tu 95 bear

7

Contra-Rotating Propellers Overview

Contra-rotating propellers are a pair of propellers mounted inline that rotate in opposite directions from each other.

ALERT: Do not confuse with counter-rotating propellers

The first contra-rotating prop design was prototyped in 1931 in Fort Worth, TX.

The inventors, M. M. Egan and D. W. Evans saw that this design increased speed and eliminated torque.

Operational Advantages

When a single prop engine spins, a significant amount of tangential or rotational airflow is created, especially at low airspeed. The energy used to create this tangential flow is entirely wasted as the flow does not contribute to thrust. Additionally, at low airspeed, this flow can wash over control surfaces and cause serious stability issues.

When contra-rotating props are installed, the rear prop turns this tangential flow into increased thrust while at the same time mitigating it so that prop wash is more linear. This gives increased thrust and eliminates potential instability.

Another operational advantage is the elimination of torque and P-factor.

Torque

The torque of an engine spinning can and does cause an equal and opposite twisting action on the body of an aircraft.

This torque is compensated for in a wide variety of ways including asymetrical wings.

Contra-rotating props compensate for this torque by producing two equal torque forces in opposite directions which compensate perfectly for each other at any airspeed. This is advantageous as many torque control mechanisms are only optimized at a single airspeed.

Torpedoes commonly use contra-rotating propellers to counteract torque that would otherwise cause the torpedo to spin. 

P-factor

P-factor is a complex aerodynamic phenomenon wherein differential airflow over prop blades causes a sidelong shift of the center of thrust of a prop.

I am going to try to explain P-factor because I believe I have that obligation but it is complicated and if you need a better explanation then scope this jawn.
https://www.youtube.com/watch?v=Zf7-nSMLnMo


In order to imagine P-factor, imagine an aircraft pointed straight in the air moving northward at 100 m/s. Said aircraft has a simple two blade prop that is spinning with an angular velocity of 200 m/s at the tips of the blades. Take a sec to imagine this situation.

Now focus in on the prop which is the only thing that matters. In fact, you can literally forget the rest of the aircraft and just imagine a prop spinning parallel to the deck, moving northward. Now at any one point, one of the two blades will be spinning northward and the other southward. Imagine the prop superimposed upon a compass. At all times one blade is moving angularly from the S mark to the N mark and the other blade is moving from the N to the S. Given all of this we can see that while the entire prop including both blades is moving northward at 100 m/s the blade that is at any moment spinning northward is imparted with an additional forward velocity while the blade momentarily spinning southward has a little less velocity (assuming clockwise rotation). In other words, while the hub of the prop is moving at precisely 100 m/s in the North direction at any given moment, the tips of the prop blades are moving at dramatically different velocities at any given time ranging from +300 m/s North to -100 m/s North.

Now what does that even mean??? 

It means that because the prop blades are airfoils and because airfoils produce more lift at higher airspeed, the prop blade that is moving with a higher airspeed, that is the prop blade moving into the wind, will generate more lift or thrust. This means that that side of the prop will produce more thrust and the center of thrust will be shifted from the hub of the propeller to towards that side, ever so slightly.  

Bear in mind that in the demonstration we were imagining the prop spinning straight up. P-factor is most severe when the prop is parallel to airflow and is absent when the prop is perpendicular. In other words, as angle of attack increases, P-factor increases, maximizing at 90 degrees.

Now that you know the mechanism behind p-factor, I’m gonna explain how it affects an aircraft. P-factor shifts the center of thrust away from the center of the prop towards the right in the case of clockwise prop rotation which causes a leftward yaw opposite for counter-clockwise. Its gonna look like this.

Now p-factor isn’t always a problem, in level flight, for example, p-factor is negligible to non-existent, but at low airspeed and high angles of attack, conditions found during takeoff and landing, p-factor can cause problems.

Contra-rotating props minimize p-factor as a result of their geometry since there are prop blades are mounted opposite the p-factor forces are cancelled out.

Disadvantages

Contra-rotating props are louder than single prop layouts. This is seen with the Tuplolev Tu-95 “Bear” which has extremely loud engines.

The second and most serious disadvantage of contra-rotating props is that of mechanical complexity. Added mechanical complexity can marginalize gains in efficiency.

A Soviet Tu-95 “Bear” in flight. The long range bomber debuted in the 1950s, fulfilling a similar role to the American B-52. As with its cousin, the “Bear” has a reputation for longevity, remaining in service with the Russian Air Force and retirement still in the far off future.

(Butowski Collection)