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 Posted: Mon Dec 27, 2010 12:12 pm Post subject: Thoughts on flying at the edge of the envelope |
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I have thought about this for a few days and if I should share this or not, but I think it will provide an experience to think on. We hear pilots talk about the edge of the flight envelope in movies and tv shows, and think it sounds really cool but what does it mean? Each aircraft has a flight envelope based on several factors, speeds, G-load, weight, configuration like flaps, ect. One I would like to think about is Maneuvering speed or Va, and what happens if you over G the aircraft below it? We all know a plane can stall and any attitude and any airspeed as long as the critical angle of attack is exceeded. When at or below Va a full and abrupt control movement results in the aircraft reaching it’s load limit, it will most likely stall, or at least they are designed to, several things are happening here but the long story short is if below Va the plane will stall before exceeding the load limit and breaking. The lightning is a responsive aircraft in all control axis but stable. The wrong control movements can result in the above at an unexpected time.
I became aware of an instance recently with a Lightning that experienced this. The pilot flying the aircraft was not familiar with the aircraft but was flying it with no problems during normal flight, there also was a check pilot onboard. The aircraft became dangerously close to another in-flight, a near mid air. The pilot flying (not check Pilot) promptly pushed the stick full forward while in normal 80knt or so climb out. The resulting pitch attitude was unexpected and undesired as you can imagine, the pilot than abruptly pull the stick full to the up stop. What happened next correlates to the discussion above. The aircraft shuddered hard than pitched down, the pilot did not change the stick input but kept the stick full up at the stop. Several oscillations resulted until the check pilot neutralized the stick. Neutralizing the stick unloaded the aircraft, decreased the angle of attack and allowed the wing to fly again.
After this happened I was told that I needed to check the design over for a flutter problem because that is was the shuttering and pitching was about. Clearly that is not what happened, had it been truly flutter we probably would not have gotten to talk about it with the pilot as they would have heard a high frequency vibration followed by a nice quite sound as the control surface departed the aircraft. That is a discussion for another time, but considering the pitch system is a push pull tube, and the related components do not allow any slop in the system, and the flight testing done prior to release of any kits, makes flutter very unlikely.
The pilots aboard experienced a high speed/ accelerated stall. The pitching was a result in lost tail down lift and the aircraft attempting to correct the pilot induced condition. The vibration or shudder that was felt was a stall buffet.
The POH explains that proper stall recovery is to relax the stick pressure not to pull or push the stick to the control stops.
The designed Flight envelope kept the aircraft in one piece, this happened below maneuvering speed and resulted in a stalled condition instead of an in-flight break up. The Pilots of the aircraft got to experience the edge of the flight envelope, thankfully it was not the other end.
Nick Otterback
[quote][b]
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Bill Strahan
Joined: 20 Aug 2008
Posts: 145
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| Posted: Sun Jan 02, 2011 11:10 am Post subject: Re: Thoughts on flying at the edge of the envelope |
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Reading this story made me think of a few things. First, on my flight home from SYI, with about 25 hours on the plane, I did something I've done many time in my Grumman. I twisted around, reached in the back, and grabbed my backpack. I pulled it up over the seat-back and dropped it into the copilot seat.
Again, it's a motion that felt so familiar. The next few moments were not something with which I was familiar. Everything suddenly stuck to the top of plane and the view out the front no longer included a horizon, it was all earth! And as rapidly as it happened, it unloaded and I was just pitched down sharply.
I realized that I had dropped the backpack in the seat, and it had simply fallen forward onto the stick. It's a big, heavy backpack that I have my 18" laptop in, and was undoubtedly several pounds of forward stick pressure.
And just like that I realized I had to change some habits. Even if the laptop had hit the yoke in the Grumman, the response would have been a tiny pitch down and I would have pulled the backpack off. In the Lightning it was at least -1g. It was pretty unsettling.
Which brings me to the next comment, I sure would like to see a removable stick option for the copilot side. I've considered pulling the stick out and having a pro welder modify it for me. Just a larger diameter tube to couple the stick, and a pin to slide in place.
For flights with non-pilots who don't want to actually fly I'd pull the pin and put the stick in the back. Most of the time I'd leave it in place, but having the option to remove it might be handy.
Glad all turned out for the pilots mentioned above. In a panic, it's easy to over control. Good thing the check pilot was there. I'm sure that other pilots in similar situations have held the stick back all the way to the ground.
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