Exceeding airspeed limits

[crashed_05]

I'm studying up on some JetRanger stuff and noticed a warning in the flight manual that says, "Airspeeds in excess of airspeed limitations door(s) off will cause cyclic control reversal of fore and aft position gradient and fuselage buffeting."

Can anyone here explain how this works exactly? I can understand the fuselage buffeting, but what kind of science is going on to cause reversal of fore/aft cyclic control?

[AStarB3]

Control reversal pertains to a loss of airflow over the horizontal stabilizer. When the doors are off there is a very steady flow of air from the nose of the helicopter to the tail. When the doors come off that airflow is disturbed and the effectiveness of the stabilizers is lost. Your fore and aft control will not be reversed but more forward cyclic will be required.

[iChris]

"Airspeeds in excess of airspeed limitations door(s) off will cause cyclic control reversal of fore and aft position gradient and fuselage buffeting."

 

Can anyone here explain how this works exactly? I can understand the fuselage buffeting, but what kind of science is going on to cause reversal of fore/aft cyclic control?

 

WARNING

Airspeeds in excess of airspeed limitations door(s) off will cause cyclic fore and aft stick reversal and fuselage buffeting.

The last haft of this warning, "fuselage buffeting, is understandable. With doors off, the streamline airflow along the fuselage and horizontal stabilizer is disrupted and turbulent. That follows along with Mattkreps post above.

 

However, the "cyclic fore and aft stick reversal" is misleading. It certainly does not mean, in this case, that if you go to 100 knots that you should push forward to bring the nose up or pull aft to bring the nose down.

 

What is more correctly being referenced is the horizontal stabilizer effectiveness in maintaining Speed Stability or Longitudinal static stability vs. cyclic stick position (position gradient).

 

The reversal comes at the point (speed) that the horizontal stabilizer becomes ineffective. Therefore, the helicopter becomes unstable with respect to speed stability. As speed increases cyclic trim position moves aft, which is in reversal to a longitudinally stabilized helicopter with an effective horizontal stabilizer that would have require a finally trim position forward of its initial trim point.

 

 

As an example: Noting your cyclic stick position at 87 KIAS and with constant collective then accelerating to 97 KIAS, the stick position to maintain 97 KIAS will trim out slightly aft of that to hold 87 KIAS. If you tried to go to 107 KIAS with the same collective position the stick will trim out farther aft to maintain 107 KIAS than that to maintain 97 KIAS. The amount is very small, and hardly noticeable by most pilots.

 

The effect is best stated in the book Helicopter Performance, Stability, and Control by Ray Prouty in Chapter 8, The Helicopter in trim (pg 527):

 

"To a pilot, speed stability is seen as the change in stick position required to maintain a new speed. For example, on an unstable helicopter, an increase in speed will initially require a forward stick motion to accelerate; but when finally trimmed out at this new speed, the stick will be aft of its initial trim point."

 

REF:

 

Extracts from Helicopter Performance, Stability, and Control

 

http://www.sendspace.com/file/kgz3wf

 

 

Extracts from The Art of the Helicopter

 

http://www.sendspace.com/file/tvnx0y

Edited September 25, 2010 by iChris

[Eric Hunt]

Yes, you will feel a little buffeting, but the aircraft is fully capable of going to 130kt with the front doors off, no dastardly "control reversal" or any of the warnings. The horizontal stabiliser is still effective. The machine does not behave in any unexpected manner.

 

However, the flight manual does publish a limit, and that's what you are supposed to stick to.

 

By the way, regarding one of the above posts, remember that if your torque is above 85%, you are limited to 80kt to minimise the stress on the main driveshaft and transmission from the forces trying to pull the mast forward and the forces trying to hold the tail down.

[crashed_05]

Thanks a lot for the replies. I completely understand the importance of not exceeding published limits and would never put them to the test.

 

I've heard the reason for the extremely slow airspeed limit for the front doors off (69KIAS) was due to the pressure inside the cabin actually blowing out the rear doors...any one ever hear of that?

 

By the way, Chris, that was an awesome explanation. I'll have to check out that book.

[iChris]

Yes, you will feel a little buffeting, but the aircraft is fully capable of going to 130kt with the front doors off, no dastardly "control reversal" or any of the warnings. The horizontal stabiliser is still effective. The machine does not behave in any unexpected manner.

There's general misunderstanding along this issue of "Control Reversal". As I stated in my post above: The amount of instability is very small, and hardly noticeable by most pilots. This is something measured and plotted during flight-testing and has little noticeable effect on your ability to control the helicopter. The term "Control Reversal" should be removed from the RFM. It dose not relay any useful information to the pilot.

 

However, it dose exist and during this phase the Speed Stability shifts from stable to neutrally stable along to unstable. You must remember this was part of the certification and Airworthiness flight test that required the aircraft to display a minimum level of static longitudinal stability during conditions of climb, cruise, and autorotation by requiring a stable stick position gradient through a specified speed range.

 

Bell found during doors off flight there was a phase were the horizontal stabilizer became ineffective in providing enough down load to provide the desired positive static stability. It was also found that any level of instability past this point did not present adverse control issues. However, the doors-off VNE was set conservatively.

 

To find out how this type of flight-testing is conducted on current Part 27 Helicopters, check out Advisory Circular AC 27-1B (Certification of Normal Category Rotorcraft) Pages B-54 - B-58 (AC 27.173 - AC 27.175)

 

Extract from AC 27-1B:

The all-engines-operating VNE is established by design and substantiated by flight tests. The VNE limits are the most conservative value that demonstrates compliance with the structural requirements (FAR 27.309), the maneuverability and controllability requirements (FAR 27.143), the stability requirements (FAR 27.173 and 27.175), or the vibration requirements (FAR 27.251).

 

AC 27-1B

Edited March 7, 2017 by iChris