Clearance with an 80" Prop

[JimParker256]

Finally found the reference for the prop clearance design standard, although it's the older CAR 3 version. Still probably good to think about with longer props...

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.

I had not known about the clearance being measured with the "landing gear strut bottomed" part, until I read this. But the clearance requirement is 9 inches in normal level takeoff position, and "positive clearance" with that the critical tire (main, for us) flat and that same. strut bottomed out, and the airplane in "level" attitude.

Since this is for "thought guidance" only for us experimental builders, I'm probably going to just leave it at this, and not try to find the Part 23 version... Hope this is helpful

[whee]

Oh, I agree Jim, it is definitely a good guideline. I'm told that the DAR the used around my area checks things like this and if it is way outside the norm then he won't issue an airworthiness cert. That's the kind of situation I was referring to.

[JimParker256]

...you won't have an issue from a regulatory standpoint...

That's a true statement, since those requirements for Part 23 certification absolutely do not apply to Experimental Amateur Built aircraft. It was offered in the interest of perhaps providing a "reasonable guideline" for us to consider. As the builder, it's your call.

[Sebastian]

I finally had a chance today to take Whee's advice and check the prop clearance using a bubble lever between stations C & D. I came up with 13". That's 3" difference from what I originally measured. I also put a second lever on the tubing above the door again to see what if any the difference between the levels. The level on the door was about half a bubble off (like it's owner part of the time) LOL

[whee]

This is a really good conversation and not that I'm close to flying my plane (4place, o-540 and 84" prop, on 800x6 tires) I thought I would take a break from the landing light work and try to figure out my prop clearance.

I started by measuring the prop from end to center (42.5"), then put my drywall square on front of the plane centering the 42.5" mark as best I could with the center of the flywheel and clamped it there. I measured from the end of the square to the floor (23"). I called this taxi position. May be a little more than that since the prop will be a little further out. See the pic.

Next I put a level on the tubing above the door (see pic) and raised the tail till it showed level bubble. Measured this time and got 10" to the floor.

I did a 3rd measurement and I called this one eyeball level since with the bubble level it looked nose low. That measured 12". See pic.

Now this may be fine "I DON'T KNOW" but as you can see in the pictures I don't even have the wings on yet so I am nowhere close to having this plane loaded. I don't think this is a problem for the test flying but as I get better and venture off airport it seems this will put my prop to close to the ground. It does have me concerned though. Any thoughts?

There is a fair number of guys running a 84" prop on the 4-place with small tires...you won't have an issue from a regulatory standpoint. If you start going off airport you'll be on larger tires anyways.

The tube above the door isn't the official place to check level. You need to use the lower longeron between stations C and D.

[Bdflies]

Ooohhh! CJ, I think she's saying you shot a fawn!... Granted, a fawn with a nice rack.

The MT is 205 cm, or 80.7" diameter. She's sitting on 8:50x6:00's that are aired to 25 psi (which is WAY more than I'll fly with). Center to center of the tires measures 71" right now. It'll spread more when loaded. I'm sure the 12" clearance is fine. Early on, I figured to put 31's on the plane because I kinda like the "Clown Plane" look, as Jim described it. Added insurance for the 'fragile' propellor, has furthered my intention.

Bill

[JimParker256]

Just a quick reminder that unless your bushwheels have larger-diameter hubs, they won't impact the "worst-case scenario" clearance with two flat tires...

[Flygirl1]

Bill, is that an 80" prop? What is your landing get spread as it sits now? Sebastian, nice looking start on your Bearhawk. Your working on the landing lights, must not be to far from completion!! Or as they say--90% done and 90% to go! Mark G. told me a quote that seems to be true--" Perfection is the greatest threat to completion" ;-) D. P.S. That head your holding up in your profile photo looks just like the one I feed wet cob to every day. AKA: Bucky ;-) OOPS, just took a closer look at your profile--Bucky lost his spots long ago!!

[Bdflies]

Well, I've got about the same (12") clearance on my Patrol. For initial and phase 1 flights, this is fine. After that, my mind is pretty much made up to get a set of 31’s. Yes, they're expensive. But, not as expensive as a MT prop!

Bill

[Sebastian]

This is a really good conversation and not that I'm close to flying my plane (4place, o-540 and 84" prop, on 800x6 tires) I thought I would take a break from the landing light work and try to figure out my prop clearance.

I started by measuring the prop from end to center (42.5"), then put my drywall square on front of the plane centering the 42.5" mark as best I could with the center of the flywheel and clamped it there. I measured from the end of the square to the floor (23"). I called this taxi position. May be a little more than that since the prop will be a little further out. See the pic.

Next I put a level on the tubing above the door (see pic) and raised the tail till it showed level bubble. Measured this time and got 10" to the floor.

I did a 3rd measurement and I called this one eyeball level since with the bubble level it looked nose low. That measured 12". See pic.

Now this may be fine "I DON'T KNOW" but as you can see in the pictures I don't even have the wings on yet so I am nowhere close to having this plane loaded. I don't think this is a problem for the test flying but as I get better and venture off airport it seems this will put my prop to close to the ground. It does have me concerned though. Any thoughts?

[Patrol28]

Get 31" Bushwheels. Problem solved. ( I know there isn't a problem but bushwheels look cool)

[Mark Goldberg]

The 74" spread is max allowed at full gross. Normal flying weights should spread the gear to 72". If you are worried about prop clearance - bringing the tires closer together by threading in the shock strut rod end bearings would help quite a bit. I have flown my 4 place down to 69" spread. It handles fine but hurts visibility over the nose. Mark

[JimParker256]

The sad thing is that I'm spending all this time online instead of building because of plantar fasciitis in one foot that is keeping me from being able to work on my plane. So this is "builder withdrawal therapy" until I can get back to it. Like you, I can't even go fly because of the weather. When it's not IFR, it's blowing 35 gusting 45... Two days ago it was 12°F for the high, and today we're at 79° (59° will be our low). Can't wait for this crazy weather pattern to cycle out...

But I know what you mean about the "standard size" being male-oriented. I have a friend who is just tiny - maybe 4'8" tall, and has to buy children's shoes to fit her feet. She has found online sources for clothes for work, but for a long time she had to buy kids clothes for that as well... Aside from the obvious challenges with reaching the pedals and seeing out of the car windows, she also had to have a footrest custom made for every car she drove, so she could reach the brake pedal without holding her leg up in the air. Imagine doing that for a 1.5 hour long commute each way, every day, in the horrible Houston traffic! She was an amazing person, though. Never complained — well except when our group started to walk too fast going back and forth between work and wherever we are lunch... She was taking at least two steps for every one we took...

[Flygirl1]

Thanks for taking the time to look this stuff up Jim. Your a wealth of information and a real asset to this Bearhawk community!! My biggest complaint on these planes is that they were made for the male frame. It's a reach to the step on a good day. A real concern for me is the reach to the toe brakes. I've minimized it by raising the floor, so we'll see soon enough if that's enough. AARGH!! Pray for better WX. We want to get these birds in the air!!!!! D.

[Flygirl1]

oops, it didn't take the post, then it did.