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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.
On a semi-related tangent: I've been watching the financial news concerning Briggs and Stratton since I got interested in these engines. It would seem pretty important that the company remain in business and healthy to keep these engines in production. Anyway, the company's stock (ticker: BGG) has been in quite a slump and now trades at $6.25 per share, less than half of its 52 week high of about $15. The company is closing a mower factory in Kentucky and taking some other steps to cut costs.
I was pleased to see this article that was bullish on the company's future, indicating that some of the trouble was due to one-time events (closure of Sears stores, unusually poor weather, tariff issues, etc). I still have concerns.
Note: I don't own any of this stock and have no interest in promoting it. I'm just interested in the overall health of the company, which may or may not be reflected in the price of this particular equity.
That is interesting.
Random data point: When my daughter’s family moved a couple of years ago, they left their gasoline powered riding mower in Florida and purchased a battery powered  riding mower for their hilly yard in Georgia.
Is that an electric riding mower? I haven't seen any, and I assume they are not inexpensive.
Yes, a riding battery powered lawn mower for a yard with steep hills. I edited my post above to clarify.
I was tempted by a cool Ryobi electric riding mower, you can talk yourself into it if you estimate a high enough cost for maintenance of a gasoline powered riding mower (esp annual maintenance done by someone else of the stuff under the deck-- belts and spindles, etc). The electric mowers have direct drive motors for the blades, they'll probably need less attention. Still, the out-the-door price for an electric rider is double the cost of a gas unit.
I suspect the cordless push and self-propelled walk behind mowers will be the first place where electric takes a significant bite out of the gas engine market. That's happening already if the display area at Home Depot is any indication of relative sales.
The 810cc vertical shaft engine that we've discussed a lot in this thread is primarily used in beefy zero turn radius mowers most often sold to commercial users. I don't think there's much risk of electric power making inroads there (though there have been a few attempts to sell electric ZTRs to homeowners). B&S should be making them for a long time.
This is the reason the 810, or equal, will be around for a long time. Much like our airplanes battery energy density just isn't there for commercial use. I mow my personal yard with a corded electric and have for well over a decade. Even though I have the tools and knowledge to maintain a gas mower the electric de-complicates my life.
I have a friend in the commercial lawn service and he has gone to all battery trimmers and blowers because his employees can't destroy them as fast as the gas. He has lots of batteries. I have both gas and battery string trimmers. I use the battery at home but need the gas for heavy stuff. Getting hard to find a good 4 stroke trimmer for a reasonable price.
I also have a gas rider that I can't even imagine how I'd replace with battery for the same reason my friend is still using gas mowers commercially. Battery might be fine for a small yard but for large yards, or commercial use, we still need the energy density of liquid fuel.
With the trend going to outsourcing and distributed use - like Uber, Lyft, and AirBnB - I expect battery powered equipment demand to be reversed and the percentage demand for gas going back up..............except for those progressive and enlightened cities where battery is mandated.
Vertical shaft mower engines will be with us far longer than the Corvair was, and probably the VW as well.
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