Control reversal speed

5 Answers

1. 
   

   
   

   Wes Beard on Oct 08, 2012

   This is a rather interesting question. I am unsure if you mean one of two things so I will attempt to explain both.

   If you get slow enough while maintaining altitude (we call this slow flight) the technique to speed up is to lower the nose and not to add power and the technique to climb is to add power and not pitch up. Above the speed where lift to drag is the greatest, we add power to accelerate and pitch up to climb.

   I think you may be asking about this. As an aircraft accelerates past its Vne speed or (Vno / Mmo speed for jets), the center or pressure on the top of the wing can possibly move aft of the ailerons. In any aircraft the center of pressure where theoretically measure where lift is coming from slowly moves aft as the airplane accelerates. This is one reason why the pitch attitude at higher airspeeds is lower than in slower speeds in level flight.

   If the center of pressure moves aft of the wings, the ailerons will effectively act opposite what input the pilot is introducing to the system. I wouldn’t worry about this to much as the ailerons will experience fludder well before this speed and rip off the airplane. This is one factor the engineers use in determing the Vne speed for the aircraft.

   0 Votes 1 Votes  1 Votes

2. 
   

   
   

   Steve Pomroy on Oct 08, 2012

   Hi Rizwan.

   Above the control reversal speed, one or more of your primary flight controls will work backward because of flexing of the airframe. Control reversal, along with flutter and divergence, fall into a branch of engineering called aeroelasticity – meaning it’s all about how aerodynamics interacts with the elasticity (or flexibility) of the aircraft.

   Ailerons don’t just change the lift of the wing, they also create twisting moments. The down-going aileron, which increases lift, creates aerodynamic loads that twist the wing nose-down – tending to decrease lift by decreasing the wing’s AOA. Vice versa on the wing with the up-going aileron. Aerodynamic loads, such as these twisting moments, increase with airspeed. However, the stiffness of the wing is fixed. So at higher airspeeds, the twisting of the wings due to aileron deflection increases.

   At some airspeed, a balance is reached where the lift gained (or lost) due to aileron deflection is completely offset by the lift lost (or gained) due to wing twisting. At this speed, which is called the aileron reversal speed, the ailerons have no effect at all. At lower speeds, they work the way we expect them to (right aileron, right roll). However, at higher speeds, the ailerons will be reversed – right aileron will result in left roll.

   Long, skinny, swept wings are particularly prone to aileron reversal. That’s why you’ll see inboard ailerons (sometimes referred to as “high speed ailerons”) as well as outboard (“low speed”) ailerons on many high speed / high altitude jets.

   Similar effects occur with the elevator and rudder due to fuselage bending. But in most aircraft designs, ailerons reversal occurs at a lower speed.

   As noted by Wes, control reversal is one of the criteria used to determine Vne. Others include the above noted flutter and divergence. Which of these three occurs first (i.e. – at a lower airspeed) depends on the aircraft design, and can also vary with altitude. Flutter speed, for example, is reduced at higher altitudes. So it’s possible for a given aircraft to encounter control reversal first at low altitude, but to get flutter first at high altitudes.

   Cheers,
   Steve
   http://www.flightwriter.com
   http://www.skywriters.aero

   +5 Votes 5 Votes  0 Votes

3. 
   

   
   

   rizwan on Oct 09, 2012

   thanks Wes and Steve. Got it . is there any possibility that after control reversal speed , if we roll the Aircraft left and it does roll at all???

   0 Votes 0 Votes  0 Votes

4. 
   

   
   

   Brian on Oct 10, 2012

   I’ve not much to add here except I’ve never read anything about control reversal being a factor for Vne. Perhaps someone can direct me here?

   I’ll see if I can’t dig up a brief history lesson of this in my design book, I know I read it quite recently during some fun review. To sum it up, sometime early on in large airliners it was discovered that controls could reverse at certain speeds, speeds that are often below what airliners of today cruise at.

   The solution is quite simple, and you can see it if you ever have a window seat in an airline and watch it turn at a cruise speed. The ailerons are disabled or only partially used. At higher speeds the spoilers provide the majority of roll control. The shorter arm for the moment eliminates the issue of wing twist.

   0 Votes 0 Votes  0 Votes

5. 
   

   
   

   Steve Pomroy on Oct 11, 2012

   Hi Brian.

   From the Canadian Aviation Regulations (CAR’s), CAR 523.629 applies. The numbering is parallel for the FAR’s, so in the US, you’d be looking for FAR 23.629. It’s probably identical. Partial quote here:

   ———-QUOTE———-
   523.629 Flutter
   (a) It must be shown by the methods of paragraph (b) and either paragraph (c) or (d) of this section, that the aeroplane is free from flutter, control reversal, and divergence for any condition of operation within the limit V-n envelope and at all speeds up to the speed specified for the selected method.
   ———-END QUOTE———-

   For the sake of brevity, I’ve edited out most of the details, the reg pretty verbose. But you can Google it for the rest easily enough.

   Cheers,
   Steve
   http://www.flightwriter.com
   http://www.skywriters.aero

   0 Votes 0 Votes  0 Votes