Please bear with me, I need to set the context first. If it's TLDR for you, just skip down to the link and take a look.

When I first joined this site, the general tone on helicopters was that they are "just the same as fixed wings except that they rotate their wings and, except when hovering, have only minor differences in flight".

Thanks to the efforts of people who actually fly them or have other detailed knowledge, we've completely changed this and I believe the site is better for it. Folks like rbp, mins, aeroalias and korinstormast (thank you guys) have added value and helped a lot of people to better understand rotor craft.

I am the kind of person who when I see something wrong somewhere I can influence, aim for one of two outcomes. Either I change my position because someone explains why I'm wrong or, I correct the mistake. My preference is the former. I believe that this is especially important here since our primary goals are to help the most people possible and provide the best Google hit. Letting bad answers live, without resolving disputes on them, harms that goal.

Finally, emotion. I hope that the old hands around here view me as helpful, respectful, good humoured and accepting of the wishes of the community. I strive to do this by keeping emotion well out of it. I learned the lessons of emotions on the internet a long time ago. I occasionally slip, especially when I've been enjoying Scotland's finest export when I get a little bit cheeky, but my personal goals when I come here are as above, and in that order.

So, the point of all of this.

Centrifugal force stiffens the blades.

My simple approach. Centrifugal force is fictitious. It exists only as a result of calculations required to explain real effects observed when looking from a rotating frame of reference to some other. Please, do not debate this further here. If you doubt the truth of this, visit Physics.SE or other canonical reference to satisfy yourself before proceeding. Centrifugal force can have precisely zero effects in the real-world. Rather, it arises as a result of observing and explaining the effects of other forces.

"Stiffens". The only way to stiffen an object is to change it's physical characteristics. Aside from some relatively minor side-effects arising from the change in shape of the blades when rotating, their physical characteristics do not change.

Unfortunately, Ray Prouty used a similar phrase in one the helicopter worlds most respected works which is being used to justify the answer. Sadly, we can no longer ask Ray so I am guessing when I say that I suspect "stiffen" was a poor choice of word to mean "resists and counters the bending force" and "centrifugal force" in recognition that the majority of people do not understand (or care about) the correct explanation. Perhaps a better phrase would have been that "the blades resist bending forces as the vertical component of the centripetal resultant force acting towards the hub and lift increasing as the blade accelerates, causes them to rise".

Had this phrase, or similar, been used in this answer, I would have accepted it except for my final objection to this answer which is that it does not answer the question posted.

To close, if I am wrong, then I will enjoy my slice of humble pie, walk away happy having learned something and make any apologies I need to. As it stands, no-one has offered any help as to why I am wrong so my objections stand.

I've already flagged as "does not answer the question" but I can't see the outcome and there is nothing I am aware of to let me see if the community has already decided to accept the answer or that it simply did not make it to the review queue.

Forty plus years of education and experience in technical and engineering disciplines yells at me the that this is wrong but my physics education is only just past the point of understanding the centrifugal force fallacy since my degrees do not major in physics.

Please help me to resolve this dispute.


3 Answers 3


I think it is unfortunate in this case that the many of users here on Aviation.SE lack knowledge on helicopters (me included, I have learnt so much from you guys!)

It is not an issue which a mod should step in: the user has broken no rules. It is just an answer with incorrect content.

For the moment at least, we cannot vote to delete it as it has a positive vote count. There is a high number of negative votes, but it is offset by a even higher number of positive votes. Stack Exchange does not have a mechanism to deal with controversial answers where the opinion of the community is split. Vote counts are not visible to low rep users, so they may mistakenly take the answer as "correct" and up-vote it.

I think down-voting is the only way to go, until it reaches a negative score.

  • 4
    $\begingroup$ This is important from the aspect that "does not answer the question" is not supposed to be used for an answer with incorrect information on SE sites, but just those that don't even attempt to answer the question at all. $\endgroup$
    – Lnafziger
    Jul 10, 2017 at 21:30
  • $\begingroup$ @Lnafziger Which describes this answer. $\endgroup$
    – Simon
    Jul 13, 2017 at 17:39
  • $\begingroup$ @Simon No. It tries to answer the question by saying that the situation is impossible because the non-spinning blades would be too flexible to stop the helicopter tipping over. $\endgroup$ Jul 13, 2017 at 23:16
  • 1
    $\begingroup$ @DavidRicherby On reflection, you are right. $\endgroup$
    – Simon
    Jul 14, 2017 at 17:47

We seem to be conflating two separate things here.

  1. Is the post correct?

  2. Does the post make an honest attempt to answer the question?

Flagging as "not an answer" should only be used in case 2 so all the discussion here about whether centrifugal force a) exists and b) stiffens rotor isn't actually relevant to the question of whether or not the question should be flagged. The answer under discussion attempts to explain why a helicopter couldn't rotate while standing upside-down. As such, it is an answer to the question. It might be an incorrect answer, but it's an answer. (Analogy: "4" is an incorrect answer to "What is 1+2?"; "dog" and "Arithmetic -- yaaaaawn!" are not answers.)

Incorrect answers should be downvoted; once they have a negative score, high-rep users can vote to delete.

  • $\begingroup$ The answer is not incorrect in itself, it is just a bit brief. $\endgroup$
    – Koyovis
    Jul 17, 2017 at 22:50
  • $\begingroup$ It's a yes/no question and I couldn't tell from reading the answer whether the author thinks the answer is yes or no, so I don't believe it is an answer to the question. $\endgroup$
    – Dan Hulme
    Jul 18, 2017 at 16:03

Everything is relative. A laundromat centrifuge exerts a centripetal force upon the items of clothing inside. Once it stops doing that, if a hole suddenly appears in the centrifuge, the shirts & shorts fly out in a straight line. They have been feeling a force pushing inwards, but they have been exerting a force themselves pushing outwards.

A bucket of water swung overhead fast enough, results in the water staying in the bucket. The bucket exerts a centripetal force, but the water itself pushes against the bucket so it does not fall out even when overhead. The person swinging the bucket feels his arm being extended outwards, away from him: in daily life called a centrifugal force. It's all caused by the balance of forces in the rotational reference frame.

In the answer in question, the term Stiffening is in itself correct. There is pre-tension in the blades, and there is no yield until the pre-tension level is exceeded. The main bit of stiffening is the hub itself though: the flapping hinge is just a hinge at rest, allowing no bending moment to be exerted. When the blades are spun around, they pull the hub in the general direction of the rotor disk, effectively transferring hinge moments. So from stiffness zero at rest, the hinges now have the ability to cause rotation as a result of a moment. What the answer tries to express is that without rotation, the blades cannot support the helicopter when upside down, although it would have been much clearer to everyone if that would have been explicitly stated.

That is all from a technical standpoint. If an answer is not perfectly phrased but expresses the gist of the matter, is it worthless and should it be removed? In my view, that is something that lawyers do, but not pilots and engineers. Personally, I don't feel the need to pursue or demand rectification, that is really not why I'm here. I visit this site for enjoyment, and appreciate the brilliant answers that can be found here and that expand my personal knowledge, and hope to do the same for others who may be interested in my experience. If they are not, that is fine as well, so be it. And of course sometimes I'm just plain wrong, like everybody is at times.

  • 1
    $\begingroup$ It's a shame that you chose to ignore my request not to debate the physics here, since it is the wrong place. Physics.SE is a great place to do this and, if you did, you would hopefully learn more about why both your explanation of what happens to the water in the bucket and your definition of stiffness (incorrect from an engineering/physics point of view) are wrong. Oh well. I get that you are only here to enjoy yourself and can accept that your priority on precision differs from mine. All said, I've done the right thing and can't be bothered to pursue it further. Have fun. $\endgroup$
    – Simon
    Jul 13, 2017 at 20:34
  • $\begingroup$ @Simon, thanks Simon, you too. $\endgroup$
    – Koyovis
    Jul 13, 2017 at 23:16
  • 1
    $\begingroup$ @Simon, and to give another perspective on bending stiffness, the Wikipedia page may help. Now according to that definition, the bending stiffness of a rotating blade is higher than of a static blade. And it is due to the centrifugal force (when observed from a rotor centric rotating reference frame). $\endgroup$
    – DeltaLima Mod
    Jul 17, 2017 at 13:20
  • 1
    $\begingroup$ Well, @Simon, since you stated in your question: Either I change my position because someone explains why I'm wrong or, I correct the mistake. My preference is the former, I am surprised that you chose to close the door to people who in fact attempt to get to your preferred solution. When you start to discuss physics in your questions, you should not be offended when people discuss physic in their answers. You may want to have a look at this answer on physics.se for a different perspective on centrifugal forces. $\endgroup$
    – DeltaLima Mod
    Jul 17, 2017 at 13:26
  • $\begingroup$ @DeltaLima I'm already familiar with these articles and the concepts they discuss. I still see nothing to support the assertion that centrifugal force stiffens the blades so I don't yet understand how I've "closed the door". I've asked the physics guys to help me out. $\endgroup$
    – Simon
    Jul 17, 2017 at 19:53
  • $\begingroup$ @ Simon we'll let the answers there speak for themselves then. $\endgroup$
    – Koyovis
    Jul 17, 2017 at 23:31
  • $\begingroup$ Sorry, in addition just wanted to point out that this answer has an arrow pointing away from the axis of rotation, representing a force, labeled "Centrifugal". $\endgroup$
    – Koyovis
    Jul 18, 2017 at 5:19

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