This is why I went 84" for mine. My airplane will see 10F frequently, and the 84 from Mark was pretty cheap.

schu

whee wrote:
The two prop shops I've talked to agree with Halestorm on how a CS prop 'should' be set up; just touching the fine pitch stops at max static RPM. Once the plane starts rolling the governor takes over. This may not be reality because if the engine turns too high of static rpm mechanics will lower the rpm by adjusting the governor. How often that happens I don't know and neither did the prop shop I asked.

If they are adjusting the RPM by adjusting the governor, there's a good chance they are doing it wrong. For a hartzell top prop you adjust static rpm by moving the high rpm stop on the end of rhe prop hub. This ensures you are on the stops at max static rpm...

Sent from my SM-G870W using Tapatalk

Whee, very much this thing:

hotrod180 wrote:A quick check on wiki indicates that all but one version of the IO-360 Cont make 210hp at 2800.
I know you didn't ask, but I would gear my prop choice based on turning up to that speed,
since that's where the max power is.

Be careful comparing the IO-520 vs IO-550 example here. Those are two different engines that likely have pretty different power curves.

The two major components of thrust from a prop are the efficiency of a particular prop at a particular RPM, and the amount of horsepower available to the prop at that same RPM.

Since most of my flying takes place in the summer I'm not sure it makes sense to shorten the prop so it isn't so noisy in cold temps, but maybe it does. This is something I've been thinking about for a while. The performance difference will probably be lost in the weeds.

I'll try to find a load cell and do some tests when I finally get the plane together.

Every prop has a designed peak efficiency at a specific RPM and HP. Prop manufacturers don't like to publish that info because it may cost them sales for varying aircraft applications. Any prop that will bolt to your flange will fly the plane. But the amount of experimenting it would take to find the prop that has its peak efficiency at your HP and RPM specs will break the family budget.

So here is what we have and I'm interested in peoples thoughts.

The engine on my BH is a Continental IO-360KB (2600max rpm). I've installed a prop governor off a IO360-D (2800rpm) and adjusted the fuel system to -D specs. Basically equivalent to the Skyhawk XP Isham STC to increase the XPs horsepower rating from 195 to 210.

The propeller on my BH is an 88" McCauley C203 Seaplane prop off a C180G. Prop is in good shape and no changes/adjustments were made.

My engine will only turn the prop at 2550-2570 rpm static. On takeoff roll it takes a couple seconds for the engine to turn up to 2800.

I don't plan to change anything for a few hours but I'm trying to decide if I should trim the prop to 86" or 84" or leave it be. Maybe I should adjust the fine pitch stops so I can get 2800 rpm static and leave it full length.

Why wouldn't you just adjust the stops? I can't imagine there is a downside unless you find out the tip speed is just sucking away the extra power.

lesuther wrote:Why wouldn't you just adjust the stops? I can't imagine there is a downside unless you find out the tip speed is just sucking away the extra power.

Top speed would be the other stop.

Just typing "88in prop at 2800rpm" makes me cover my ears. In a good way.

This is a fun discussion. So prop speed of .92 mach is actually quite high; you are into transonic flow at this point, and while you haven't technically exceeded a rotational tip in excess of Mach 1, the local flow velocities are almost certainly exceeding it as the air accelerates over and around the tip. Also the air starts to become compressible in the transonic regime, so even absent a fully developed shock wave, you are dramatically increasing drag.

This also fully ignores the effects of helical tip speed. Unless you plan to sit in one place and run your engine at max RPM all day, your airplane is moving forward at some velocity. Static RPM is a neat starting point, but the airflow over your propeller also has a tangential component produced by the forward motion.

Basically the tangential velocity will effectively increase the tip speed, meaning that if you didn't bust Mach 1 on the ground, you very well may in flight. What happens when you get transonic/supersonic? You expend a lot of fuel to make more noise with little if any practical benefit. I've witnessed this with a local C180 watching him fly from a spot adjacent the runway. As he accelerates through the climb on takeoff, you can distinctly hear his prop go supersonic without any increase in RPM at all. So the big prop helps while starting out slow, but prop efficiency falls off a cliff as the plane continues to build speed unless you pull it back.

For those interested, I have a Google spreadsheet I can send you (PM me please). It takes your prop diameter and OAT as inputs, and spits out the helical speed in ft/sec and Mach as a function of RPM and true airspeed in a chart. I'm about to data log the shite out of my 182 with an MT prop and get real world comparisons of best power against helical prop speed, using this chart as a starting point, because empirical evidence to back up the math is always nice to have (more to follow soon on how I'm accomplishing that part). Anyone else wanting to mess with the spreadsheet is welcome to do so, I just don't want to post the link here until I get it into website form.

Pponk's tip speed calculator says an 88" prop at 2800 rpm
results in a tip speed of .963 mach on a 59 degree standard day.
An 84" prop under the same conditions shows .919.
Even on a 100 degree day, the 88 incher is at .926.
The sweet spot is .88 to .92 mach.
I'd think about going shorter with the prop (first choice) or lower on the rpm (choice #2).

https://pponk.com/props/#1463774934977-1d7f57b4-6b2b

asa wrote:Just typing "88in prop at 2800rpm" makes me cover my ears. In a good way.

It is painful to stand near the airplane when doing a static run up.

Colopilot, It will be interesting to see your results.

I've very familiar with the pponk calculator and the normal recommendations. The things that make me scratch my head are examples such as the 90" prop available for C185s and the MT prop put on the C185 but the STC limits the rpm to 2700 and claim that it pulls harder at that rpm than at 2850.

Today I made an adjustment to my fuel injection and had to do full power run-ups. The engine is pulling 2600rpm static. When taking off there is no delay in achieving 2800rpm when I push the throttle in when rolling for a standard takeoff. My procedure has been to roll back the prop to 2600 shortly after breaking ground. It's still pretty loud though.

I don't want to be the noisy guy in the neighborhood, currently I am, but I also want to use all the performance available. What prop setup will get me there I'm not yet sure.

I'm currently setting up a 79" LUGA 2 bladed, grd. adj. prop for the Rotax/S7-S combo. That is HUGE, nothing bigger in dia. in the Rotax world! It has curves in all the right places, being a scimitar planform, is all carbon fiber with a stainless leading edge, is super light, and is (no other word for it) sexy as hell. I have an airplane savvy neighbor I will use as my sound dummy, he was the first to tell me my Prince made such a deep tone it almost sounded like it was radial engine powered . I don't expect the LUGA to be obnoxious, but do find the sound a prop makes pretty important

Interesting tradeoffs.

I was under the impression that there was no advantage in general for going sonic, that the L/D of the prop was slashed, and that lower efficiency from higher disc loading was generally better than pushing the diameter.

Does anyone have data on measured thrust at any other point than at rest (static)? The comments above on inflow speeds is interesting.

I run an 88" prop on my 180, it redlines at 2600 rpm and I'm told it's pretty loud.
A longish 3 blade prop on a Ponk'd 470 (2700 rpm I think) or an IO520 (at 2850 rpm) are quite loud.
It's not just loud but the sound seems to project forward of the airplane quite a bit,
so you get to enjoy it longer.

Here is my opinion and keep in mind that it is only the opinion of someone who may be and probably is wrong. Prior to the McCauley 200 series props Cessna 180's came with C66 props and 185's with C58 props. The prop blades are the same and both start as 90 inches but are trimmed to length. The dowel placement in the hubs are different to index the prop differently on the crankshaft. Cessna for the O470R (2600 rpm) seaplanes used 88 inches and for the IO520D (2850 rpm for takeoff, 2700 rpm after 5 min) used 86 inches and for land plane versions of both used 82 inches. I would trim the blades to 86 inches on the basis that Cessna or McCauley in all probability did testing to determine the length which offered best performance for seaplane operations. Keep in mind also that this was before FAR 36 (Noise levels) so noise may have been a consideration but was not the controlling consideration.

Tim

Love my 88" Mac and IO470 @ 2625+

whee - Great thread topic!
It must be nice to work in the Experimental world, where you can bolt-on any size propeller that you want to test, or that happens to pop-up for a great price!

But, if the theoretical "sweet spot" is Mach 0.88 to 0.92, why are so many factory airplanes equipped with propellers that turn their tips much slower than that?

For example, the Cessna R172K Hawk XP uses the same Continental IO-360-KB as whee's Bearhawk, and it can be tuned-up to 210hp @ 2800rpm using the Isham STC. But, the two propellers that are fitted by Cessna at the factory are the two-bladed 76" McCauley 2A34C203/90DCA-14 for landplanes, and the 80" McCauley 2A34C203/90DCA-10 for floatplanes.

Using the P.Ponk calculator:

The 76" land-prop @ 2800rpm barely enters the "sweet spot" with a tip speed of Mach 0.8878 on a really cold -20*C day, and then only produces Mach 0.8115 on a hot 30*C day.

The longer 80" float-prop @ 2800rpm only has a tip speed of Mach 0.8538 on a hot 30*C day, and doesn't quite reach the center of the "sweet spot" at 0.8988 on a brisk 0*C day, when lakes start to freeze-over and float season is ending!

And that's only if you buy & install the Isham STC! Back at the standard factory propeller setting of 2600rpm, the tip speeds never even come close to the theoretical "sweet spot". What's up with that? Doesn't Cessna know about the "sweet spot"?

It sure would be nice to bolt-on a 82-84" propeller to a Hawk HP, but alas, that's not approved.

Thanx, Dave.

Dave, I think it comes down to prop clearance issues. You strap an 84" on and with a flat nose strut you'll probably be rototilling...