How much ground clearance??

[Scott Richlen]

With my fuselage center-line set up parallel to the ground and the measurement from the ground to the center of my engine shaft at 7 inches, how much minimal ground clearance do you retain in choosing prop diameter?  i.e., what maximum prop diameter can I use?

Thanks,
Scott

[Tim Wescott]

I think it depends on what you fly off of, and how good a flier you are.

On my trike-geared Twister, I have about one inch of clearance from prop tip to ground.  Flying off of very short grass, I need to wipe green slime off the prop at the end of the day, but I haven't had problems.

When I had 1/2" of clearance I was chipping prop tips and I nearly murdered my flying buddy with a rock, but I never had a problem on pavement.

I'd probably try for 1" of clearance with the plane level, but I'm sure that folks will weigh in both for more and for less.

[Steve Fitton]

Hi Scott,

I just measured my Dreadnought, its just over 1" (maybe 1.25") with the airplane in level attitude.  I have run as little as 1/2" with the 13 inch two blades in the past.

[tom hampshire]

Scott - For real world flying on fields of varying surface, both blacktop and grass, with mow heights varying all over the place, you might try multiple sets of gear held in place with bolts.  Say an inch of ground clearance with the mains 15 degrees forward of the CG for pavement and golf green type grass.  1.5 inches of ground clearance and the wheels slightly ahead of the LE for a typical grass field.  Trying to get one set of gear to match all surfaces is too broad a compromise, and performance suffers accordingly.  Another thought is to change to big wheels for a rough grass surface.  It's a good feeling to arrive at a new field and know you can pick and install gear which will fit conditions as found.  Especially if pre contest conditions prevented the hosts from mowing as they would have liked.

[proparc]

On Autocad, I go for 1.5 inches minimum. On the Saito 72, I go for 2 inches. Prop strikes on 4 strokes are not good.

[Tim Wescott]

Prop strikes on 4 strokes are not good.

What happens with a 4 stroke that makes it worse?  I'm just curious -- I've never flown one.

[Joe Yau]

I have about 1/2" clearance with a 13" 2 blade on the Saito .56 (with fuse level)  but I don't fly on grass.  not sure why it would need 2" clearance..

[Dane Martin]

What happens with a 4 stroke that makes it worse?  I'm just curious -- I've never flown one.

Milton flys a saito .72. with all that torque, a prop strike punches holes in the run way.... 
lol

[Steve Helmick]

I'd expect bent pushrods and piston/valve conflicts from a "shaft-run", wouldn't you, Tim?  Steve

[proparc]

Milton flys a saito .72. with all that torque, a prop strike punches holes in the run way.... 
lol