What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Wrinkled firewall, weldments fine - concern?

K

kleecker

Member
This is related to a pre-buy inspection of an RV-4. I will be contacting Van's on Monday, but would also like to hear the voice of those who are flying these planes.

http://www.kleecker.com/Misc/Videos/Misc/100_0342.jpg

An A&P familiar with the weldment fracture issue said there was nothing at all out of the ordinary in the regions of all four (4) weldments.

The firewall damage was observed during the current owner's pre-buy ......... he decided it was not structural, based on his research. It has, at a minimum, 100 hrs on it (as shown) and appeared the same 100 hrs ago as now.

Much appreciated,

Mark

(If no link shown, then I am in process of
 
If possible can you take a photo inside down in the lower corner where the bolt goes through? I agree the distortion shown on the SS is not structural but I don't see how it got like that. Have the lower corners on that airplane been rebuilt and motor mount replaced? If that is simply a dent it's a non-issue. That's what it looks like from the 1 photo.
 
Last edited:
asav8tor said:
If possible can you take a photo inside down in the lower corner where the bolt goes through? I agree the distortion shown on the SS is not structural but I don't see how it got like that. Have the lower corners on that airplane been rebuilt and motor mount replaced? If that is simply a dent it's a non-issue. That's what it looks like from the 1 photo.
Click to expand...

No, no, and no.
 
I personally wouldn't be comfortable until I knew what caused the dent.
 
By the looks of it, I'd say something broke at one time and has been replaced. Do you have any knowledge of how it got like that?
 
A lot of repairs get done on airplanes without being documented in the logbooks.
I would definitely look deeper.
 
As a side note you have a fairly significant fuel leak on the carburator main fuel line, if that is fuel stain I'm seeing.

There appears to be an oil leak from the oil cooler. The breather tube looks too close to the exhaust for me.

Any plane needs tweeking though.

Mel has it right, dig deeper on the dent, something caused it.
 
Last edited:
Geico266 said:
As a side note you have a fairly significant fuel leak on the carburator main fuel line, if that is fuel stain I'm seeing.

There appears to be an oil leak from the oil cooler. The breather tube looks too close to the exhaust for me.

Any plane needs tweeking though.

Mel has it right, dig deeper on the dent, something caused it.
Click to expand...

thanks for the observations; the breather tubes have since been relocated.
 
szicree said:
By the looks of it, I'd say something broke at one time and has been replaced. Do you have any knowledge of how it got like that?
Click to expand...

Unfortunately neither I nor the current nor the previous owners know - just speculation of a hard landing(s), though the previous owner (not the current owner - it was there when he purchased it) 'claims' such was never the case.
 
Maybe irrelevant, but my firewall looked something like that when it was damaged during shipping. Structurally that is a stainless steel bent edge, and some serious forces is needed to get a damage like the one in the picture.
 
Looks alot like a friends -4 that was the victim of a hard landing. His also pulled the two center supports out a little and the tire hit the fuel tank leavinga small dent. That's stuff I'd look for anyway.

On a related note I had an FAA maintenance inspector in my jumpseat at work recently who told me that the vast majority of RV incidents he's investigated were -4's with damage after a hard landing. He said the landing gear design on the -4 was the only week link that he'd noticed in inspecting RV's over the years.

I would bet that dent is from a hard landing and I'm assuming that's a replaced engine mount and gear leg.

Good luck,

Chris
 
Looking at your picture made me refer back to a picture of a -4 with known heavy landing damage. There are two creases. They go from the junction of the fore and aft rib/fuselage/bottom skin to the point where yours is crinkled. So out and upwards at about 30 degrees. There is a little crinkling on the corner in the same way, so I would have thought yours was heavy landing damage also. Do you have the 30 deg creases also?

You might want to check if it has/had the old lighter weldements, though even the new ones will not make it immune.

I put a picture of the current ones here.

Good luck!
 
My wifes RV-4 was damaged in a hard landing a few years ago and has bends in the firewall just like your picture from the landing. The motor mount and new tall gear legs were installed along with the new beefier weldments Vans came out with for the RV-4.
 
FWIW:

I e-mailed the pics to Van's, talked to Scott there. His response was basically "this is typical of what we see on -4's with over a few hundred hours. Do your due diligence in examination of the weldments and nearby locations on the firewall front side. If all looks good, do not worry about it."
 
Fuselage skin...

Tom (and others)...

How does the firewall crinkle so much with no damage to the external skin of the fuselage?

gil A

rvator51 said:
My wifes RV-4 was damaged in a hard landing a few years ago and has bends in the firewall just like your picture from the landing. The motor mount and new tall gear legs were installed along with the new beefier weldments Vans came out with for the RV-4.
Click to expand...
 
az_gila said:
Tom (and others)...

How does the firewall crinkle so much with no damage to the external skin of the fuselage?

gil A
Click to expand...

Hi Gil,
Good question. I dont know why, just that it did. I can take a picture of her firewall the next time I have the cowling off. I added additional bracing on the top and bottom weldments of the new RV-4 I am building like the RV-8 uses.
 
I should say too that my friends RV-4 also had no damage to the weldments, but the engine mount was shot. He's puting on the new gear legs, new engine mount and installing a new firewall. I should also say his looks considerably worse than yours which as I mentioned before includes a dent in the fuel tank from the tire.
 
az_gila said:
Tom (and others)...

How does the firewall crinkle so much with no damage to the external skin of the fuselage?

gil A
Click to expand...

Gil, not quite sure how to explain this but I think I understand. I suspect the actual movement ,in the case we saw in the picture below, is very slight, and if the firewall were flat you would never know. Because the f'wall has these right angle bends, it becomes very stiff. The frame clearly does not take all the load. Some is transferred to the very stiff part of the firewall. Hence the crumple. It is similar to a tube of paper. Quite light paper you can make into a tube and place a book on top, but when it goes its a mess.

The flat(ish) sheets of the 'fuse hardly move. My guess is at the moment of impact the side of the fuse bows out a little in the middle, but well within its elacticity. That corner moves up under the impact and must twist also. I say that because the diagonal marks that I have seen several times and mentioned below are indicatve of stretching along their length.

I have a friend who ran an analysis of the -4 engine frame in the CZ republic. He said after they had studied it, their conclusion was 'it just should not be designed like that!' Still I am building one. I'll just have to try and keep the wheels away fromn the tank.
 
Thanks...

Steve Sampson said:
Gil, not quite sure how to explain this but I think I understand. I suspect the actual movement ,in the case we saw in the picture below, is very slight, and if the firewall were flat you would never know. Because the f'wall has these right angle bends, it becomes very stiff. The frame clearly does not take all the load. Some is transferred to the very stiff part of the firewall. Hence the crumple. It is similar to a tube of paper. Quite light paper you can make into a tube and place a book on top, but when it goes its a mess.

The flat(ish) sheets of the 'fuse hardly move. My guess is at the moment of impact the side of the fuse bows out a little in the middle, but well within its elacticity. That corner moves up under the impact and must twist also. I say that because the diagonal marks that I have seen several times and mentioned below are indicatve of stretching along their length.

I have a friend who ran an analysis of the -4 engine frame in the CZ republic. He said after they had studied it, their conclusion was 'it just should not be designed like that!' Still I am building one. I'll just have to try and keep the wheels away fromn the tank.
Click to expand...

Thanks Steve... that actually does make sense....:)

I think I was considering a point impact load, rather than a bending of the entire side skin... and the firewall vertical side angle would spread the bending quite evenly.

gil A
 
A recent crumpled firewall experience

I, too, just recently (early Feb. 2008) experienced the worst landing I had ever made in any airplane!

The damage pictured here:

http://picasaweb.google.com/afulmer1/RV4FirewallDamage

When we taxied to our tie down spot, I visually inspected the plane externally and it looked like we had escaped with no damage. I thought this was just a tough little airplane.

I flew the plane over the next 6 weeks with 9 takeoffs and landings and did not notice anything out of the ordinary. Imagine my surprise when I took the engine cowl off for an annual condition inspection and saw the crumpled firewall. If I had not already had the cowling off this past fall and was sure no damage existed then, I might have thought the previous owner was trying to stick it to me. But I knew immediately I had done it myself.

Sent the pictures in an email to Van's and this was Ken Scott's reply:
------------------------------------------------------------------
"Allen: we've seen this before and yes, it is possible that the only damage is to the stainless
firewall. That contributes almost nothing to the strength of the corner, so if you want to
replace it (painful) you can, or you can ignore it until a better time.

The points that need careful inspection are the Wd-403/402 brackets behind the firewall.
These receive the engine mount bolts and attach the fwd longerons. They can be cracked
and it takes a bright light and a careful probing to find out for sure.

OF course, the engine mount itself should be carefully inspected, but it's doubtful that the
gear legs themselves are hurt if they aren't visibly bent and the tires track ok."
-------------------------------------------------------------------------
I still can not figure out HOW the firewall could crumple like that when there is no other evidence of damage? Maybe I can figure it out when I get the engine off.
 
Yep - -had the same sort of wrinkling on the -4 I built and owned many moons ago -- I recall the landing was 'firm' but not really that bad. We checked everything over, and decided that the SS didn't 'come back' after some slight flexing.

I think we were correct -- that ship now has 2000+ hrs on it, and no further work was required in that area. The current owner had installed a 180/CS/new mount/long legs about 3 yrs ago, and the new parts fit right up to the existing attach holes. No cracks have developed in the lower attach weldments to date, but these are the newer, heavier versions, installed in the original assembly.

So, judging from previous posts, this sort of deformation is probably not uncommon, but I would check the lower weldments for cracking. If there is none, and the mount and legs are OK, I would suggest that this is a non-issue.

If you have a -4 under construction, you might talk to the engineers at Vans about beefing up this area via larger lower longeron material or a skin doubler in that area, but you and they will need to figure in and agree upon where the stresses will transfer to! It could be as simple as changing the fwd fuse side skin to .040, but talk to the engineers 1st.

The Rockets use .040 skin throughout the cockpit area, and a have very large formed lower longeron, along with .090 4130 fittings that are installed with screws, not rivets, in this area. We've seen no firewall deformation as has happened to the -4s, and believe me I've seen some whacky landings that really should have produced some damage!

Carry on!
Mark
 
Removing tail dragger main gear?

In case I remove the engine, anyone got some experience with removing the gear legs on a several year old plane?
 
AFulmer said:
I still can not figure out HOW the firewall could crumple like that when there is no other evidence of damage? Maybe I can figure it out when I get the engine off.
Click to expand...

The key to this failure is dissimilar materials. Aluminum has a significantly higher modulus of elasticity (measurement of "springynes") than stainless steel. So if you had an aluminum rod next to a metal rod, bend them both say 2%, the aluminum rod would require much less force.

All that beefed up 2024/6061 around the engine mount are designed to take all that landing and fibrational stress. All that aluminum won't actually take all that load without deflecting (not talking inches here, like millimeters). So as the load applied to the aluminum deflected it relatively easily. That stress was then transfered to the stainless which couldn't get out of the way without buckling.

Imagine glueing a rubber band to toothpick. Bend it, which breaks first?

Earlier folks discussed how it would have taken a lot of force to crumple this. For arguments sake, lets see... It was a real stiff landing, so let's say 2g's, lets say after the aluminum bent out of the way, it left 90% for the firewall to "pick up"... That's 1,400 lbs on each side... .010 (from memory) sheet, yeah, it would buckle.

-Bruce
 
Last edited:
This is very interesting. The firewall not being a structural member, but acting as one. I see one problem with this. When the firewall buckles and deforms, it will not get back to it's original shape, or overall dimensions, without a much larger force the opposite direction. Let's just say for the sake of the argument that the landing is 4g. Half of that is taken by the fuselage and half is taken by the firewall and this causes it to buckle. Then, because it won't stretch back (it is deformed), this causes a pre-stress in both the firewall and the fuselage of about 1-2g or something from thereafter. This is not good for fattigue (in theory at least). I also think really hard landings are more in the order of 6-7+ g's, depending on the definition of "hard landing" :)

The problem is not the modulus of elasticity, the same would happen if the firewall was made of aluminium. The problem is that the fuselage is designed to flex, the firewall is not. Structurally the fuselage simply flexes, while the firewall is being compressed to the point of instability and permanent deformation. The fuselage can flex more than the firewall, and instead of the legs, to such a degree that the firewall will deform at a hard, but not "structurally damaging hard" landing. In other words, the fuselage is simply too flexible (but obviously strong enough).

Buckling or not, somehow I feel that this is a no-issue structurally. The fattigue limit will be lower due to this, but by how much? 10% maybe?
 
Modulus of elasticity

All steels have a modulus of elasticity around 30 million psi. All aluminums have a modulus around 10 million. Strength varies greatly for different alloys within these each of these classes of materials, but stiffness is basically the same.

That being said, the SS firewall is annealed, so it does not take much deflection for the material to yield (not return to its orignal shape).
 
Just how fragile is the -4?

What I have never managed to find out are just how heavy the landings are required to be to achieve the degree of deformation we see. I am concerned because I fly into a 1020' strip.and some landings could undoubtedly be tidier if I had another 300'.

If Tom V's wife's landing involved the wheel touching the tank, and the sort of damage we see in these pictures was all that happened, then I have no concern. But, if A Fulmer's "the worst landing I had ever made in any airplane! " involved no more than dropping it in from 12 to 18" I am worried. I would be interested in your comments Mr Fulmer? How heavy was it? Any others also.

Also, when people report damaged -4 it would be really interesting to know which generation of firewall weldements they had. Everything I tracked down in the UK appeared to be using items from before the current issue.
 
No weapon too short for a brave man...

Steve,
In 1500 hours my RV4 landings were 88% grass/gravel/other than asphalt. I operated my RV4 off some VERY rough strips during those 12 years and never had one wrinkle, dent or crack anywhere. I routinely flew mine from a one-way 750' strip that was anything but smooth as well, no worries. The key on rougher strips is to land 3 point full-stall, and distribute the weight evenly. The 380X150X5's help too...(Thanks)
I have seen some guys force the mains on and porpoise on and off the ground, or stall 3 ft off the ground, not good either. I believe this is where the damage is inflicted, hard asphalt contact. A super Cub driver like yourself should have no problems mastering off pavement with your 4.

Rob Ray
RV4ormerly
HR2
 
how hard

I haven't flown my -4 yet, I have 400hrs on an RV-3 and 10hrs in various other folks RV-4s (front seat). I've been in direct contact with several folks transisioning to more demanding aircraft. RV's, KIS & Pulsars.

One thing I've seen over and over, if one is a hand-fisted pilot unable or unwilling to learn how to properly fly and land an aircraft to a high standard, you will eventually bend your RV. It's just a matter of finding the right x-wind or plunking onto the mains hard and setting into motion that tortuous buck that bends firewalls.

As a CFI, I know all to well that 90% of active pilots today learned to fly on 172s and think every airplane should take this punishment. RV's just can't. they just aren't built for it, they have to be flow like their built. Well!

To any pilot that want's to learn how to fly well, here are a few techniques.

#1 - Find your local x-military runway that's 10,000' long. Setup to land, and try to fly slow flight 10' off the deck from one ILS runway markers to other. Do that a few times getting slower and lower. This will teach you a great deal about precision flying. the key is to teach yourself how to put the plane exactly where you want it, exactly how you want it.

#2 - Take gliding lessons. There's no better landing aircraft than a glider.

#3 - Do a lot of dead-stick landings. Fly to the perch and cut throttle to idle. If you can consistently hit the numbers, you understand approaches.

One more specific tip to early tailwheel RVs... DO NOT try to stall out 3-pts, like you would in a Champ or a Cub. Angle of stall in an early RV is W A Y steeper than it's sitting attitude, you will hit tail first and you will slam onto the mains. From those of us who have done it... don't do it!

-Bruce

Just a few tips from a guy who's done his share of prangs too.
 
firewall

I am dealing with the same issue, pics enclosed. In my case the previous owner claimed no knowledge of the damage but is willing to pay to have it fixed. I know its a big job but feel for resale it would be wise. Any opinions? Estimates of time required?
Whay annoys me is that I paid to have the airplane inspected by an RV4 expert and it went unnoticed.

Thanks..
IMG_0312.jpg
 
Dave- I feel for you. Clearly he was not an expert.

I would be interested to know which generation of weldemnts you had. The current generation, delivered I think since '94, can be seen at item 3 here.

There are two earlier parts which VANS shipped and then beefed up to try to stop this happening. I wonder if any of the pictures we have seen are with the current post '94 parts?
 
Hash Marks?

Dave,
What are the four lines on the MLG receptacle? Are those scratches, gouges or jjust markings?
 
weldments

Most people seem to feel the weldments are of the heavier type and possibly were replaced. They seem to be ok but I am going to make sure. I have had about 10 different builders/experts have a look at the airplane and everyone feel s it is serviceable but I would like to repair the damage. I intend to fly it this summer and plan to fix it in the fall...
I believe the marks on the engine mount are scratches. The mount looks ok.

Dave
 
777Dave said:
I am dealing with the same issue, pics enclosed. In my case the previous owner claimed no knowledge of the damage but is willing to pay to have it fixed. I know its a big job but feel for resale it would be wise. Any opinions? Estimates of time required?
Whay annoys me is that I paid to have the airplane inspected by an RV4 expert and it went unnoticed.

Thanks..
IMG_0312.jpg
Click to expand...

I was quoted $5K by someone who owns an RV-4 which I looked at. Now this was to replace all the 4 weldments as well.
 
I don't see how the weldments will fix this unless they are considerably larger and will make the fuselage stiffer. Does anyone have any pictures of the two versions to see the difference?
 
Bjonar, I think it works like this. VANS have twice increased the thickness and design of the weldement. Most recently in '94.

The early weldements were just cracking up so not doing their job of keeping everything else in their relative position, thus releasing unintended forces into the rest of the structure. The structure while it is naturally flexible since it is a tube, was then flexing even more than intended. And as you summarised somewhere below, the firewall which in the area of the right angle bend, but in fact was never designed to resist the compressive force then buckles, or worse, cracks.

So I think VANS intent was to provide a part that would not crack. What I dont know is if all the buckled firewalls are on aircraft that started out with the flimsier part. No one ever says.

There is in fact (I think) another part that was changed. the triangular fillets on the engine frame, where it bolts to the weldements were not there originallly. Again they stiffen the whole structure.

Having said all that, I think in fact the firewall is still being stressed by heavy landings with the new parts, so I am sure if you give it enough bad treatement you will still get firewall damage.

I had a conversation with a Czech who has asked the Technical Aero Uni in Prague to run a stress analysis on the -4 U/C. The point they came up with allegidly was the design clearly was not ideal. I think the issue is because in a heavy landing, because of the aft sweep of the gear legs there is a very strong twisting force. The design of the frame is triangulated to try to resist this, but it is quite a light structure with short resisting arms.

The -8 I think in contrast has an u/c which is almost perpendicular to the landing forces. (Possibly bolted to the spar?) The -7 and -9(a) do have swept u/c but it attaches to heavy steel parts which bolt very directly onto the spar, so little load transmits through the fuse'. The -4 is unique in that the loads are taken into the frame, with the engine trying to bend it in one direction, and the entire fuse' in the other.

I think Smokey's comments are about right. Nice smooth low energy landings. I have done 1086 landings in my Supercub to date. I had better do some more and you work on your score. The cub you are involved in looks fun!
 
I was just looking at my firewall and it occurs to me that placing something small and heavy (like a battery) right on top of the firewall shelf could produce an additional tendency to crumple the lower firewall in the event of a hard landing. Also, I'm wondering if the addition of wooden gearleg stiffeners might make the situation worse.
 
Steve, first off you wont do hard landings!

Did you put yours on the shelf? I think Smokey's is/was that way. I have put mine above, but not on the shelf. I put a piece of angle across which ties in to one of the vertical angles on the inside of the firewall, and the rudder pedal brackets. Both of them, with AN3. I can find pictures if you are interested.

In all honesty I would think it makes little difference. The shelf is 'hung' on the firewall and the angle cross member above, so little extra strain I would have thought.

The way that I visualise the movement is all about the bottom corner weldements moving up wrt everything else.

How is it going? You must be nearly ready to fly? I am 6 or 8 weeks off I think.

PS I dont think VANS are so enthusiastic about the wood stiffners these days. I didnt have them on the -9a and never missed them and I even flew that a few times on concrete. Why not leave them off unless you need them. Its all more work.
 
Last edited:
Steve,

I actually put the battery between my feet, but I still can't quite get my head around how the firewall gets so bent. I know everyone is focusing on the corner weldments, but I can't understand how the firewall gets wrinkled without also deforming the firewall framework, particularly the vertical members.

As far as my completion date, I really have no idea. All my plane needs at this point is for me to get the cowling and spinner in paintable shape and then to paint the whole project. That being said, work has to come first so things are going slowly. I'm hoping to be done perhaps by the end of summer.
 
Just a thought - Is there some pitch (Y) axis rotation going on at the lower mounts during a hard landing that would only cause elastic (recoverable) flexing of a flat 6/7/9 firewall but which causes plastic (permanent) buckling of the footwell folds on a 4? It might even happen on the rebound.

There is after all a huge pry-bar/torque arm in the form of the landing gear leg pivoting roughly about that point.

Simple compression of the firewall to cause buckling seems unlikely since it implies that the engine mounting frame compresses too.

Beefing up or cross-bracing the vertical tube of the engine frame might help to reduce the rotation and avoid the buckling.

Sounds like a neat problem for a college student with access to non-linear, dynamic FEA - and a fast computer.

Jim Sharkey
 
You must log in or register to reply here.
Back
Top