Production Flight Testing

[733driver]

I was watching this documentary LearJet - National Geographic Documentary on Mega-Factories - YouTube about the learjet factory and was wondering about production flight testing. In the video they conduct a stall and recovery around the 36 minute mark. I was wondering what the test pilot was doing with the yoke after the jet was stalled. It looked more like a "full&"free" than a stall recovery. Was he trying to see what happens during a mis-handled stall recovery or was that what was needed to get it out?

Also, it looks to me like he is doing the test flight single-pilot (including the readios and system operations) and the guy in the right hand seat "only" runs the checklist. Is that how it's done?

[dereknf]

It looks like he is using full right handwheel to stop it rolling off to the left. Yikes!

[ICT_SLB]

It looks to be a LJ60. Like all classic Learjets, the stall characteristics are governed by a "stall strip" only about 6 inches long between the fuselage & the wing fence. This must be adjusted by hand after a qualified pilot conducts the initial stall testing. My guess is that this is a video of that initial testing.

For an illustration, google "learjet stall strip" and you'll see the device in "Flying the Classic Learjet".

[733driver]

Thanks for the posts guys. Had a look at the suggested book (on google books) and it said that the stall strip's job is to disturb the laminar flow at high AOA so that the buffet can be felt through the elevator. Ok, makes sense to me but that doesn't really mean that the stall strip affects the stall charateristics of the aircraft (when and how it stalls) but purely how and when the buffet is felt.

Any input regarding the apparent single pilot ops during this flight test?

[Zoom-56]

Some great questions and observations! As a former military test pilot with a type rating in the Lear 20-series, I can offer a couple of thoughts on some of this. For commercial ops, virtually all biz-jets are certified with a two-pilot crew, but the fact is that they can be safely flown by a single pilot with the only added risk being the loss of "meat servo" redundancy. The guy in the other seat on the film may have been another rated pilot, or may have been a flight test engineer or even a maintainer. You'd have to ask the company about its policies on that.

The early Lears had crappy full-stall characteristics, so they had stick shakers/pushers installed to keep you from getting that far, but recoveries involved using coordinated rudder and aileron to get the wings back to level. The pilot's feet were not visible to me as I looked at the video, so there is no way to tell what he was doing, but consider this: at about 36:39 of the clip you see the yoke being worked frantically, but if you look carefully, you will also note that the attitude indicator is frozen solid at a wings-level, nose-high attitude. In Hollywood, sex sells, and so does pucker-factor... real or not. Cut to the simulator? Your guess is as good as mine.

[sycamore]

Z56,are the Learjets all manual controls ,ie,no hydraulic assistance.?
Looks like they still have `crappy` characteristics at the stall,or just plain `overcontrolling`instead of a proper `unloading`,then add power..

[Zoom-56]

I'm not typed in the current-generation Lears, but I know that several bizjet manufacturers have started going to fly-by-wire. Unless something has been added recently I don't think the Lears are so equipped. Previous generations had hydraulics, but only for utility systems like flaps, spoilers, and gear. The primary flight controls were mechanical, with a bit of boost from stability augmentation like the yaw damper.

[BrasiliaCaptain]

I have a friend who has produced some reality TV shows. They can (and do) cut, paste, and parse footage to make things look more dramatic and drawn out than they really are. So I wouldn't take this footage to be real-time documentation of what happened on that flight.

[italia458]

Ok, makes sense to me but that doesn't really mean that the stall strip affects the stall charateristics of the aircraft (when and how it stalls) but purely how and when the buffet is felt.

What causes a buffet? Separation of flow - you're not going to have a buffet without it. What is an aerodynamic stall? Separation of flow.

Ever had ice on your wing?

[John Farley]

Ok, makes sense to me but that doesn't really mean that the stall strip affects the stall charateristics of the aircraft (when and how it stalls) but purely how and when the buffet is felt.

Spot on 733driver.

The whole point about the nature and position of stall strips is that they give warning without affecting stall handling. Indeed with some aircraft types the position of the strip to achieve this was only finalised during production flight test.

[WeekendFlyer]

Simply put, the stall strips induce unsteady flow (usually vortex shedding of some description) in to the airflow over the wing, which can be felt as buffet, when the wing is a few degrees of AOA below the stall AOA. The resulting buffet will not cause a stall because it does not lead to dramatic and sudden separation of flow across the majority of the top surface of the wing. However, it does provide a warning to the pilot that he can feel through the controls and his seat, in time for him to avoid the stall. The Hawk has similar devices but still gives very little warning of stall in the landing configuration, which sadly has been a contributory factor in the loss of some of them.

Stall, on the other hand, is marked by the separation point moving forward rapidly towards the leading edge of the wing at the stall AOA, leading to separated flow over most of the upper surface of the wing. Buffet and stall are not the same thing.

Given the learjet's rather unpleasant stall characteristics, one can see why they opted for stall strips! Sadly, such is life for swept wing aircraft. Witness also the wing fences and the boundary layer energisers (vortex generators) in front of the ailerons, and one can see that spanwise flow and tip-stall was a problem for these aircraft.

[italia458]

John F,

The whole point about the nature and position of stall strips is that they give warning without affecting stall handling. Indeed with some aircraft types the position of the strip to achieve this was only finalised during production flight test.

Don't some airplanes use stall strips to ensure the airplane stalls 'evenly'? I've seen a number of airplanes that have stall strips only on one side.

Weekendflyer,

Simply put, the stall strips induce unsteady flow (usually vortex shedding of some description) in to the airflow over the wing, which can be felt as buffet, when the wing is a few degrees of AOA below the stall AOA. The resulting buffet will not cause a stall because it does not lead to dramatic and sudden separation of flow across the majority of the top surface of the wing.

The stall strips induce unsteady flow into the airflow over the wing. Surely the stalling AoA is affected by stall strips?

If a certain airplane were to have no perceptable buffet before stall, stall strips would be added to provide a buffet. If you compared the critical AoA of one airplane with stall strips to an identical airplane, except without stall strips, wouldn't you find that the critical AoA is higher on the airplane without stall strips?

[John Farley]

italia458

Are you asking if stall strips are used to improve handling by eliminating wing drop? I guess they may have been but I have not experienced it. If you can get the warning and handling you want with one side only why do both?

If your type is one that has an unacceptable tendency to drop a wing at the stall I would think one of a variety of leading edge fixes to improve the breed would come to mind before stall breaker strips.

Since it is clearly impossible to produce wing after wing where both sides are identical the question of whether some stall handling fix/trimming is needed during production test will depend on the degree of variation and the basic characterisics of a given type when it comes to accepting wing profile/rigging variations.

In my experience the types that need trimming at production test are a small minority. But I am over three decades out of date.

[dereknf]

When I was involved with the BAe125, production aircraft on test used to be stalled "naturally" with stick pusher off. If the aircraft rolled one way or the other, the stall triggers were adjusted and the aircraft reflown. When the stall handing was satisfactory the instruction was issued to rivet the triggers in place. If I recall, the triggers didn't introduce any buffet but were there to actually stall the inboard wing first (rather than the tip).

The BAe146 also had breaker strips but as far as I recall, these were never adjusted. There was little or no pre-stall buffet or nose drop at the stall either (hence the stick shaker and stick pusher).

[safetypee]

Hi Derek, as you say the 125 invariably required some adjustment, but the 146 never.
The 146’s stall breakers (the only external wing appendage, and how many of today’s-aircraft have no VGs etc anywhere?) were removed for one test flight. Marketing thought it would reduce drag (marketing – aerodynamicists?) and the MD thought it would look better.
After a short flight of stall tests, the strips were nailed back on very quickly. The pilots debriefed marketing and the MD with some well-crafted words of advice!