Contra-rotating propellers, turbulence?

Question

First, since contra rotating propellers have been used, not sure if I can say widely but used... I'm not sure if I can ask this question but I would like to know.

Doesn't contra-rotating propellers introduce a great probability of turbulence occurring if not guarantee because the blades "smash" into each other as they rotate in opposite directions on the same shaft? And isn't air compressed between the blades? Or is it simply sucked through? I am concerned about the abrupt change in direction I would think of the air following the curvature of the blades...

Can someone shed some light for me?

Answer 1

the blades are NOT meant to smash *into each other*.



yes, there are indeed some problems with accelerated flow beyond propellers, HOWEVER prop aerodynamics roughly estimate that the acceleration of the air reaches reasonable differential well beyond the propeller diameter. meanwhile, the counterrotating propellers are usually a fraction of diameter away.



edit: regarding the acceleration and pressure

http://s624.beta.photobucket.com/user/mindworms/media/propellers0.jpeg.html



so, as the flow approaches the propeller, pressure drops, speed of air increases. AT the disc, there's a pressure increase (this IDEAL disc behaves like a hovercraft) and the pressure difference in the theoretical aerodynamics is what creates the thrust of propeller. at certain distance behind the propeller, the airflow compensates the pressure again and resumes ambient pressure, moving at higher speed ... (obviously dissipating after some time).

the theoretical aerodynamics assume that the air pushed through propeller terminally achieves speed equal to the original airspeed plus TWICE the acceleration at the propeller disc.



there are some things to consider (among others, greater pitch of the second set of blades) but that's marginal to the technical aspect of producing the hollow dual shaft and assembly.



it's usually visualised like this

http://lyle.smu.edu/propulsion/Pages/Images/propstep1.gif



the flow ammount through the prop disc is constant (what goes in, must go out)

accelerating the flow is usually visualised by narrowing the volume of the air (also happening in reality, by sucking in ambient nonmoving air)







edit:

now, a little explanation to the Fig III. you need to remember that helicopters (and prop driven aircraft) are RANDOM ASSEMBLIES of parts flying in formation.

thus.. you start with AIR flying against the formation (i.e. airplane). you vector add the rotation of propeller. from now, you see the air from PROPELLER blade element point of view. each prop blade acts like a wing. pushes air little bit down, tilting the vector producing reaction voilla creating lift.. pardon, thrust.

then, we deduct the rotation again. you have the air going against the airPLANE again (in Fig III it's vector labelled 6. compare 6 to 1. the difference is the net influence of the first propeller. the vector is leaning to one side.. that creates the spiralling of the air passing through the propeller (part of what's calle Pi factor). then adding 7 rotation in opposite direction, you see the air from point of view of the second prop blade. again, air is "processed", i.e. pushed, pulled and beaten, leaving even more accelerated and spinning in other direction.



when you compare vector 1 to vector 11, the difference in speed (multiplied by air mass passing through the propeller) makes up the total thrust of the propeller set.

Answer 2

Of course there are design issues when dealing with contra rotating propellers.



But, it's all taken into account since there's the need to add up the effect of the first sets of blades and how it affects the second set.



Let me tell you tough that it's design while it's very much alike an axial compressor's design, it's not the same since front blades only "shadow" the back blades in much lesser way than in the compressor.

Answer 3

Counter-rotating propellers, are stumbled on on twin-engine, propeller-pushed airplane and function propellers that spin in opposite instructions. Counter-rotating propellers frequently spin clockwise on the left engine, and counter-clockwise on the appropriate. the ease of counter-rotating propellers is to stability out the end results of torque and p-component, removing the situation of the severe engine.Counter-rotating propellers should not be at a loss for words with Contra-rotating propellers.