Elevator Rock Rash

[Nev]

I've recently clocked over 500 hours on my BH. One thing that has been a recurring theme is getting rock and stone damage on the underside of my elevators - both sides. Given that it's a bush-plane, and a very capable one at that, my purpose for building and owning it was to get out "off-piste" and explore the backcountry. So while I make every effort to look after the airplane and keep it in great condition, I also made a conscious choice to use it as I intended and that includes some gravel bars and on occasion higher rough strips that by their nature have a large quantity of sharper rocks of all sizes.

My question here is, do any of you know if take-off and landing technique has an effect on rock damage to the empennage. For example, I used to keep the elevators full forward to help raise the tail clear of the ground. However this obviously presents the elevators squarely in the way of flying rocks. So for the past couple of hundred hours I've tried to keep the elevators neutral when operating on rocks - however it hasn't made much difference. I've eliminated heavy braking (on nearly all landings) and have always performed rolling take-offs. With a tailwind, I'm very careful with ground handling and thrust application especially on loose surfaces to avoid picking up rocks in the prop vortices.

It's possible that getting this type of damage is simply "par for the course". But if anyone has any good suggestions then I'd welcome sharing them here.

[Steve W]

When I started flying for a “new to me” outfit I asked the chief engineer how he wanted me to treat his machines. “Don’t do any short field take offs unless its a life or death situation.” He was extremely serious at the time so I paid attention. “Ease forward with speed and power. No WOT until airspeed is alive and if there is plenty of runway, wait a while longer. As the tail comes up add full power and I better not see any marks on that prop.”
If he found a nick in the prop at the next 100hr inspection, I could feel his eyes across the hangar floor if I was in the building. I got the same lecture when we went to floats about taxiing too fast down wind when the water was rough. And again he would say, “slow with the power on takeoff.” I think those instructions helped me keep the tail of my Patrol kinda damage free but I didn’t operate on many gravel strips before switching to amphibs.
Again my 2 cents worth.

[Nev]

That's exactly what I was after. Thanks.

[Richard E]

Nev,

I had the same advice after putting a hole in the underside of the elevator. Avoid stones/gravel and put the throttle in gradually to get momentum first. I've never had a hole since.

Its pretty rare that you really need to do a short field take-off in a 260hp Bearhawk given that it is normally off the ground within a few feet !

[TJ_Slice]

Just curious, is there any pattern to the location of the damage, or is it random?

[rodsmith]

I can't add anything to the great advise Steve W gave. When I was flying my Maule in Alaska on some very rough strips, I not only had damage to the elevator fabric, I had several dents in the leading edge tube of the horizontal stab from gravel. That prompted me to build my Bearhawk stabs with a .049 leading edge tube instead of .035. When I recovered my Maule I did a double layer of fabric on the underside of the elevators. This was actually outlined in the Polyfiber manual, they called it bulletproof fabric. Just a thought if they get bad enough to recover them.

[Nev]

I'm thinking of doing the same double skin on the underside Rod. Good idea.

[Nev]

Just curious, is there any pattern to the location of the damage, or is it random?

Most of the damage is to the underside of the elevators - as you'd expect - and pretty much directly behind the main wheels. The occasional rock appears to have "bounced around" and required several repairs. On one occasion I discovered the under-side of a horizontal stabiliser rib had been deformed and was evident by loose fabric in that area. Fortunately we were able to remove the stab, make a tool on a long arm, and fix it using access through the lower trim tab arm cover.

Interesting there is no damage to the flaps at all, and only one nick to the fabric in the aft fuselage. The horizontal stabiliser struts have taken a few direct hits, being directly in line with the main wheels. Early on I purchased a second pair of struts. They're quick to change out, and can then be repaired without down-time.

One of the local Mechanics showed me how to make up a piece of fabric on a frame, and prepare it right through to painting. I now keep a prepared roll that can be cut into patches for repairs when needed. The longer term plan is to do a double skin covering on my spare set of elevators for longevity. I recall that Mike Patey also did a video on this. I've always done rolling takeoffs on gravel, but I think I'll try a far more measured application of power and see if I can reduce the number of airborne rocks being flung enthusiastically rearwards !

[Pat Fagan]

All those techniques are good, but they didn’t fully solve the problem until I switched to bald tires. The tread was picking up the rocks and propelling them at the stab.

[kestrel]

I think that Nev is already on ABWs? I find that it is ice crust when ski flying that puts the worst cuts in the under side of my stab and belly.

[Bcone1381]

https://www.supercub.org/forum/threads/retread-bushwheels.35717/

Screenshot 2025-03-28 at 9.25.10 AM.png
This is wondering post....

I've thought about tires, tread, stones and how they can pick up and throw stones into the tail. I have this theory. I stress theory, not fact of what I am about to say.

When I look at the slot in the tread above a tire, it is opened up and a dimension of maybe 6 mm at the top. When the tread rotates around to the gravel surface I can see the slot in the tread squeezing together, down to a smaller dimension, maybe 4 mm. A 5 mm stone might get grabbed by the squeezing slot. The tire rotation will pick it up and then relax back to 6mm then release the stone. But its got energy and the release is like a pitcher releasing the baseball towards home plate, except it hit my tail.

Second thought. If we did not have slots, it would not pick up stones.

Third thought. If I got slots because they dont make a 8.50x6 6 ply tire that is smooth then can I fill the slots to make it smooth and remove that risk? What should I fill it with? Will the filling hold and what will be the failure mode if the filling is released? Will it do damage?

The Supercub guys have discussed retreading bushwheels by the a truck bed liner product. I wonder about permatex silicone gasket maker. Can I put a few dots in to test its hold ability?

We are experimental and we get to wonder....

[rodsmith]

Hi Brooks, I am using the Aero classic 4-ply smooth 8.50x6 tires. They are rated at 1800#s each and I now run them at 15 psi, I started out with them at 20 psi. I am happy with them.

[Nev]

I've got smooth Alaskan 29'ers. They do still flick stones up unfortunately. A while ago someone had a video of me landing on a gravel bar. We played the video in very slow motion and then froze the frame just after touchdown. You could clearly see what looked like about 20 rocks rising into the air behind the tires as they touched down, and falling back to the ground in the vicinity on the elevators.

[Battson]

In my view, most (if not all) the damage is probably occurring at touchdown.

I think we all agree that easing the throttle in and applying good off-airport taxing practice helps, but the underlying cause of 90% of the damage is probably touchdown, assuming all those other things are sorted. Smooth tires are not the issue here, but a very valid comment.

Once you've watched (and filmed) far too many off airport landing videos on gravel surfaces, you notice what's happening. When the tires touch the ground for the first time, they are not spinning, so you're effectively hitting the ground with huge rubber pads, moving at between 70 and 85 km/h (40 to 50 mph), with almost unlimited mass / energy compared to the individual masses of all the rocks and gravel they are impacting. It's easy to understand that this collision sends a shower of rocks and debris flying forwards.

Some of the collision energy goes into spooling up and deforming the tires / suspension, and the rest goes into throwing rocks and debris forward at a speed less than the aircraft is moving. Equal and opposite reactions. Many of the rocks bounce and impact other stationary rocks, as they fly forward. They are deflected upwards into the air. This is about the time the tailplane flies through the scene, travelling through a shower of rocks which are themselves moving at a slower speed than the aircraft. The tail impacts several of these rocks, especially on the leading edge and underside (as the rocks are bouncing upwards), causing rock rash in those areas but almost never on the upper surface. If that's where you're seeing damage, then that's probably the cause.

It all makes sense, hopefully?

Having established those facts, solutions become easier to conceive. Getting the whole tail assembly up, away from the ground before the main wheels touch the ground, is possibly the easiest solution. How to achieve that?

- You can check forward in a positive way, before the aircraft is really finished rounding out, to keep the tail up.

- You could also try holding a little more airspeed to keep the tail up.

- Avoiding three-point landings or tail-low wheelers, and opting for a true wheeler landing is another choice.

Overall, I have found that the above options can greatly reduce the amount and rate of damage, but you'll never avoid it entirely unless you avoid the locations with loose rocks.



You can also try these. This may be possible sometimes, but in some locations it could introduce more serious risks:

- Touching down on non-rocky surfaces like a patch of sand, a vegetated area, or pea shingle to get the tires spinning without dislodging rocks large enough to cause damage.

- Softening the moment of touchdown, just kissing the ground softly, to reduce the rate of energy imparted to the rocks themselves. This has the added benefit of keeping the tail higher, usually, by the nature of the control inputs and wing angles required.

[huntaero]

We operated Dornier 228 in Manitoba operating out gravel strips and they had a gravel kit installed which was a small framework around the back of the 2 tires and a peice of belting attached to it> I couldnt find a pic of it but found a pic of a similar installation. They have a gravel kit for the Cessna Caravan 208 series so may give some ideas to create one.

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