Dry Ice for Control Surface Hail Dings

[Steve65E-NC]

Can anyone give a report on the successful application of Dry Ice for smoothing out Control Surface hail dings.  I am an ME but cannot get my mind behind the theory for this process.  I have often used light ball peen hammer dents on the inside radius  to straighten out a very heavy plate or industrial drive shaft.  I understand in that application how the compressive cell left around the many cold work dents tend to straighten out the item.

Back to the Control Surface:  It seems to me that a hair dryer to gently  heat up the surrounding area of aluminum might assist what ever process is at work on the hail ding.  However, I am not clear if the dry ice goes right on the ding or if the immediate area around the ding is more important.  If I better understood the physics of this process it would be easier to plan. Does any one know what is happening at the metal grain level and what, if any, residual stresses are part of the action.       

[Hank]

Skipping the part if your question about the grain structure, think back to the basics:  things expand as they warm, and contract as they cool. Seems that putting dry ice (much colder than water ice) will make the dent contract, and as the metal contracts the surface area reduces, drWing the dent out because a flat plane with a sudden radius has more surface area than a flat plane. It won't be possible to place the ice only on a dime-sized dent, but even a hand-sized area around the dent should suffice.

but then again, this is a thought experiment conducted without ever performing or observing the actual process. Just another ME whose material science classes were a long, long time ago, but I have stayed at several Holiday Inn Expresses since then.

[Yetti]

Seems like liquid nitrogen like they use to burn off the pre cancers would be a lot more pin point application.  Shrinking metal would be the theory

[Hank]

Dry ice is easier to obtain, and to handle without injury,than liquid nitrogen.

[Steve65E-NC]

  Hank and Yetti, so you are suggesting that we can tension the aluminum past its yield point?  That might recommend area heating with more localized cooling?  I think a calculation could be here involving thermal expansion coefficients.  I agree that if you can pull the Aluminum past yield (thus wiping out any local/dent stresses)  while constrained to flatness, that just might do it.

  I wonder if anyone on the list has actually successfully done this trick and can confirm that it works for whatever reason?  Seems I have heard it talked about a lot, but no direct report.

[Releew]

Before the Mooney I had a 172 that was in a minor hail storm.  After attempts with either Dry Ice, Heat Guns and even a Vacuum pump with a suction cup in conjunction with the dry ice and the heat gun the most success I had was with a .5" Dia. rod 36" long with a rubber tip on the end.  Most of the dents were reachable through inspection hatches and could be pushed out.  Be very careful.....  One can do more harm than good.  Get a light positioned in a place in which you can really determine EXACTLY where you are pushing up!

There are sever folks who remove dents like this for a living.  Depending on the severity, they usually charge from $25-50 per dime dent.  A lot cheaper than breaking the Paint!

The theory behind the dry ice is it is supposed to reverse the impact stress by rapidly cooling the surface area in a specific targeted location. I have seen this work with small imperfections on thinner control surface skins.  I have had limited success, but it did actually work when I used a small shop vac after cooling.  You can hear the pop when it does actually work!  Few and far between!

I have always wondered what a metallurgist would have to say about using this methodology???????  Does aluminum fully recover after being hit with such a rapid D/T?

Rick

[Hank]

I would think recovery from brief cooling would be simple--don't we fly in sub freezing temps at altitude? How cold are the ailerons on jets at FL410? On the other hand, just a brief time with a torch can permanently alter the molecular structure of the aluminum, and thereby its properties (to say nothing of accidently scorching the paint or melting a hole . . . ).

[Bob]

I have specialized in paintless hail repair, on cars for the last 20 years.  Dry ice, suction cups, let is heat up in the sun then putting cold water on it, the infomercial ding king tool and heat guns are all MYTHS!  Please do not try to push the dents out yourself, you will mess them up!

The problem with paintless dent repair (PDR) on airplanes is the access.  The thinnest panels on the Mooney are the ailerons and there is no way to get a tool in them and also have proper leverage.

When hail hits a panel, it stretches the material.  The only way to remove it is by shrinking the aluminum.  This is done by a whole bunch of very small pushes.  In order to push the dents, a pivot point is needed and unfortunately in a aileron for example, that pivot point would be the lower skin.  So if you can get a PDR rod in the small holes, you will be pushing out the dents on the top and also doing damage underneath the panel.

FYI- I do hail repair for a living and I have light hail damage on some of my control surfaces on my own plane. 

[Hank]

Well spoken, Bob! A professional voice of reason.

[N601RX]

Has anyone tried to put a steel ball on the inside and use a magnet on the outside to roll the steel ball back and fourth over the ding?  The theory behind this is that the steel ball has a very small contact area with the dent and can concentrate a lot of force in a very small area.  There are some u-tube videos of dents in musical instruments being removed this way.

There is also this system, it would be interesting to know how expensive it would be if you delivered the plane to him.

http://www.fluxtronic.com/index.php

[Yetti]

I always like the idea of steel balls as it would mimic the english wheel process.   Hand working metal fascinates me

 

[Steve65E-NC]

Ok, Has Anyone Ever Satisfactorily  Repaired Control Surface Hail Dimples Short of Re-Skinning.  So far nothing reported to justify a process effort. 

[Releew]

Yes.... but the end result is based on if you can get behind the dent.  Like I mentioned.....  Few and Far Between.  Re-skinning is a very expensive process.  If a paint job is on the horizon, filling the dent is the best option.  I've repainted several aircraft with multiple dings and dents and only use 1/2-1 quart of filler. It really doesn't take much.  If you go that path be sure to use etching filler designed for aluminum surfaces. The correct filler sticks very well to aluminum and feathers nicely.  

Just an opinion but it saves time and A LOT of money.....  If a paint job is near!

Rick

[Bob]

If dry ice could help me remove dents, I would have been a long term buyer for the last 20 years.

With the ball and magnet idea, How are you going to get the steel ball in the aileron?  The only holes are tiny drain holes.

601RX, I did talk to the owner from  http://www.fluxtronic.com/index.php about 10 years ago.  The machine when I got a quote was about 300K.  Basically it is very interesting.  You place the tool on the outside skin, it is energized and forms a magnetic field under the panel, the power is shut off and the field collapses.  This causes a impact or push from underneath the skin and shrinks the dent.  The process works poorly on steel panels, due to its magnetism pulling the energy into the steel rather than under the panel.  Their process actually favors aluminum vs steel panels due to aluminum having poor magnetic properties.  I believe they may have some sort of traveling demo and service schedule.  It may be good to talk to them.

Steve, it looks like you are down to 3 options to remove the hail damage. Re-skin, filler or the electromagnetic dent puller.

[jetdriven]

3 hours ago, Releew said:

Yes.... but the end result is based on if you can get behind the dent.  Like I mentioned.....  Few and Far Between.  Re-skinning is a very expensive process.  If a paint job is on the horizon, filling the dent is the best option.  I've repainted several aircraft with multiple dings and dents and only use 1/2-1 quart of filler. It really doesn't take much.  If you go that path be sure to use etching filler designed for aluminum surfaces. The correct filler sticks very well to aluminum and feathers nicely.  

Just an opinion but it saves time and A LOT of money.....  If a paint job is near!

Rick

What specific brand and part number of filler would you recommend?

[Steve65E-NC]

Bob, The fluxtronic looks kind of interesting and I am not that far from Nashville, Tn.  Think I will give them a call.  Will report back to the group.   Thanks

[Releew]

23 hours ago, jetdriven said:

What specific brand and part number of filler would you recommend?

I use a product call USC Aluminum Filler or Ever-coat Gold.  With the new FORD Aluminium Body trucks on the market aluminum filler is readily available.  Go to a Paint and Body supply store.  A typical auto parts store probably won't carry it. Bottom line.... make sure its formulated for aluminum adhesion.  If not, it will lift over time and you cannot feather-edge into the surrounding metal.  In other words, it will look like you filled the crater but the circular ridge will be view-able.......

Rick