Chromemolly vs mildsteel?

[bergdwerg]

I understand that the bushbaby frame is not out of chromemolly, how strong is it, what is it made of , pro,s and cons etc
Regards
Bergdwerg

[Morph]

Moerse strong,

I have a Bush Baby Explorer, unfortunatly recently ended up on it's roof after loosing a front wheel on landing

However the airframe took the impact very well. There was slight bends on the lower longerons and on the front fork anchor points. Both of these very easy to straigthen and repair.

The pros, price, simple to repair
The cons, weight and having to protect it really well from corrosion. I had my entire airframe powder coated, definately worth it.

[Miskiet]

Chromoly 4130 is a lot stronger than mild steel but it is very expensive. It is common in the AC industry because of it's good strength/weight ratio.

Drawbacks is that it is prone to cracking (your frame needs to be METICULOUSLY engineered - only buy plans/frames from people with a long track record) I heard NASCAR banned chromoly because of this cracking issue (hearsay)

Normal mild steel will be a little more"bendy" and a lot easier to fix. It is cheaper too - Price usually means the little weight penalty is easy to live with (I think BB frames use a slightly thicker pipe wall thickness to compensate for the lower strength of chromoly vs mild steel so your plane weighs a few kg's more but is just as strong)

[bergdwerg]

Thanks guys, that nice to know.If i go that route, i will def get it powder coated.

[skybound®]

Powder coating great for the outside but what about the inside. Perhaps not a good alternative for an aircraft to be kept anywhere near the coast.

[grostek]

Crop sprayer fuselages work in a very corrosive environment (Chemicals)

When the fuselages are manufactured, all intersections where pipe meets pipe, have a small hole drilled in them. These holes are invisible after welding because they are inside the pipes.

After welding the fuselage is complete, a hole with a thread is put into the main longerons Usually at the front where the Longeron interior is blocked off with a round disc. The threaded hole is in this round disc.

The main longerons are then filled with Raw Linseed Oil, and the holes plugged.

The fuselage is turned a couple of times over a period of some hours.

The Raw Linseed Oil will have seeped into (Hopefully) all steel tubes in the structure.

After this the excess Raw Linseed Oil is drained off and the threaded hole plugged with an allenscrew.

Then the fuselage is cleaned on the outside , sand blasted and then given two coats of Zinc Chromate primer, then given an undercoat and finally a coloured coat of this lot.

IMHO Powder coating is fine, but it is prone to cracking if Fuselage is subjected to heavy loads, (Heavy landings etc.) You then have a place where corrosion can "Grow" without being seen under the powder coating.

Paint is applied in Micron thickness whereas Powder coating is much thicker probably to the order of 1mm or possibly even more. Powder coating is also not flexible. So when the base tube moves for any reason you may get cracking in the Powder Coating.

Kind regards,

Gunter Rostek.