RV-7 engine mount cracks

Yesterday after completing an oil and filter change I took some time for inspection and found this:











The crack is just under the top weld at the rear of the plate that attches the bolt tube. The left side has progressed a little more than the right.
The plane has done about 220 hours but most of this has been off grass strips some of which are quite bumpy. No hard landings that I know of.
Here in the UK there have been quite a number of mounts cracking. I'm going to check out what others have done and add reinforcing if necessary.


Peter

Cracking Undercarraige

Engine mount cracking
Sent PM

Strip the powder coating off and verify it is actually the metal cracking and not the powder coat.
I am not a big fan of powder coating. There are many examples of adhesion problems due to poor factory prep. It is also more difficult to inspect as a crack can propogate under the coating without showing.

Jon

I plan to strip off the powder coat and dye penetrate. But I'm pretty sure given the large number here in the UK that have cracked, that's what it is!
The grass strips seem to be hard on the mounts.

Peter

Peter,

The slickest way to strip the powercoat is to use Permatex Gasket Remover in the aerosol can. Wear glasses and cover your arms and hands. Protect the powdercoat you do not want to come off. Spray it on and wait about a minute; it will bubble up and wipe off with a papertowel or shop cloth. The mount will be clean down to the metal. I cleaned my nose gear leg this way and it worked perfectly.

Good luck!

Jim

My RV-6 mount cracked last year and I pulled the whole front end of the airplane off to fix the mount. I split a 1/8" thick tube to wrap over the bottom of the gear leg socket, welded it on, and built up the welds such that I think its nearly indestructible now.

gear

Both the round and flat spring gears were designed by Steve Wittman. The round gear was used on the Wittman Tailwind which first flew in Jan 1953. The airplane weighed 685 empty with full electric and an antique radio that weighed at least 25#. Gross weight was 1235. That gear had a "small diameter" of 3/4" vs 7/8" for the current gear. The RV gears are also longer. So we have RV's with empty weight approaching the gross weight of the first Tailwind. Is ANYONE surprised that the mounts are cracking???

jrs14855 said:

Both the round and flat spring gears were designed by Steve Wittman. The round gear was used on the Wittman Tailwind which first flew in Jan 1953. The airplane weighed 685 empty with full electric and an antique radio that weighed at least 25#. Gross weight was 1235. That gear had a "small diameter" of 3/4" vs 7/8" for the current gear. The RV gears are also longer. So we have RV's with empty weight approaching the gross weight of the first Tailwind. Is ANYONE surprised that the mounts are cracking???

Click to expand...

I don't think the Tailwind comparison is a valid explanation of the very small cracks that can occur on the RV-4, -6, and -7 gear. The cracks are compression failures and haven't proved to represent an actual service issue. It seems the cracks are due to the metal shrinking at the weld and stresses arising as the gear is loaded and unloaded. But the affected area is under compression when the gear is loaded and the welds that are in tension have not proven to be a problem.

No crack regardless of how small should be ignored, but these cracks haven't resulted in gear failures due to their location on the mount. I have first-hand experience with this matter having watched a small crack on my RV-6 gear mount for several years.

jrs14855 said:

Both the round and flat spring gears were designed by Steve Wittman. The round gear was used on the Wittman Tailwind which first flew in Jan 1953. The airplane weighed 685 empty with full electric and an antique radio that weighed at least 25#. Gross weight was 1235. That gear had a "small diameter" of 3/4" vs 7/8" for the current gear. The RV gears are also longer. So we have RV's with empty weight approaching the gross weight of the first Tailwind. Is ANYONE surprised that the mounts are cracking???

Click to expand...

The numbers you quote are the gear leg diameters. The gear legs aren't what is cracking. The mount is.

Different issue entirely.

gear

I understand the issue completely. The smaller diameter gear legs were softer and put less load on the mount. If Van had copied the Tailwind mount as closely as possible there likely would be few if any MOUNT CRACKING ISSUES. The longer gear leg of the 6 and 7 and the heavier weights put considerably more stress on the MOUNT. More later on the multiple design defficiencies of the 6 and 7 mounts

jrs14855 said:

More later on the multiple design defficiencies of the 6 and 7 mounts

Click to expand...

Now this should be interesting.......

Sam Buchanan said:

Now this should be interesting.......

Click to expand...

I don't understand why you would dismiss this gentleman's opinion on engine mount cracks before he has even presented it, especially considering the fact that you have just mentioned your mount is cracked. Constructive criticism is a good thing and should be encouraged. I'm looking forward hearing what he has to say, but when people make comments like this it can easily discourage someone from participating.

Having designed and built engine mounts that are now standard on the F1 kits when they were being produced, there are design deficiencies that were addressed that I felt were a problem with RV mounts. Thicker walled gear leg sockets, increased bolt tube thicknesses and much improved load bearing on the bottom bolts have resulted in ZERO failures on the F1 mounts, even though the airplane is considerably heavier with much longer gear legs.

gear

First of all I am not an RV builder or owner and never will be. I do like the RV's and admire what Van has accomplished. I am reasonably familiar with all the RV's except the 10 and 12. I started building my first airplane 49 years ago last Dec. It was completed in July 1964 and is still flying. It has been an interesting journey. I have built two sets of round gears myself, turned on a lathe out of bar stock, hand polished with emory cloth and heated and bent. I don't mind the potshots, if I can provide some useful advice to just one person on this forum, I consider it a worthwhile accomplishment.
The gear- some definitions based on the pictures in the first post:
The location of the intersection between the gear socket tubes and the bushing for the mount/fuselage bolts I'll call A
B is the 1/2" outside diameter??? bushing for the mount/fuselage bolts
C is the small gusset below B
D is the location on the gear legs where the legs come in contact with the lower portion of the socket. This should be an area between the two tapers where the gear fits fairly tight in the socket. It is only about 1/4" long.
The vertical tube which in the picture is offset substantially toward the firewall should be moved forward to center on point A. This will better distribute the landing loads between the lower and upper attach points to the fuselage. It would probably be beneficial to increase the od of tube B to 9/16. This would reduce flexing and better transfer loads into the fuselage. A channel gusset between the vertical tube and tube B would be at least an improvement, an alternative to relocating the vertical.
Not visible in the picture is the lower crossmember between the lower ends of the gear sockets. This tube, like the vertical, is offset to the rear. It is well gusseted to make up for the offset, but it would be SO MUCH BETTER if it were on the centerline of the gear sockets. It might need to be moved upward slightly to maintain clearance on the cowl.
Ideally point D should be in line with point A. This will result in optimal transfer of loads from the gear leg to the mount to the fuselage.
The gusset below tube B is completely useless IF D and A are properly aligned. If I were to install a gusset in this area I would install a gusset cut from a rectangular tube between the vertical tube and the socket tube, above bushing B. Aircraft Spruce has 3/4 x 1 1/2 x .49 rectangular tubing which would be ideal for a gusset.
If the distance between point A and the lower end of the socket tube is too great, in a moderately hard landing the gear leg is trying to bend or flare if you will, an area at the bottom of the socket. I believe that the inner reinforcing tube is edge welded in the socket at the bottom edge. If the distance is too great the bottom of the socket could be shortened and rewelded. It is likely that the gusset cracks in the picture are caused by too great a distance between D and the bottom of the socket.
Finally some comments about the round gear and the flat gear. Lets consider a hypothetical paved runway with a significant pothole. We will make multiple landings, touching down just before the pothole. Prior to each landing we will have someone dig the pothole slightly deeper. The round gear will take this in stride until the pothole becomes quite deep. The flat gear however will at some point cause damage to the attach structure. The flat gear obviously has very limited tolerance for forces in a rearward direction. The downside of the round gear is that it is only in perfect alignment for one load/pitch angle. As the static load is increased the axles toe out and the camber is reduced. With the flat gear toe is adjustable and does not change significantly with different loads.

Drawing?

Jim

Thanks for a detailed reply. I think I understand what you are saying and have planned to do a mod which is common in the uk which braces the inside of the leg to the middle bolt. I'll try to post a photo of this.
Would it be possible to show a drawing, even hand done and scanned or email a drawing?


Jim (JAT)

Thanks for the way to take the powder coat off, excellent idea.

Thanks

Peter

rocketbob said:

I don't understand why you would dismiss this gentleman's opinion on engine mount cracks before he has even presented it, especially considering the fact that you have just mentioned your mount is cracked. Constructive criticism is a good thing and should be encouraged. I'm looking forward hearing what he has to say, but when people make comments like this it can easily discourage someone from participating.

Having designed and built engine mounts that are now standard on the F1 kits when they were being produced, there are design deficiencies that were addressed that I felt were a problem with RV mounts. Thicker walled gear leg sockets, increased bolt tube thicknesses and much improved load bearing on the bottom bolts have resulted in ZERO failures on the F1 mounts, even though the airplane is considerably heavier with much longer gear legs.

Click to expand...

Bob,

I am not aware of service problems with the RV-6 and -7 mounts beyond the small cracks that are discussed in this thread. Perhaps my post was inappropriate but the phrase I found interesting was "multiple design defficiencies" of the mounts. It seems if there were multiple problems we would be seeing many instances of failure other than the small cracks described herein. I'm not talking about theoretical or opined deficiencies but rather deficiencies that result in widely publicized problems. Using unmodified RV mounts on a heavier F1 is beyond the scope of this discussion.

Glad to see my post didn't discourage further discussion.

Do you have gear leg stiffners fitted?

Hey Kiwipete,

As a data point - do you have the wood gear leg stiffners fitted like Van recommends?

WRT the RV-6 engine mount *most* do not have the gusset/web bracket in which you have the cracking. Same mount without this additional piece. RV-6 cracks generally appear in the weld between the gear leg socket and the bolt tube. You do not appear to have any cracks at that site. Even so, these cracks once initiated, do not seem to progress.

My personal opinion (unqualified - I am not a ME or LAME/A&P) is to grind out the crack in the forward edge progressing from the weld material and keep an eye on the mid web cracks.

Doug Gray

gear

I agree with the last post, but I would very carefully grind away the entire gusset, then watch far additional cracks at frequent intervals.

Doug said:

Hey Kiwipete,

As a data point - do you have the wood gear leg stiffners fitted like Van recommends?

WRT the RV-6 engine mount *most* do not have the gusset/web bracket in which you have the cracking. Same mount without this additional piece. RV-6 cracks generally appear in the weld between the gear leg socket and the bolt tube. You do not appear to have any cracks at that site. Even so, these cracks once initiated, do not seem to progress.

My personal opinion (unqualified - I am not a ME or LAME/A&P) is to grind out the crack in the forward edge progressing from the weld material and keep an eye on the mid web cracks.

Doug Gray

Click to expand...

My experience exactly. The small crack in my mount at the location referred to (no gussets, no stiffeners) has been there for many years. I look at it each oil change but it seems the stress has been relieved by the crack and it isn't progressing.

Having said that, I intend to pull the mount...sometime....and weld the crack just because.....

Location of cracks

Just to clarify the crack on each side is at the rear of the gusset just under the top weld. The Mark on the front the the gusset is not a crack.
My plan is to add reinforcement as this is quite a common thing here in the UK where we do a lot of grass strip flying. Maybe only one in ten of my landings is on a hard runway.

Peter

I frequently operate off our rough-in-spots grass strip at gross weight. Over the years I have had cracks develop in the lower bushings for the bolts that are on the lower outboard corners. I TIG welded the cracks as they came along. One day last summer I took off and landed on pavement and it felt like a brake was dragging and so when I got back I pulled the wheelpant off. When didn't find anything wrong I pulled the cowling my eyes about popped out of their sockets when inspecting the mount. There is a tube welded to the lower side of the gear leg socket, about an inch long and this piece is rosette welded to the outer socket as well as around the end of the socket where the main gear leg enters. All welds broke and this piece fell down the gear leg, ending up on top of the gear leg stiffener. I will post a pic after we get back from vacation. The right gear leg, for lack of a better term, was flopping around which caused the alignment to go out of whack. There was a huge crack on the back side of the gear leg socket that was hard to see develop.

Upon pulling the mount I had some puzzled friends ask how the heck I was going to fix it. I made a new inner sleeve, welded it in as the factory did. Clamped the socket and welded the crack, and ground down the weld. I then made an outer sleeve which I mentioned in another post and welded it on both gear leg sockets. Stripped and repainted the mount and it is now better than new.

Over the years I have queried various folks at Van's and they have acknowledged they crack and that's why you see the small gusset on the latest mounts from the factory. Original RV-6 mounts did not have this gusset. I don't think the gussets do much to help prevent cracking as one can see from previous photos in this thread.

If the gear leg socket were machined out of a tube as we did on the F1 "Indy" mount, and made the lower outboard bushings thicker, there would never be problems with cracking.

As I am now rebuilding my RV-6 I am seriously considering building a new engine mount to take a shortened version of the titanium rocket gear legs. I would make them slightly longer than stock, which would require angling them forward a bit. What this would do is allow for a "real" full stall 3-point landing. Titanium gear on the rockets are a bit more forgiving. With lower axle sockets the toe and camber can be set to zero/zero. Lots of benefits of doing this!

gear

Bob- Your last post is really interesting. The Tailwind originally had the socket doubler tubes on the outside. Putting them on the inside is a significant advantage in that there is much less distortion/welding scale to clean up. The downside of the inner tube is that the tubes are not a snug fit, so a lot of load is being placed on the rosettes on the lower socket. Also the Tailwind socket doublers are 3" long on the bottom and 1" long on top. Do you have any recollection on the length of the upper socket doubler when you repaired your mount. I am wondering about the possibility that some of the socket assemblys were inadvertently placed in the welding jig upside down during manufacture??? Does anyone have an uninstalled mount that they can check the length of the socket doublers. Another issue that I partially addressed before is the relationship of the gear leg to socket on the lower end of the socket. An optimum design would be to grind a slight taper on the ID of the lower socket below where the gear leg makes contact with the lower socket mating point. This would reduce/prevent bending forces on the lower/aft portion of the socket on a hard landing. I like your idea of zero camber on the Rocket gear. On the Wittman gear if there is significant camber the toe is going to change significantly as the pitch angle is changed. I think the Rocket gear is also much better able to handle varieing weights, etc without the alxes moving around so much.

gear

Another point on the socket doubler breaking loose is that to produce a good rosette weld takes a bit of time and patience. It is very easy for a production welder to fall into the trap of rushing the process with the result that the rosette may look ok but is a very poor weld.

Doug said:

Hey Kiwipete,

As a data point - do you have the wood gear leg stiffners fitted like Van recommends?

Doug Gray

Click to expand...

Hi Doug, Kiwipete has an RV7. It is my understanding that Vans do not recommend a gear leg stiffener for the RV7 (A).... nose or mains. In fact I think they recommend AGAINST a leg stiffener for the RV7. Not that that necessarily means anything. There seems to be ample evidence that when it comes to the landing gear on the 2-seat models Vans may not be the font of all knowledge.

Stiffeners

Doug.

I don't have the stiffeners, as I've never had any noticeable vibration when landing on hard runways I never bothered.

Bob in my RV7 manual (about 6 years vintage) vans does have a section on stiffeners and talks about shimmy on the hard runways. So my impression when building was if you experience shimmy, then fitting the stiffeners might be a good idea.

My impression from talking to a few people that cracks are indeed quite common if you operate off strips.

Peter

kiwipete said:

Doug.


Bob in my RV7 manual (about 6 years vintage) vans does have a section on stiffeners and talks about shimmy on the hard runways. So my impression when building was if you experience shimmy, then fitting the stiffeners might be a good idea.

Peter

Click to expand...

Pete, I checked my RV7A manual (about the same vintage as yours) and you are exactly correct.

My mistake arose because I have a friend with a stiffener on his RV6A nose gear (it was apparently called for in the RV6A manual). I contacted Vans and asked them if it was required also on the RV7A nose gear but they recommended against it.

Shimmy

The Goodyear Flight Custom III or the top of the line Michelin tires, properly balanced, should eliminate shimmy problems. The less expensive tires are typically horribly out of balance. Also the gear leg must be tight in the lower socket.

I don't know if the wood dampers are a factor here but there is some opinion in this part of the world that they are.

One local RV-6 (>400 hours) that has dampers, is operated on paved runways almost exclusively, with a very skilled pilot has cracking. Not progressing but cracking none the less.

My RV-6 does not have stiffners. By reducing the tyre pressures the shimmy I had at 40psi pressures seems to be under control. Even so I have been told there is a substantial amount of gear leg shake on rollout that I cannot feel through the structure.

If I had the resources I would place a series of strain gauges on the legs and log data under a variety of conditions. Something tells me there is more to this than just landing on rough strips.

Doug Gray

Shimmy

If the gear leg attach hole is not a reamed hole with a very tight fit there will be movement there regardless of how tight the bolt is torqued.

My RV4 Engine mount frame is cracked too!!

My frame has cracked in exactly the same place on each side, right next to the top of the main gear leg tubes.
Im sure its reparable but can you still buy a new RV4 engine mount frame? I cant find one on the Vans website.

Cheers

Paul (G-RVNS)

pics?

I can't see any pics in the very first post?

It's from 2011 so maybe that has something to do with it?

ao.frog said:

I can't see any pics in the very first post?

It's from 2011 so maybe that has something to do with it?

Click to expand...

Many of the "free" image hosting sites from days gone by that claimed they would host images indefinitely did not live up to their end of the bargain.

This has left forums like this littered with broken image links...

RV...Phorrrr said:

Im sure its reparable but can you still buy a new RV4 engine mount frame? I cant find one on the Vans website.

Click to expand...

Yes you can still buy them.

Call Van's middle of next week and ask for Daryl, or talk with him in person this week if you attend the Aero-Expo in Friedrichshafen.

Local Repair

Paul,

Local repair of the mount is feasible. Another option is to send the mount to McCutcheon Metal Fabricators here in the states. We locally repaired similar cracks on our -4 mount (discovered at 1100 hours in service). Obviously the engine and gear legs have to come off.

At the advice of the folks that build the mounts, I primed and painted the repaired mount in lieu of powder coating after repair to facilitate future visual inspection. I used a dark colored epoxy primer and white paint for contrasting finish. Yup, not at durable as powder coat.

There is some good discussion about a related topic, lower firewall deformation, if you do a search. With the mount off, it's fairly straight forward to repair or replace the lower firewall. If you're not familiar with issues with the longeron/engine mount weldments (part numbers W-402 left and right) now is a good time to reference that as well. DWG 27 shows details. If all of this old news--disregard!

Drop a PM any time if you'd like to know more about our experience/lessons learned.

Cheers,

Vac