Request for Uncle Keithmac!

RE: Request for Uncle Keithmac!

Ah.

Clear as mud, but thanks anyway.

(note to self – stick to props, they’re easier…) 😉

RE: Request for Uncle Keithmac!

Hi Steve, The difference between a prop and a turbine is that the prop is converting engine power into thrust, while a turbine is extracting power from the Gas flow to drive the Compressor. So no, the angle of attack on the turbine blades does not progressively increase. The number of turbines you have depends upon the number of compressors you have to drive and how much of the power you want to extract from the gas flow.
Uncle KeithMac

RE: Request for Uncle Keithmac!

KeithMac, one of your points has got me wondering. You say that the rear props have a slightly increased angle of attack, to effectively cope with the airflow coming off the front disc.

So, would I be correct in assuming that the same principle would apply to the blades in a jet turbine?

And if so, if the Angle of Attack increases for each disc as you work rearwards, presumably there must be a limit to the number of discs you can effectively have in a turbine…?

RE: Request for Uncle Keithmac!

Owwww…. my brain now hurts. I think I’ll stick to dismantling and rebuilding my Bristol Hercules cowl gills and leave the actual engines to people who understand ’em… 🙂

RE: Request for Uncle Keithmac!

And what exactly is “a jet”

Are they those whistly noisy smelly things?

They are a fad. They’ll never catch on, you mark my words.

Moggy C

RE: Request for Uncle Keithmac!

OK Folks, good answers. Here’s some info:
The two props are to convert all the engine power to thrust without having very big discs.
They rotate in opposite directions to cancel out the torque on the aircraft.
The front prop has a greater diameter so that the tip vortex does not hit the rear prop – this would cause vibration and noise.
The pitch is controlled by the front prop and is transmitted to the rear prop by a translation unit.
Because the airflow leaving the front prop has been acceellerated, the rear prop has to have a coarser pitch, usually around 1 degree coarser so that it will have the correct angle of attack to produce it’s share of the thrust.
Hope that’s clear!!

Would you like one on Jets for a change?

KeithMac

RE: Request for Uncle Keithmac!

Surely the purpose of contraprops was to convert the power of extremely pokey engines into thrust without the need for the props to be so long that they would carve a series of chunks out of the runway or going supersonic at the tips.

The most obvious difference between the front and back props (The original question)is that one has the blades set for clockwise rotation and the other has them set for counter-clockwise, but surely it’s not that simple?

Moggy C

RE: Request for Uncle Keithmac!

Was it something to do with counterracting torque (by having equal clockwise and anti-clockwise torque forces throughout the rpm range), therefore making the aircraft more stable and safer to fly during the take-off / landing phases of flight?

RE: Request for Uncle Keithmac!

Hi KeithMac,
I´d like to start with a thanks for Your answer to my Centaurus question. I didn´t think that the situation is that serious.

I´ll also have a go on Your contraprop question and this is what I´ve learned about the Seafire 47:
Seen from the pilots seat the front propeller rotates counter clockwise and is the master to which pitch is applied from the pitch control unit and the pilots lever in the cockpit. Pitch is transferred to the rear propeller via a translation bearing. If this bearing fails the rear propeller goes into coarse pitch and will provide nothing but drag. The front propeller will have to do all the pulling against the will of the rear one and full power has to be used to maintain flying speed.

Is the rear propeller different to the front one in other respects? Maybe they differ in pitch range since the front one rotates the airflow against the rear one? Maybe the airfoils differ due to different angles of attack? I don´t really believe in the last one but maybe the first!?
As You probably have understood by now, when it comes to me the questions always outnumber the answers.

Regards,
Christer

RE: Request for Uncle Keithmac!

i agree with ja on the front one turning clockwise and the rear one anti_clockwise…. I think it maybe to counteract the shuddering
Am i close? thanks keithmac for the questions …im starting to learn, but i dont think i’m quite ready for questions like this

RE: Thanks Uncle 😉

NO! But I’ll leave the answer a little while to see if anyone else has any thoughts.

Thanks Uncle 😉

She’s now a member known as Goodness! Another female to add to our little band 🙂 The number is growing 🙂

As for your question; The front usually turns to the right where as the rear prop turns to the left, I can’t remember exactly what the difference is but I remember it had something to do with using lower power for the same speed!
(Am I near)?

“We who grew up tall and proud, in the shadow of the mushroom cloud.
Convinced our voices can’t be heard, we just want to screem it louder and louder”! – Queen from the song Hammer to Fall

RE: Request for Uncle Keithmac!

Hi Nephew, I’ve been away for a while. Here’s one for your friend, hope she likes it! On an aircraft with contra rotating co axial props (ie Wyvern, Gannet Shackleton etc.) what are the differences between the front and rear propellers, and why are they different?