Straightening propellor blades

The guys at the Freeman Field Recovery Team have a lot of experience in straightening German and Japanese blades dug up from the burial pits on the site.

Have a look through the photos on their Facebook page for examples of their work.

https://www.facebook.com/pages/Freeman-Field-Recovery-Team/100776559984890

We still do get them straightened, years ago we had a Seneca hit a rabbit hole on take off from some horse racing course, he bent all the tips 90* fwd! they looked like proper Q tips, and because on grass they were not marked. Having flown straight to us with bent blades he was not happy with the quote so flew away again! Still sporting the bent blades to get another quote…… He’d even mentioned it was rattling his teeth…. Stupidity is often ruled by ones wallet :apologetic:

The father of a friend of mine used to fly a Rapide for the Army Parachute display team in the 1960’s . One busy weekend, they took off for a display jump, but on the take off run, with the tail raised, they ran through some hidden undulations in the grass, and nibbled the prop tips along the ground.

Take off abandoned, they inspected the new profile blades, now sporting a rakish lean to the rear. Pilot chappie goes off to find a phone, and on return, is told by the paras that ‘it is now fixed !’ Being very ‘press-on’ in their attitude, they had pounded the blades with a lump of iron, and whilst not straight, were in deed ‘a bit straighter’

Though tempted, the pilot had a sudden attack of common sense, and binned it for the day ! In a Rapide, the driver is pretty much in line with the props, and bits whirling off would make anyone a bit thoughtful.

A number of years ago, I watched a WWII film that covered the repair of metal bladed propellers. The film started with Queen Mary’s arriving with some seriously bent, holed, and generally trashed props. The whole process of repair was covered, including cold working, straightening and pitching these props. IIRC, they followed one particular Hydromatic prop from start to finish, but also showed other props undergoing repair. This Hydromatic prop had bent blades AND a 20mm hole in one blade about one foot from the blade root. The hole was blended and dressed, but not plugged, and the bent blades were cold pressed straight. At the end of the process, the prop was placed on an arbour over a balancing pit, and a feather was placed onto a horizontally positioned blade. The prop rolled under the weight of the feather.

Although most of what was done then is considerably out of civil allowance for repairs now, if you can find the film on the web, it is most enlightening.

Bet the holed blade whistled like crazy.

Camlobe

Thanks for the reply, very interesting to see that it is done cold, i would have thought that heating them up first would make it easier.

Metal blades were routinely straightened in the war but there were prescribed limits beyond which they were sent for scrap.

I’ve straightened blades before but, as Merlin Pete says, they take a lot of hyd pressure to do it and the nearer the shank the more they need. In fact, less than two thirds down from the tip would require such excessive force (probably 50-100 tons-plus)that the blade surface would probably be damaged.

Once I’d straightened a blade, the problem was trying to re-introduce the pitch twist accurately and this was next to impossible – at least within the limits of my experience.

In the book Lincoln at War (Garbett/Goulding) there is a story of a Lincoln crew who went out to an aircraft to fly it to the scrappers. They found six inches of the tip of one of the prop blades bent back. Not knowing quite what to do and knowing the aircraft was taking its last flight they got a couple of sledge hammers, belted it straight and flew it away.

Anon.

Yes you can, you straighten them cold, but you might need a sizeable press if it is near the shank.
If they have age or work hardened then the area can be annealed by marking with a permanent marker pen, then heating with a propane torch of the kind used to do flat roofs, until the pen line starts to fade, then allow the blade to cool.
The wartime DH propeller AP’s show blades with a near 90 degree bend being straightened in a press.

Pete