Contra-Rotating props

I think flight sims have a role (or two – training and fun…) but it’s the assumption of accuracy in the machine that I take issue with –

Like when I sit a tranee down and give them a calculator – “add that list of numbers. Ta.”

And then they take whatever the machine spits out as gospel, despite any evident absurdity (e.g. adding 10 nos each less that 10, and getting a 250+ figure…)

Back to contraprops?

Have you actually tried to fly the Wright Flyer in MicroSoft Flight Sim 2004.

No, Turbo_NZ, I haven´t. I don´t think flight simulaters are that accurate …… 😀 …… so, in this case I definitely agree with James!

A few years ago, a friend in our gliding club twisted my arm and forced me down in front of his computer. It was the first and only time so far I´ve ever flown a desktop.
He put me in a Mustang to fight a Me-262. It took me a minute or so to learn how to fly the Mustang and another few minutes to shoot down the Me-262. The flight sim was flying the Me-262 and it was pretty predictable. It would probably be more fun to fly against another fighter, flown by another person on another computer.

Maybe some day …… :rolleyes: ……

Christer

Originally posted by Christer
James,
I know what You mean!

However, if the Wright Brothers had a computer and a programme (see, I caught the correct spelling) to simulate how the Flyer would behave, would they dare strapping themselves into it?

Christer

I agree !!
Have you actually tried to fly the Wright Flyer in MicroSoft Flight Sim 2004. The difference between stall speed and top speed is bugg’rall 😀

James,
I know what You mean!

However, if the Wright Brothers had a computer and a programme (see, I caught the correct spelling) to simulate how the Flyer would behave, would they dare strapping themselves into it?

Christer

Thanks Christer,
Your point is good. What I’m saying is that the real world is a lot more complex (and to use some over-used terms fractual, co-incidental and with lashings of chaos theory)

Yes, you can predict what’s in the analysis – but that doesn’t include the bug in the pitot head, the cloudburst on approach, the revolution in the state that takes the power out to the airfield, the pilot’s tuna lunch rebelling – you get the idea.

Part of human hubris to to make models of the real world and then say how perfect and real they are – yes, up to a point…

GI GO. Garbage in = garbage out. What you can get out of a computer is limited by what you put in – which is’t ‘everything’ ever. Please remember this.

To create/design a computer simulation, a lot of input data AND known output data corresponding to the same are needed. Then the “bit in the middle” can be created/designed. If new input data is given, then the “bit in the middle” can calculate/predict the output data.

Any good computer model (simulation) is based on empirical test data and actually gets pretty accurate.

Christer,
who will remember that lousy science put into a computer is lousy science en masse!

So that emergency prop feather switch is completely non-standard. If so, good on them for having the foresight to fit that. Would save a multi-million dollar (euro) irreplaceable aircraft me thinks…:)

Very sensible indeed, weren’t both Precious Metal and Red Baron lost partly because of the props going flat? Obviously the first one is flying again, but I remember a few undignified photos of her lying in the desert!

Seafire VP441

I noted this little addition to the standard panel in Steve Patterson’s cockpit shot of our now airworthy Seafire 47 – an emergency PROP FEATHER SWITCH. A carry over from the DH prop fitted to the Shackleton, it will do wonders for the glide performance in the event of an engine failure – six blades on fully fine pitch make a pretty effective air brake.

For those with a passion for control column tops, that is not a Seafury item fitted, as has been mooted, but a ‘D’ ring spade grip with the Dunlop electric gun firing system uniquely fitted to this last of the Spitfire line. I suspect our US colleagues, with their dislike of the British pneumatic systems, have converted to US brakes as the brake lever is omitted and modifications are apparent on the rudder pedals, now fitted with toe brakes it would appear.

Mark

Originally posted by turbo_NZ
I guess the other drawback is the added complexity of the gearbox and prop-shafts for this setup.

Indeed, and complexity always adds failure modes to the equation. In Alex Henshaw’s ‘Sigh for a Merlin’ he writes elaborately about the problems they had with translational bearings. These transfer the prop pitch setting from one prop to the other, and when it fails the rear prop goes into flat pitch, providing a very efficient airbrake! Quite a few Spits were written off because of this I think. They got it sorted in the end, but a contra prop will still be a complex arrangement that needs more maintenance than a single prop.

But then, the pilots love them!

The other advantage is the zero torque reaction from having both props spinning in opposite directions, cancelling out the twisting motion. Very desirable in 2000hp+ carrier launched aircraft like the Seafire FR.47.

Interesting listening to the Jack Stafford at Wanaka talking about take off in the Tempest.
Full opposite rudder trim and then rudder just to keep it in the right direction when accelerating down the runway !!!

C.R. Props would have sorted that.
I guess the other drawback is the added complexity of the gearbox and prop-shafts for this setup.
As if it wasn’t already complicated by the massive complexity (and sometimes unreliability !!) of the Napier Sabre !!!

Chris.

Makes sense, thanks archer.

The reason for the contra props on the Shack lies in the power of the engine. To transfer the available horses to the surrounding air you need surface, much in the same way that you need larger wings to keep a heavier airplane aloft. That’s why you see props with an increasing number of blades as the hp of the engines increase throughout the Spitfire family. Now the problem with a lot of propblades is aerodynamic interference, and that decreases the efficiency again, thus throwing away your added thrust. Another option would be to increase prop diameter, but that has constraints based on airframe geometry (you’ll need longer gear legs or a Corsair solution) and tip speeds. The next step then is to create a contra prop, thus keeping the prop size reasonable but adding lifting (really pulling-) surface.

I just love the assumption that the computer programme can ‘simulate everything’

GI GO. Garbage in = garbage out. What you can get out of a computer is limited by what you put in – which is’t ‘everything’ ever. Please remember this.

Contraprops on Shacks? The cynic in me (it is Friday night) says “As we can’t afford a jet, let’s go for the max no/ of props…”

Cheers

Just what were the advantages of counter rotaters on an aircraft such as the Shackleton? Was it just a matter of availability?

Well, those were the good old days when NOT everything could be simulated in a computer program …… 😉 …… and the test pilots job was often boring iterations of the same task. But then at other times, their job could be unnecessarily exciting!

Christer

Wow, very informative and interesting, Christer.

Must have been a real science for the RAF engine techies during the war.
Thanks for the info.

Cheers
Chris
🙂

About discrepancies in performance figures, small changes to the propeller in terms of its condition and the airframe as a whole can produce different performance figures.

In the early stages of testing, Rolls-Royce often experimented with different supercharger gearing in MS and FS which produce differences in performance, especially at which altitude the maximum speed was reached.
At full throttle, the maximum Indicated Airspeed is approximately the same within a certain range of altitude and finding the “sweet spot”, by adjusting the gearing to the altitude just before IAS starts to drop, then the IAS translates into the highest True Airspeed. Then, MS is maintained until TAS drops and the gear change to FS occurs.
The unknown factor that prompted these experiments was ram air pressure due to actual speed and air density.

One example is the Mk.XIV, which is quoted to attain 439 mph at 24,500 ft from one source and 446 mph at 25,400 ft from another and 448 mph at 26,000 ft from a third. These figures are probably not from the same aircraft but indicate that there were differences as mentioned above, probably in supercharger gearing.

The maximum speed is attained in FS gear. At lower altitudes, the differences in MS gear may be even bigger since different gear change aneroids could be fitted to suit operational needs. As I´ve understood it, adapting engine performance by altering the gear change altitude was not uncommon for the Mk.XIX recce missons.

Christer

Precious Metal is a wicked looking P-51 !!
Looks fast, even parked up.
🙂

Ain’t that for sure !!
Bill Gunston’s normally onto it and so is William Green.
Both well respected aviation researchers.
They both come up with different figures from what I’ve read so that means they both did their own research ??