Prop Clearance on Wheel Penetration Skis

80" or 83" prop?

I know that wheel penetration skis aren't the best, BUT, I ordered a cub crafters carbon cub FX3 recently and would like to try the Summit penetration skis next year. (straight skis not really possible due to the hangar situation).

I have to choose between the 80" or 83" Hartzell Trailblazer composite constant speed propeller in a few months. The penetration skis would be used with 8.50x6 tires.

I have been in touch with cub crafters, they say the 83" prop requires 29" or larger tires, but I haven't gotten a definite answer about skis. I am thinking I will need to go with the 80" propeller for adequate propeller ground clearance (the airplane does come with 3x3 extended gear). I would hate to chop up snow or even weather it excessively due to inadequate clearance. On the other hand, I could see how the better low-end performance of the longer 83" prop would be nice for ski ops, and 1.5" of reduced clearance doesn't seem like much?

The airplane will live on 31"s in the summer, for now I think. I just wanted to at LEAST have the option of using wheel penetration skis, and after taking a few ski lessons this winter, I know it's something I want to do.

The 83" adds $500 and 1.2 lbs to the airplane over the 80". I don't have hard numbers on cruise speed & takeoff roll differences between the two.

Apologies if this has been discussed, I tried a search, but couldn't find the specific topic of prop clearance on skis being talked about. It seems that the "borer" props are the go-to for PA18 skiplanes, and those are 82", although I think a lot of people use outlaw Cattos (or for experimentals). I would imagine that the PA18's without the thrustline mod would put the 82" borer as close to the ground as the carbon cub (which as i understand it all have the thrustline mod) with the 83". Of course, type of skis will also make a difference.

You will like the summits on that machine. Good choice. They work great on any plane with good hp.

Here is what an 84" Catto looks like on 6" gear, that is 70" crankshaft height. Ground ( snow) clearance; is the least of your worry's. Datum wheel skis.

I took this picture today. 84” Catto on 3” gear. Crusty inch or so on top so the skis are not penetrating. LW2500s.

No clearance issues and when I get back to my airport I wheel land on the 8” tires.

Well, the factory is out of 80" props, so I guess I have to go with an 83" Trailblazer. Thanks for the responses, it doesn't seem like it'll be an issue with the 3x3" landing gear. Not sure if running Acme shocks affects that. Going to start out trying summits with 8.50x6's in the wheel penetration configuration (summits can also be run as straight skis, but accessing my hangar would require an extended taxi on dollies).

Wheel skis aren’t the issue. The question is whether the FX on 3x3 gear has appropriate prop clearance on 850s. I’d bet lots of FXs leave the factory on 850s and probably stay on 850s. Ask CC.

stewartb said:

Wheel skis aren’t the issue. The question is whether the FX on 3x3 gear has appropriate prop clearance on 850s. I’d bet lots of FXs leave the factory on 850s and probably stay on 850s. Ask CC.

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True, but prop clearance on skis should be even less than on just the 8.50's assuming that the tires will penetrate the surface of the snow.

I'm pretty sure this airplane had the 83" prop when they made the video (it's on 8.50's and summits too), so I'm guessing CC says it's OK, in fact they insinuated that when I asked about it a year or so ago using the answer "it's experimental, you can do whatever you want". Their order form say's 29's or larger are required for the 83" prop though, must be a liability issue or something.

If the snow is soft enough for the skis to sink it isn’t likely to do anything to the prop. And I don’t think the prop clearance takes any ski settling into account. The measurement is taken on a hard surface.

stewartb said:

If the snow is soft enough for the skis to sink it isn’t likely to do anything to the prop. And I don’t think the prop clearance takes any ski settling into account. The measurement is taken on a hard surface.

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Sounds good, thanks Stewart! Seems like there isn't too much to worry about. Supposedly the 83" makes 5% more static thrust (1000 lbs vs 950 lbs) than the 80" at the cost of 2-3 mph top end cruise, so that's a probably a plus overall.


I'd not want to be out there in this stuff with penetration skis, lol:

When the tip of the prop hits snow it can change tone. Starts to sound hollow and thrust is lost until the tip gets clear. It's not a matter of if but when during ski flying in virgin unpacked deep snow, especially if turning sharply. The best solution is finding the incidence angle that keeps the tail and prop just clear. The bigger the ski footprint and longer the gear the better for prop clearance and takeoff. Helps to snowshoe a path forward for the prop and skis before takeoff to get on top.

Gary

Narwhal said:

Well, the factory is out of 80" props, so I guess I have to go with an 83" Trailblazer.

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Don't understand that. Why can't they order an 80 inch from Hartzel. The prop goes on the airplane very late in the build process. Cubcrafters will not sell an FX-3 with 8.50 tires and an 83 inch prop. I think they base that on the regulatory requirements for prop clearance in the specified abnormal conditions (one tire deflated??). Most FX-3 leave the factory with 83 inch and 29 or bigger tires.

Your FX-3 would not leave the factory on 8.50 tires though so they would fit 80 or 83 inch then you decide if it's ok to use 8.50 with skis and 83 inch.

FWIW Cubcrafters operates their demo FX-3 on 8.50 with penetration skis and the 83 inch prop. I flew it in that configuration and was cautioned to make 3 point landings and 3 point takeoffs.

I was told static thrust 828 lb for 80 inch and 855 lb for 83 inch. Maybe they got better numbers on a colder day.

§ 23.925 Propeller clearance. Unless smaller clearances are substantiated, propeller clearances, with the airplane at the most adverse combination of weight and center of gravity, and with the propeller in the most adverse pitch position, may not be less than the following
a) Ground clearance. There must be a clearance of at least seven inches (for each airplane with nose wheel landing gear) or nine inches (for each airplane with tail wheel landing gear) between each propeller and the ground with the landing gear statically deflected and in the level, normal takeoff, or taxing attitude, whichever is most critical. In addition, for each airplane with conventional landing gear struts using fluid or mechanical means for absorbing landing shocks, there must be positive clearance between the propeller and the ground in the level takeoff attitude with the critical tire completely deflated and the corresponding landing gear strut bottomed. Positive clearance for airplanes using leaf spring struts is shown with a deflection corresponding to 1.5g.

As an FYI, I’ve seen a Borer prop on a Super Cub which had its pitch flattened by operation in deep snow. Minimal damage to blades from erosion, but those thin blades were twisted. There CAN be forces involved.

MTV

Is the Catto 3 blade an option for you? It preforms very well with the Cubcrafters O-340 engine on my 12. It allows greater ground clearance and makes a lot less noise.

Put 6” gear on it. Every ski pilot I know would prefer that.

My prop is Whirl Wind’s most aggressive and it’s only 80”. I have a crane scale. Maybe I should do a pull test.

My own adventure with penetration skis begins. I’ve never wanted penetrations but straights would be impossible where I live and I don’t need another pair of hydraulics. I’m looking forward to trying these out.

stewartb said:

Put 6” gear on it. Every ski pilot I know would prefer that.

My prop is Whirl Wind’s most aggressive and it’s only 80”. I have a crane scale. Maybe I should do a pull test.

My own adventure with penetration skis begins. I’ve never wanted penetrations but straights would be impossible where I live and I don’t need another pair of hydraulics. I’m looking forward to trying these out.

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Sounds very cool, I'd love to see a pull test on your rig! I know I'll have to pick my spots carefully with the penetration skis but you'll have the power to do pretty well with them on your widlcat, I think.

Paul Heinrich said:

Is the Catto 3 blade an option for you? It preforms very well with the Cubcrafters O-340 engine on my 12. It allows greater ground clearance and makes a lot less noise.

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I'm not sure, I guess it could be something I could put on later since this is Experimental, but I'd probably like to stick with a constant speed (I assume all the cattos are fixed pitch, but not sure).

Narwhal said:

Sounds very cool, I'd love to see a pull test on your rig! I know I'll have to pick my spots carefully with the penetration skis but you'll have the power to do pretty well with them on your widlcat, I think.

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Your plane will have a very good power to weight ratio. Probably very similar to mine.

Don’t get too hung up on your prop size. Until you get lots of time in that plane and get comfortable landing in spots where an extra 10’ makes a difference? The difference in the two props won’t be important. A man’s got to know his limitations. So it’ll go for me on these new to me skis.

stewartb said:

A man’s got to know his limitations. So it’ll go for me on these new to me skis.

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My guess is snow conditions will be a larger factor than prop length flying skis.

I would worry more about size of my snowshoes than prop clearance.;-)

Like MTV said seen many a prop damaged eating to much snow (usually crust or slush) Big skis help there.
I am a big proponent of the 80 inch props, less noise better climb and cruise.
The 5% is more theory than reality on most installs. ( and only while static not moving at all )
While there is a big difference between 78 and 80 inches the improvements drop of dramaticaly above 80" and other limitations like speed and climb.

I ended up getting an 83" on mine because I guess that was all they had when they built it ~2 years ago. Prop clearance on my penetration skis has not been an issue that I've noticed; the summit/cubcrafter skis do float really well, however. There has really only been one time that I was concerned about it, on an 8000 ft glacier that I probably should've done a few more drags on before landing. Even that was OK. If anything the prop clearance is more of an issue when I try to do the STOL contest takeoffs on bushwheels where I hold the brakes and raise the tail before rolling; not so much for fear of striking the prop, but from the vortices and disturbed FOD that the prop kicks up, especially on gravel or if there isn't much headwind to keep that stuff away from hitting the face of the prop. I'm slowly learning how to be kinder to the prop face and bottom of the tail feathers....