View Full Version : A400M supra-divergent props
20th Jul 2011, 08:43
Looking at this photo of the A400M, it would appear that props 1 & 4 are supra-convergent (as one would expect to minimise P-factor) but that 2 & 3 are supra-divergent.
Why has it been designed like this? There is presumably a substantial advantage of making the centre props supra-divergent, otherwise one would expect all props to be supra-convergent to minimise P-factor.
If the aircraft had a low tailplane I would hasard a guess that it would help reduce tailplane drag at some flight regimes, but surely the high T-tail is not affected by prop upwash from the middle engines?
Or is it to reduce the total prop downwash over the wing to reduce dihedral effect?
20th Jul 2011, 10:53
No, not contra-rotating.
I would have thought people on this forum would know what supra-convergent means!
To stop further irrelevant replies:
Supra-convergent - upper prop tips rotate inwards towards aircraft centreline
Supra-divergent - upper prop tips rotate outwards away from centreline
Why are the inner props supra-divergent? One would anticipate that normally all props would be supra-convergent to reduce P-factor.
20th Jul 2011, 11:22
A400M to have 'handed' propellers -18/05/2004-Flight International (http://www.flightglobal.com/articles/2004/05/18/181755/a400m-to-have-handed-propellers.html)
... gives the manufacturer's view, some years ago.
Airbus A400M Counter-Rotating Prop Configuration — Tech Ops Forum | Airliners.net (http://www.airliners.net/aviation-forums/tech_ops/read.main/231456/)
... if people who read this forum are not too grand for it, gives a moderately interesting run-through of the sundry pros and cons.
... gives nice pictures (worth a thousand words) describing a term which I've managed not to hear in a few years of relatively broad aerospace experience.
21st Jul 2011, 05:23
Thanks for those explanations.
The French link suggests it was perhaps it was my mistake to assume English-language pilots also use supra-divergent-convergent.
21st Jul 2011, 13:17
Love of Latin roots often gives rise to a sesquipedalian vocabulary ...
Trim Stab's term is summat that I too have never come across since I saw the first description of the beautiful de Havilland Hornet, with its "handed" propellers as a teen-ager.
You learn something every day ...
Edited to add that without terms like those to which Trim Stab has introduced us, French is a bit cumbersome (to English ears, at least) when describing, for example, rotation - "in (or against) the direction of the hands of a watch" - so "super (or contra) divergent" fully meets the 3C's Rule, that all writing should be "Clear, Concise and Correct".
Apologies too for using the long word in my first line - I used to mutter it (those satisfying "s"-sounds) during lengthy, boring and verbose lectures and briefings, but I've never before had the occasion to use it. Self-indulgent, what ???
Hopefully this will help a bit (http://www.icas-proceedings.net/ICAS2008/PAPERS/362.PDF).
25th Jul 2011, 15:24
Some nice work there, Algy, especially 3.2 et seq (a picture is worth a thousand words !).
Have you anything more for us about the reported gear-box problem ?
Under investigation in co-operation with the powerplant suppliers, but note that all four aircraft are flying.
26th Jul 2011, 15:28
Re Fig 10 Para doors AND ramp down! Can they be serious?
Trim stab, please forgive another possibly irrelevant outbust, but having (I think) understood the contents of the links above, it seems to me the propellors do rotate in opposite directions, which explains both the tip shape and my original post.
26th Jul 2011, 18:29
glum - the props are indeed contr-rotating, but this does not explain the unusual configuration of outer supra-convergent and inner supra-divergent props. All aircraft with supra-convergent props do indeed have contra-rotating props, which is why your comment was irrelevant.
Algy's post does however explain the reasoning behind the configuration, which is interesting.
As an ex-para I can see the mission flexibility of dropping sticks out of the side doors and loads out of the back ramp - though hopefully the design allows the loads to be dropped first:uhoh:
27th Jul 2011, 10:02
The A400M was designed to have a Para Wedge system capable of double the amount of stores the Herc could drop and we insisted on a flat floor between the rollers so that Store preceeding men systems could be dropped over the ramp.
The point is that men exiting the paradoors will have their canopies damaged if the ramp is also open.
However, back to the thread subject. One reason given by Airbus for the prop rotation was to provide an airflow that would reduce the 'Crossover' risk to Sim stick paratroops. Having seen the difference made to the airflow and parachutists crossover by the change of props on the C-130J. I remain sceptical of Airbus's claims.
"All aircraft with supra-convergent props do indeed have contra-rotating props, which is why your comment was irrelevant. "
Ah I see. Thanks for the education!:ok:
28th Jul 2011, 23:16
T-S and Glum, Contra- rotating props are those that are either on the same drive shaft,or concentric drive shafts,a la ,Shackleton,Seafire 47,Tu-95,Gannet.Those that are counter rotating are ,a la P-38 (prototype and production were different direction),de H Hornet, OV-10,etc...Please do not mix `em up !!!
29th Jul 2011, 12:12
Exactly what Sycamore said :ok:
Every day is a school day...