Response to RVGuy’s Comments
RVGuy,
Immediately responding properly to your extensive comments required time I did not have. I finally have some free time. Sorry for the delay.
To try and maintain clarity and continuity in my response, I have pasted in multiple sections of your postings in quotes.
“My response was indeed to dispel your attempt to link lack of active control margin feedback to the accident. It is important for current operators of that aircraft to know there is more than adequate control margin throughout the operational flight envelope.”
Please re-read my posts. I specifically state I am not attempting to tie unique trim cyclic to the cause of this accident.
“Now to your other comments. I was a presenter at the early certification meetings to educate the FAA as to the Sikorsky FBW approach. There is nothing in the FARs that would prevent implementation of a unique trim controller. The main issues with the non-traditional controller approach were in demonstration of FARs such as longitudinal static stability. With automatic trim follow up (a fundamental element of unique trim), the stick is always in the same position regardless of trimmed speed. So we came up with an alternate method to demonstrate stability.”
Prior to your discussions with the SW FAA Office to certify the S92FBW, I was working with the same FAA personnel on the certification of the FBW flight control system for the Leonardo 609.
Just prior to Sikorsky giving up obtaining FAA certification for the S92FBW, I was involved with developing the certificate basis for FBW system on the 525. By an interesting chain of events, in between working on both of these programs I turned down an offer from Sikorsky to work on the certification of the S92FBW. So while I do not question any of your statements validity, I believe they are based on your impressions with relatively brief meetings with the FAA. My statements are based on the sum total of over 20 years working to civil certify FBW rotorcraft. I have the mental scars to prove it ;-).
You are correct there is nothing in the FARs that specifically prevents the use of unique trim cyclics. But then again, there is next to nothing in the FARs for Part 29 aircraft regarding Fly by Wire. As you well know, the FARs are vague, and their interpretation is up to the discretion of the FAA. Recently, I have witnessed two identical Issue Papers submitted to the FAA for approval on FBW issues. The one that was sent to the NY office was approved, the one that went to the SW office was turned down.
RVGuy, you may have walked away from meetings with the FAA thinking that they had accepted unique trim as being acceptable for certification. However, prior to the 525 program we specifically asked the SW FAA if unique trim would be acceptable for Part 29 (without mentioning the S92FBW), and were told no. This is not secondhand, I asked the question myself.
“If Bell decided against a unique trim approach, that was their decision based on an assessment of the effort required to meet the certification challenges, not an FAA mandate. BTW, all the publicly accessible photos of the 525 cockpit show a sidestick with very little space for proportional control movement, at least with acceptable sensitivity. Makes one wonder, if it isn’t unique trim, what is it?”
While neither the 609 or 525 are yet FAA certified, they are both are much closer to achieving this milestone than the S92FBW ever got. This is not to say that unique trim is inadequate. But it has limitations, especially when transitioning pilots familiar with conventional mechanical controls.
You need to get the opportunity to sit in a 525 mock up or aircraft. You’ll be surprised at how much displacement the cyclic actually has. It was modeled initially on the Zulu Cobra side stick displacement. The Z is all mechanical with SCAS. With all that linkage and resulting lost motion, a very short stick would’ve resulted in an unacceptable dead band. So yes, the 525 mimics conventional mechanical controls, with proportional movement. This is best displayed on aircraft start up. Prior to rotors turning the 525 does not have hydraulic pressure to the swashplate actuators. Therefore, upon start up the cyclic stick moves to synchronize up with the swashplate position. The same goes for the 609.
As I mentioned before, the biggest concern with dual control unique trim the FAA expressed was with pilot to copilot coordination. When the FAA was asked why unique trim was acceptable on large commercial airliners but not for helicopters, pilot to copilot coordination when operating close to obstacles was the first concern. This is why although the 525 has side sticks, the two-sided sticks are mechanically linked.
Thanks to Nick Lappos’s brief stay at Bell, the 525 incorporates some advanced control features similar to what he pushed for on the S92FBW. But the FAA moves very slowly and excepting new technology. So many of the original features had to be stripped out in order to achieve certification. But it’s now only a software update to bring them back.
“There are indeed benefits and challenges to the unique trim approach, and I have presented a couple of papers on the subject. The challenges, however, do not generally have to be mitigated by incorporation of active feedback. The control strategy with a unique trim stick is quite different than a proportional controller. If you look at time histories of a flight with a unique trim stick, you’ll notice the stick is in detent about 80% of the time. The stick moves in and out of detent at a relatively high frequency. Thus requiring the non-flying stick mirror the movement of the flying pilot is of questionable value.
“I have considerable time in experimental helicopters with both active and passive unique trim sticks and, comparing the two, I didn’t find the active stick to provide a demonstrable improvement. And there are some nasty failure modes embedded in the active stick architecture that need to be addressed in emergency procedures.”
I agree concerning the potential nasty failure modes with many active stick architecture’s. This is especially a concern with dual controls, where force feel must also replicate pilot to copilot interaction. This is another reason why the 525 cyclics are mechanically linked. The single actuator that provides force feel for each axis can be kept to a very low load and rate, similar to conventional mechanical controls.
“Lastly, your contention the USMC “demanded installation of active BAE cyclics sticks” is false. Sikorsky convinced the Marines that there was potential in the active stick to incorporate tactile cueing, and if they didn’t invest in the technology now they might not be able to retrofit it later. The aircraft could handle the approximately 30 lbs weight penalty and the cost was just lost in the scope of the entire program. They are unique trim sticks and do not “mimic” the functions of mechanical controls.”
I had the opportunity to visit be BAE and get a demonstration of an earlier generation of the CH 53K active sticks. Additionally, I presented a paper at the SAE A6 conference, when NAVAIR present a paper on the CH-53K active sticks. My statement was based on what I was stated by both BAE and NAVAIR representatives. Also if you reread my statement you will see that I stated that active sticks were incorporated so that the cyclic controls ‘COULD’ mimic conventional mechanical controls, not would mimic. As was the case with the 525 certification, being able to mimic conventional mechanical controls reduced potential program risk.
Finally, I believe you were being kind by saying it was only a 30 lb hit in weight For active sticks on the CH 53K. For both cyclics and associated electronics, I believe it’s more than twice that amount.
Stay healthy,
CTR
PS John Dixon, I will respond to your posting as soon as I get some more time.
Last edited by CTR; 12th Jul 2020 at 20:27.