Propeller-powered aircraft have a "problem" called P-Factor, where the descending blade of the prop is generating more lift (a.k.a. thrust) then the ascending blade due to a higher angle of attack when the angle of attack of the wings is high. (I hope I summed that up properly.)

How about helicopters? For this example let's say the blades turn counterclockwise when seen from the top. When the helicopter is accelerated, the blades on the right side of the helicopter are faster then the ones on the left side. Thus they should be generating more lift. The higher the speed, the higher the difference in lift on both sides would be.

What mechanism handles corrections that have to be made due to this lift difference? How does it work?

  • 2
    $\begingroup$ See this and a number of questions on here that already discuss dissymmetry of lift. Search for "retreating blade" and see the corresponding answers. $\endgroup$
    – Simon
    Commented Jun 22, 2015 at 10:49
  • $\begingroup$ Okay, that's a start, I didn't know own that word yet and hence couldn't search for it... Will rephrase my question later, I am more lookin on how they achieve the corrections made. As they must be different for different speeds and pitch attitudes. $\endgroup$ Commented Jun 22, 2015 at 10:54
  • $\begingroup$ Note that P-Factor is an issue in multi-engine aircraft too $\endgroup$
    – Ben
    Commented Jun 22, 2015 at 11:17
  • $\begingroup$ For corrections, see this $\endgroup$
    – Simon
    Commented Jun 22, 2015 at 13:39

2 Answers 2


This was one of the primary problems that needed to be solved in helicopter flight.

In my example, I'll use a 2 bladed helicopter, but this holds true with all helicopters, and I'll explain some of the different rotor types later on. The blade that is moving forward, relative to the direction of the helicopter, is called the advancing blade. The opposite is called the retreating blade, and you are correct in assuming that they would generate different amounts of lift. As a previous commenter noted, the term used for this is "dissymmetry of lift", and is the same thing that limits the maximum speed of a helicopter.

A two bladed helicopter will generally have what is called a "semi-rigid" rotor system. Semi rigid systems are only possible on even bladed helicopters, and are generally only used on 2 bladed helicopters. The two blades are connected together by a hinge on the rotor hub (where the blade meets the helicopter), and if you push down one blade, the other blade moves upward, and vise versa. What happens as you move forward is that the advancing blade (the one generating more lift) will get pulled up with a greater force than the retreating blade (the one generating less lift). As the advancing blade rises up (because of its additional lift), it pushes the retreating blade down. This advancing blade, in pushing up the rotor, is reducing the angle of attack of the blade against the airflow, which in turn is going to reduce its lift. The retreating blade is hinging downwards, which is increasing its angle of attack against the airflow, which will increase the lift it provides. These two forces will work against each other until they are in balance, which means that the advancing blade and the retreating blade will even out in the lift they are generating, which removes the dissymmetry of lift.

The blades in a helicopter are spinning at a some speed, which varies depending on where on the blade you are talking about (the tip or near the rotor hub, for example), but to simplify this explanation, I'm going to assume that the blade is traveling at the same speed along its entire length. Let's say that the blade is spinning around at 100mph at its tip. As the helicopter moves forward, at let's say 50mph, then we have one blade that is moving against the air at 150mph (the advancing blade), and one that is moving against the air at 50mph (the retreating blade). As the helicopter gets closer to 100mph (the speed of the blades), at some point, the speed of the air against the retreating blade will get closer and closer to 0. When it hits 0, it will stop generating lift, and the helicopter will start to roll (now the rotor is only generating lift on one side). This effect, called retreating blade stall, is one of the reasons why helicopters are limited in their top speed.

Just as a side note, I have greatly simplified some of these explanations, so I am well aware that I have glossed over various parts of this explanation to simplify it.


The blades are hinged and left to flap. Thus, the advancing blade flaps upwards, that flapping motion reduces the angle of attack, and the excessive lift resulting from the stronger relative wind is reduced too. The retreating blade flaps downwards, that flapping motion reduces the angle of attack, and the insufficient lift resulting from the weaker relative wind is increased. In that way, discovered by Juan de la Cierva in the 20s, the lift is conveniently equalized...

  • $\begingroup$ You should have read Tom K's answer.. $\endgroup$
    – amI
    Commented Mar 24, 2017 at 21:52

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .