True, A1.
But, I'm pretty sure that there's the "required" amount of clearance for an 82-84" on floats?

Or, dare I hope to get Field Approval from the FAA for an 80" THREE-bladed MT propeller?

BluNosDav wrote:True, A1.
But, I'm pretty sure that there's the "required" amount of clearance for an 82-84" on floats?

Or, dare I hope to get Field Approval from the FAA for an 80" THREE-bladed MT propeller?

Ya, I'm not sure what the clearance requirements are down to floats. Sire would be nice though!

courierguy wrote: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

I got the Luga prop set up and flew with it about 10 hours. It climbs great, but so does the Prince, but I noticed something in the short video a neighbor made for me, as a sound check. https://www.youtube.com/watch?v=cTht968TfsQ&t=3s

Right at :29, and again at 1:20, each time as I reach 2260 prop RPM (5500 on the Rotax) the sound changes to kind of a snarl. 79" dia. in case anyone wants to do the math, I don't! Not really noisy but compared to the Prince's deep tone, with NO snarl at any RPM, I decided to put the Prince back on. Watching/listening to the ski flying video a friend made for me this winter, (https://www.youtube.com/watch?v=nmTK-fdqe_M&t=754s, right at 4:10 in I do a full power climb out) the difference in tone in obvious, and to my ears anyway the Prince has a much cooler sound, deeper in tone and generally less obnoxious. The Luga climbed a bit better, but it was also turning up faster, and I already climb great with the Prince, plus the Prince got me off shorter. It has more grunt from the start, noticed in the rpm needed to climb the hill to my takeoff site, once I was back on tires, the Luga needed 500 rpm more. If I was in a short takeoff contest, I'd want the Prince, if in a time to climb to 1,000', the Luga would win by a hair. I am cool with the Luga, it's in the hangar, pitch all set, ready to go as a backup the next time I do something stupid. Both were about the same in cruise BTW, I hit a drag wall much over 90 anyway, unless I want to burn a LOT of fuel, like over 4 GPH!!

Great topic, most interesting reading.

I believe that on question 1 that both reaching the same static RPM does not indicate same HP. The prop should be in the governed range at T/O rpm and thus more or less HP within limits should produce the same RPM. If it was at the stops then on a cold day would produce higher RPM and visa versa.

Fascinating discussion. The one thing I see missing, though, is anything comprehensive about the power (horsepower and torque) curves of the individual engines. I guess I've become a little more sensitive to this since buying my Class B motorhome on a Sprinter chassis, which has a 3 liter (183 cu.in.) turbo diesel V6 that puts out 185 hp and 325 ft. lbs. of torque, all at pretty low rpm, so that I cruise at 65 mph at 2400 rpm. Even flooring it until it shifts itself, it doesn't go over 4000 rpm. In contrast, my RAV4 puts out 170 hp, lots less torque (I've forgotten how much), but that hp comes at a much higher rpm, nearly 5000, out of a 2.0 liter (122 cu.in.) engine, and the top rpm shifts occur close to 6000 rpm.

So we get back to airplanes, and the power curves of two 360 cu.in. (5.9 liter) engines are likely to be similar, depending probably more on whether one is FI and the other carbureted, and on tuning (spark advance) than anything. If the peak power (assuming hp and torque curves are similar) occurs at 2600 rpm, there's not any benefit cranking much faster than that, unless the power curve is really peaky. But my observation of lower rpm engines is that the power curves tend toward being flatter than with a sharp peak.

Now FWIW, my Lycoming 360, carbureted, is governed to peak at 2700 rpm. I notice, because I have a Horizon digital tach, that it sometimes peaks at slightly less than 2700 rpm and sometimes a shade over, but no more than about 10 rpm either way--probably an unnoticeable difference on an analog tach. After take off, I routinely reduce the rpm to 2550, unless I have obstacles that require continuing with full power. Next time I go out, I'll make a point of seeing whether it peaks at 2700 rpm without moving--I don't usually take off that way, choosing to slowly add power as the airplane gets moving. But if the runway is clean, I'll try a full power, hold the brakes, take off. I don't know if what I learn will help the Whees of this world, but we'll see.

Oh, I'll also measure the length of the prop--I have no idea how long it is, just that it's a McCauley.

Cary

Ok guys. I need to decide what to do with my prop. It is a 88” C203 taken off a C180G. My engine will turn it to 2600 static and I have the prop gov set to 2800 so the prop engine spins up to that within a second or two on takeoff. It is LOUD. Another significant issue is I don’t have enough prop clearance. I have to be on 850s to meet the minimum requirements but that still isn’t enough, I’m picking up rock damage. So I think I’m going to trim to prop to some undetermined size. I’d like to hear your thoughts on size. Note: I’m familiar with Pponks calculator and recommended tip speeds. WE DON’T NEED TO DISCUSS THAT. The prop shop I called and will use to trim the prop doesn’t have an opinion.

Here’s what I know. My out of the hole performance is great. However, when I roll the rpm back during takeoff I can feel a significant acceleration. At this length the prop is loud. I live 4 miles from the airport and if I’m outside I know if my dad decides to go flying. We borrowed an 82” C203 off of a local C180 and flew both props back to back. Takeoff performance was less with the 82” but due to pilot technique, or lack there of, it isn’t really quantifiable. Climb rate at 70mph was 300fpm less with the 82” but cruise speed was a solid 10mph faster.

The prop clearance issue creates a problem for when I want to go on skis. Very few skis will fit 850 tires and when on snow the prop clearance will be less due to the skis sinking in the snow.

Trimming the prop may create a CG issue since it will lighten the prop. I need to keep the empty CG as far forward as possible. Maybe a 80” 3-blade Mac would be a good option and help my CG but then I’d lose the speed benefit of going to a shorter 2-blade. Not to mention the expense of a replacement prop.

I’ve been talking myself in circles for a couple months and need to make a decision.

Oh, maybe interesting to some. I wanted a easily repeatable test that could represent takeoff thrust. I came up with recording the climb rate at full power while at the edge of stall. In my airplane with flaps up that is 50mph. I did the tests at 8000msl. With the 88” prop I saw a climb rate of 700fpm. With the 82” prop the climb rate was 300fpm. Both tests were repeated several times.

Maybe they don’t represent takeoff thrust in a meaningful way but I found the numbers interesting.

About the closest thing I can think of to your airplane is a Maule M4 or M5-210.
FWIW dunno what might be STC'd but the Maule TCDS says they use a 74" Mac.
Also FWIW the TCDS for the Mac C203 says they're good from 90" down to 72".
I don't think there'd be too much weight difference between an 88 and an 80 or 82.
I went from an 82" C203 to an 88" C201, and I think there was a pound difference.
That said, I'd hate to see you cut a good 88" C203 down because they can be kinda hard to find used.
Maybe sell it off, then buy another one more suitable for your application?

I have a digital strain gauge you can borrow any time you want to put a real number on static thrust. To test climb and cruise performance, I can provide you the FAA flight test parameters required when the FAA Test Pilot is gathering data for certification approvals.


John

john54724 wrote:I have a digital strain gauge you can borrow any time you want to put a real number on static thrust. To test climb and cruise performance, I can provide you the FAA flight test parameters required when the FAA Test Pilot is gathering data for certification approvals.

Thanks for the offer John. Sorry I missed your post.

I’m still suffering from analysis paralysis. We pulled the prop and took it to the prop shop. The owner came out, looked at the prop and said “you really don’t want to do this.” He was willing to do it of course but in his opinion we would be losing performance. So we put the prop back on the plane and tried to forget about that extra 10mph we could be seeing. I’ve also added a couple nicks to the blades due to lack of clearance. We really need to shorten the prop or increase prop clearance with either big tires or extended gear.

Ill be honest, I didn't go through and read every post this time (I have in the past), but in light if the data from december, have you considered something in the middle? 10mph is hard to let go, but so is 300-400fpm. What about 86"? Lucky for you, you set your bounds and know the possibilities so you can sort of choose where you want to end up.

Were the stops on the 82" and 88" set to the same amount?

We were planning on 84” since it was kinda in the middle.

The 88” and and 82” props were set up for 0470s on a C180. I can’t remember remember the setting for the fine pitch stops on each. It something like 12deg for the 82” and 10deg for the 88”. Static rpm was similar and both achieved very close to the max 2800rpm. The 82” was a lot quieter.

hotrod180 wrote:About the closest thing I can think of to your airplane is a Maule M4 or M5-210.
FWIW dunno what might be STC'd but the Maule TCDS says they use a 74" Mac....

Did you ever investigate what these maules can use?
You might need to ditch the C203 (which I expect you can sell very easily),
and buy a Hartzell or different model Mac.
Have you talked to the Flight Resource folks about an MT?

hotrod180 wrote:

hotrod180 wrote:About the closest thing I can think of to your airplane is a Maule M4 or M5-210.
FWIW dunno what might be STC'd but the Maule TCDS says they use a 74" Mac....

Did you ever investigate what these maules can use?
You might need to ditch the C203 (which I expect you can sell very easily),
and buy a Hartzell or different model Mac.
Have you talked to the Flight Resource folks about an MT?

I did, years ago. They come with small Macs or Hartzell like you listed above. Most of the guys swap to an 80” C203 because they say it is the best and longest prop approved for the Maule. A guy in AK has field approved 82” and 84” C203s on a couple Maules and says the 84” performed the best but was loud.

So we know the C203 is the best performing prop on the -210 Maule and the limited experience has shown that longer is better. How long before diminishing returns is unknown.

Tip speed theory aside: Empirically I know my 88” prop performs significantly better than the 82” I tried. What I’m trying to do it reduce noise while not significantly reducing takeoff and climb performance. Increased cruise speed would be an attractive bonus that would make me feel better about the reduced takeoff/climb.

A composite prop isn’t an option since it would move my empty CG aft which would significantly reduce the utility of my airplane. It my even make it unsuitable for my use.

Maybe we should just take a vote. What say you guys: 86”, 85” 84” ? I’m getting the hack saw out tomorrow...

I agree that it would really suck to cut a good 88" prop down. Can you maybe find a used 86 or 84" off of a 185 that's been upgraded?

A1Skinner wrote:I agree that it would really suck to cut a good 88" prop down. Can you maybe find a used 86 or 84" off of a 185 that's been upgraded?

I can see you point but finding a different prop would cost me money and time with no benefit. Likely would end up with a older prop with many more hours and therefore less performance. The prop shop told me that a prop at the end of its service life, ie been overhauled enough times that it is barely inside service limits, has lost 40% of its performance compared to a new prop. My prop has 350hrs since new and was in beautiful shape before going on my plane. It’s a really nice prop. Seems foolish for me to trade it for something else. Cut and repaint will cost me $300 and a couple days.

Whee, have you considered leaving it at 88 inches and dialing the rpm back with the prop control to 2600 rpm, same as with a O470 on a 180, if you don't need the extra performance. Then if you are where the extra performance is required you could push it up. Some 185 guys will figure out where the 2700 rpm point is on the prop control then use that instead of 2850 rpm if they want to reduce the noise level. I have seen it posted where some of them clip a clothes pin over the prop control. You might even be happy with 2500 or 2550. The thrust numbers may be higher for the 88 inch at 2500 than a 82 at 2700. You might be able to borrow a 82 off a 182 to do some pull testing. I think limiting the rpm was how Cessna certified the 172XP at 195 hp. Just something to think about. Noise abatement can benefit from operational changes. With 180 hp 170 operating at the airport where I am based I have the flaps up the the prop rolled back to 2500 from 2700 by the time I passing thru 200 agl and am still over the runway. Because of operating this way I am quieter when I fly over any ones house.

Tim

That makes sense Jon, it is nice to have such a low time prop for sure. In that case I'd tend to lean Tim's way and just dial the RPM back, although that doesnt help with clearance issues, but I'm not convinced that 2" off each blade will help a lot with dings anyways.

whee wrote:....The prop shop told me that a prop at the end of its service life, ie been overhauled enough times that it is barely inside service limits, has lost 40% of its performance compared to a new prop.....

I call bullshit on this one.
I removed an at-minimum length 82" C203 prop from my C180,
and installed a full-length 88" C201.
Some performance increase (not at much as I'd hoped for, of course),
but no way a 40% difference.

Whee, I dunno if you've looked over the TCDS #P3EA for the Mac 2A34C203 prop.
For the Cont IO360 (210hp at 2800 rpm), page 9 shows limits for the C203 as max diameter 80" and minimum diameter 74".
Maybe that's just an arbitrary limit, but maybe they know something we don't.

FWIW I checked out a buddy's Cessna T41 today, it has the same 210hp IO360.
Uses a 76" diameter Mac D2A34C67 prop.