I have always worried that the high exhaust heat (1800F) of the rotary would do the turbo compound version in.

In Hooker's "Not Much of An Engineer" book he mentions 1200 C for the turbine blades. At first I thought he had made a serious error and it was really 1200 F. No No! Not true. 1200 C!!!!!! Wow!!! This was 1970 no less. 1200 C is on the order of 2200 F. At this temperature an AC piston engines exhaust valves are long gone.

Quote: Page 183

As for the vital HP turbine, this again went well beyond the previous state of the art. Derby ( RR) had been one of the pioneers of air cooled turbine blades in the Conway bypass jet designed in the mid-1950s. All Derby blades were forged in a Nimonic high nickel alloy, able to retain adequate strength at about 900 C. In the Conway the peak gas temperature was about 1,100°C, so the cooling air holes and passages inside the blade had to keep the blade at least 200°C cooler than the surrounding gas. In the RB21 I the gas was going to be at 1,250°C. This may not sound a very big increase over 1,100, but the strength of the blade is halved for every extra 25°C!

To make a blade at Derby (RR) the rough metal bar from which the blade would be fashioned was first pierced by a large number of long but small-diameter drilled holes. Just where these had to be was found by a laborious process of trial and error, so that, after the rough blank had been forged, the holes were all in the correct positions. To forge the blade it was squeezed while almost white hot between two dies having the correct shape to form the blade.

Finally the whole blade was machined all over to precisely the correct size and profile. The price of each blade was inevitably very high, and the whole process was an engineer's nightmare because microscopic variations between the cooling air holes in successive blades meant that the cooling would be uneven.

At Bristol we had abandoned forged blades soon after 1960. Instead we cast our blades using the famous lost-wax process used 500 years earlier - by da Vinci, for example - to produce precisely repeated sculptures. Casting enabled us to use materials basically stronger than Nimonic alloys, too hard to be forged or machined. We cast our blades to the finish dimensions, with a mirror finish, without any machining apart from precision grinding of the root. Cast blades proved to be much stronger in tension than forged and machined blades, and despite their brittleness and susceptibility to thermal stress we proved on the Proteus that they are superior. On the Olympus 593 [Concorde engine] and Pegasus [Harrier engine] we produced large numbers of eminently reliable blades with radial air cooling passages operating at some 1,250°C, much hotter than any production engine at Derby at that time.

End quote.

A turbo compound rotary is a piece of cake.

This book is a real education in aircraft engine design.

Paul Lamar

  Very interesting information about turbines and turbine blades,
    hard to find together without spending hours, now that Paul signs
    in classical Greek Alphabet, one anecdote: 25% of Spaniards do
    have the so called 'Greek foot' where the second toe is longer than
    all the rest, the same as in the statues of classical Greece, my only
    remaining doubt is if the women and men that are in the classical
    Greek sculptures where someone real, or just of 'ideal' or 'imaginary'
    beings, As far as I know, the people who built this culture (more than
   60% of the inhabitants of Athens were 'slaves') left no genetic or
    biological remains, just literature and lots of legends.

     (I guess the search for 'Golden fleece' was some kind of an 'extreme
     sportive event'. The Merino sheep have its name from a Berebere tribe,
      arriving to Iberian peninsula with the VIII century Arab invasion.
    The contest for Greeks may have been going to where the Merino
     tribes lived, in what is today Morocco and Atlas mountains, cut
     a fleece from their sheep, and come back home with it, outwitting
    the sheep's guardians)
       Thanks for your attention, best regards. Salut
             Jose Gros-Aymerich; Madrid, Spain


The merino sheep were the salvation of the early Australian shepherds and they are still the basis for Australian fleece.

Peter Duffecy



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