC site (on their FAQ page). In part:
According to this explanation (as I read it), the unflexed "relaxed" state of these props is coarser pitch and seen at higher airspeeds. They "flex" to a >finer< pitch when the blades are loaded up at lower airspeed (in response to the pull of the prop, the blade tips arc forward and this "untwists" some of the pitch that was carved into the blades. I see how that could work, but I can't say from experience that it actually does).
Yes, the prop will be important. For uses similar to the SD-1 or Luciolle (relatively clean airframe, 65 kt climb, 90 kt cruise), the props already in use by these planes and fitted to these engines would be the best place to start--or finish--the quest. But for folks doing something different (e.g. "next generation DA-11 " cruising at 130+ MPH, or a draggy slow plane, or a multi-engine design that has hard-to-accomodate requirements for good thrust at both low airspeed (SE climb) and high airspeed (normal cruise), etc)--they may need to do something different.
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