Metalworking Question for Bruce Frank or anyone else with expertise

After a rough landing, I minimally bent my experimental's tailwheel spring
and need to have it straightened. It is a piece of round solid ferrous
steel, 39" long, 1.25" in greatest diameter as it enters the fuselage
midway along its length and tapers to about 0.75" at each end. Alloy
composition and type of heat treatment are unknown. At max gross, the
static load on the tailwheel is ~205 lb.

Is it possible to have it straightened? If so, what type of shop should I
look for to do this kind of work. Does it need to be heat treated after
straightening and if so, to what PSI. Any help is appreciated. I live in
Phoenix but am glad to ship it somewhere to have the work done right.
Thanks in advance.

Dave Uhrbrock

Here is what I would do + an explanation of my reasoning:

39" long and a static load of 205 lb gives a bending moment of 39*205 = 7995
in lb.

Assume a ground reaction factor of say 3 which is not unreasonable based on
experience - this can only be a guesstimate because you didn't provide
enough info to calculate it properly. Thus the bending moment at limit load
is 7995 x 3 = 23985 in lb. In order to calculate the actual ground reaction
factor the deflection of the spring has to be calculated, the decent
velocity of the airplane during landing estimated (function of wing loading)
and an analysis performed based on energy of the impact cf strain energy
stored in the spring. Beyond this dicussion.

The max moment will occur where the spring enters the fuselage where the
diameter (D) =1.25" (note I am assuming that this is the critical location
but on some tapered rod springs the maximum may be at some other location on
the spring - it needs a lot more work to establish if that is the case).
The second moment of area (moment of inertia) of the spring at this location
is

I = (pi * D^4) / 64 = 0.1198 in^4

Thus the stress is

s = My/I = 23985 * 0.625 / 0.1198 = 125130 psi

where y = 1.25 / 2 = 0.625"

Applying a safety factor of 1.5 which corresponds to the reserve energy
requirements of FAR 23.723/726 the ultimate stress is 187695 psi.

Assuming that the spring is 4130 / 4140 etc then it is clearly going to need
heat treatment. This approximately equates to Rockwell C 41 (191 ksi). I
would probably heat treat the spring to more like Rockwell C 47-48 to
provide some additional margin (these are typical heat treatment levels for
a landing gear spring).

You will need to anneal the spring prior to straighening. Then re heat treat
following straightening. I would just take the spring to a shop who does
heat treatment work for aircraft - heat treatment is relatively cheap to
have done so their is no sense in trying to save money by taking it to an
auto spring shop etc.

See MIL-H-6875 for specific process specifications for the heat treatment.
Get that from http://stinet.dtic.mil/str/index.html

Only catch is that you are going to have to figure out what alloy the metal
is before you try to anneal and heat treat. Can the kit supplier or builder
help with this ?? Otherwise there are some ways to figure this out but I
won't elaborate on that yet.

Hope that helps .....



"David H. Uhrbrock" wrote in message
news:SWP1d.5756$Xc.546@lakeread01...
After a rough landing, I minimally bent my experimental's tailwheel spring
and need to have it straightened. It is a piece of round solid ferrous
steel, 39" long, 1.25" in greatest diameter as it enters the fuselage
midway along its length and tapers to about 0.75" at each end. Alloy
composition and type of heat treatment are unknown. At max gross, the
static load on the tailwheel is ~205 lb.

Is it possible to have it straightened? If so, what type of shop should I
look for to do this kind of work. Does it need to be heat treated after
straightening and if so, to what PSI. Any help is appreciated. I live in
Phoenix but am glad to ship it somewhere to have the work done right.
Thanks in advance.

Dave Uhrbrock

On Wed, 15 Sep 2004 20:14:44 +1000, "smjmitchell"
wrote:


Here is what I would do + an explanation of my reasoning:

39" long and a static load of 205 lb gives a bending moment of 39*205 = 7995
in lb.

Assume a ground reaction factor of say 3 which is not unreasonable based on
experience - this can only be a guesstimate because you didn't provide
enough info to calculate it properly. Thus the bending moment at limit load
is 7995 x 3 = 23985 in lb. In order to calculate the actual ground reaction
factor the deflection of the spring has to be calculated, the decent
velocity of the airplane during landing estimated (function of wing loading)
and an analysis performed based on energy of the impact cf strain energy
stored in the spring. Beyond this dicussion.

The max moment will occur where the spring enters the fuselage where the
diameter (D) =1.25" (note I am assuming that this is the critical location
but on some tapered rod springs the maximum may be at some other location on
the spring - it needs a lot more work to establish if that is the case).
The second moment of area (moment of inertia) of the spring at this location
is

I = (pi * D^4) / 64 = 0.1198 in^4

Thus the stress is

s = My/I = 23985 * 0.625 / 0.1198 = 125130 psi

where y = 1.25 / 2 = 0.625"

Applying a safety factor of 1.5 which corresponds to the reserve energy
requirements of FAR 23.723/726 the ultimate stress is 187695 psi.

Assuming that the spring is 4130 / 4140 etc then it is clearly going to need
heat treatment. This approximately equates to Rockwell C 41 (191 ksi). I
would probably heat treat the spring to more like Rockwell C 47-48 to
provide some additional margin (these are typical heat treatment levels for
a landing gear spring).

You will need to anneal the spring prior to straighening. Then re heat treat
following straightening. I would just take the spring to a shop who does
heat treatment work for aircraft - heat treatment is relatively cheap to
have done so their is no sense in trying to save money by taking it to an
auto spring shop etc.

See MIL-H-6875 for specific process specifications for the heat treatment.
Get that from http://stinet.dtic.mil/str/index.html

Only catch is that you are going to have to figure out what alloy the metal
is before you try to anneal and heat treat. Can the kit supplier or builder
help with this ?? Otherwise there are some ways to figure this out but I
won't elaborate on that yet.

Hope that helps .....



try straightening it cold.
if that doesnt work(and I've seen my undercart legs bent through 45
degrees without hint of straightening the kink) then do it hot.

to straighten the spring you just need an oxyacetylene torch.
hold the fuselage end of the leg in a vice well padded with bits of
sheet metal to prevent marking of the spring. (take it off the
aircraft if that isnt obvious)
heat the area of the bend plus about an inch or two each side, getting
the place where you want to bend it nice and cherry red hot.
bend it to the position needed. in the red heat stage it will be
reasonably pliable. I bent mine just using a leather gloved hand to
pull it straight.

DO NOT QUENCH IT !!! you risk cracking it.

now take it down to the tempering shop and tell them that you want it
tempered to 200,000psi ultimate tensile strength.

my 4140 tailsprings are tempered according to steve wittman's figures
for 6150 and havent given any problem in service.

I straightened 9 bends in one 6150 main leg and 6 in the other using
the technique given above and had the local truck spring place temper
it.
beadblasted they have now been in service for two years.

you can err with the hardening temperature by going over temp a little
but the tempering temperature is critical for the final strength.

btw when the hardening is being done the quench must be done by
dropping the spring vertically into the quenching oil. you mustnt hold
it horizontal and drop it into the oil. if you do the side hitting the
oil will shrink. in my mates case the double taper tailwind tailspring
ended up straight on the side that hit the oil first. he managed to
cold straighten it.

the tempering consists of heating it to a particular temp. holding it
at that until saturated with heat evenly then letting it air cool on a
rack.

once you heat the spring the spring temper is lost in the heated area.
re tempering it from scratch erases all previous tempering.
Stealth Pilot

Try Harmon Lange:

http://www.langair.com/

He'll set you straight.