Hello,

I was wondering if anyone had a rule of thumb on what the maximum breakaway power that should be used to get the airplane moving on gravel/rocks before the pilot should turn the motor off and attempt to reposition the airplane by hand. Obviously the concern is damage to the propeller and other parts of the airframe aft of the prop.

I realize you can probably get away with more with bigger rocks so there can't be a hard and fast rule, but especially with a tricycle gear, I would like to know at what point propeller damage is most likely to occur, since it can take 1300-1400 RPM just to breakaway on flat asphalt if the airplane is heavy (182).

Also, what part of the propeller will display damage first? I imagine the leading edge and front surface are most critical for thrust production (like a wing) but am unsure where gravel damage will usually manifest.

Thanks in advance.

Flying navajo's off gravel is educational because you can see the vortex of dust/dirt/rocks underneath the props as your give it power to get moving. My work (local-ish to you) tells us that rather than easing the power in to breakaway at minimum RPM, just give the throttle the onions to get moving as quickly as possible, whether twins or single engine, and whatever RPM that takes. They are the one buying the props so surely they believe what they teach. The worst case is you slowly easing in power which is sucking up more and more rocks the longer you go on. A better case is a big burst of power to get rolling then never let that sucker slow down. As you know,the best mitigation strategy is thought when parking, etc.

Prop damage will rarely be on the front face. When the prop is moving in the normal direction, that's the relative downwind side so it's hard to hit. Leading edge and backside are more prone to damage. Turbines with reverse thrust on gravel... everywhere has rock damage always.

Thanks, interesting. I will keep the in mind your recommendation. It sounds like a second or two of moderate power close to 2000 is better than 10 or 15 seconds at 1200-1500 rpms that just barely gets you crawling.

My experience is with gravel runways not big rocks, I would classify big rocks as surface type closer to off strip operations. ASA advice echos my technique, sufficient power to "break out" and establish a rapid walking to light running pace and thereafter normal take off power settings for the remainder of the take off roll. Once your moving forward at a reasonable pace anything the props kick up will be left behind and present limited harm or risk to the prop. Now that said full disclosure, I was operating aircraft that had sufficient HP to establish a pace very quickly, this may not be the case for a single or light piston twin.

Keep moving, apply power smoothly first , and once it starts rolling,apply full power, I have had my 2 blade MT prop in my Cessna 182, for more than 2,000 hrs operating flawlessly mostly from dirt strips, gravel, grass and rocks.

Asa covered it pretty well. One other point: Once it’s moving, don’t stop and start again. Try not to move till you can roll right onto the runway and do a rolling takeoff.

That’s not always possible, of course, but when possible.....

The aft surface of a prop blade is referred to as the “Face” of the blade. The leading edge and the face of the blade will take the most damage.

Be advised that dressing out nicks in a prop leading edge CAN do more damage than good. IF you’re going to do so, get with a good mechanic and ask them to show you how to do so correctly.

And be advised that some FAA types consider dressing a prop to be “maintenance” that is NOT permitted by a pilot or owner, must be done by a licensed mechanic.

Not saying I agree with that, but there IS a right way and a wrong way.

As a side note, losing a piece of a prop in flight WILL be a deeply moving experience.

MTV

The concern about getting it to move at all that was indicated in the OP makes me think we are talking about true soft surfaces, not just gravel like on a prepared gravel strip.

Anytime I have a tricycle gear aircraft on a soft surface, I push back in my own tracks so I can start up with some packed tracks in front of the airplane. That way you can start moving easier, and then make sure you don't stop...

I got brave enough to fly into Picnic Strip @ Knik in the 182, which is why I ask. First time I've ever landed any place that isn't an official "airport" (in an airplane anyway). The plane does have 8.50x6's all around and the 206 airglas nose fork, landing and stopping was no problem and the strip looked to be almost half a mile long, it was the condition of the surface that was the concern, very loose "pebbles". Mud strip was a lot better even though it was only ~1000 ft or so!

I tried to find a good spot to park off to the side. Airplane would not really budge by hand after parking, just skid on the loose rocks whether I was pushing from wing struts or with the tow bar. Eventually just had to fire it up and power it out. Prop was fine after, and it probably took about 1700 rpm to get moving, but I was definitely worried about it at the time. That said, the airplane was based on a gravel field for decades and the 82" McCauley definitely has some nicks that have been filed out before.

Building on @asa’s recommendations, always try to park into the wind, facing downhill, and on the firmest possible surface. You won’t know where to find those the first time in to a new strip, but do your best.

When other guys are starting up and taxiing out watch for the mini vortex of gravel and sand behind their prop; the first time you see it you’ll be surprised, and you’ll better appreciate why you always want to park facing downhill!

If you haven’t already put a piece of heavy rubberized tape on the leading edge of your horizontal stabilizer, you need to do that right away. They take a heck of a beating. Some guys also put it on the leading edge of their gear fairings. And if you have any belly-mounted blade antennas you want to build up a generous bead of clear RTV on the leading edge.