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A System A Day Keeps The Troubles Away 4

Tue Nov 26, 2002 10:19 pm

Hi folks, here I am with another session about the 35. Today'e speech will deal with the wing design, with the empennage and with FAR Part 25 certification.

The Learjet 35 is endowed with a swept back, all metal wing mounted to the lower fuselage by 8 fittings. Each wing is fitted with ailerons attached to the outboard rear spar at three hinge points, with single slotted Fowler flaps attached to the inboard rear spar by a system of tracks, rollers and hinges, with a spoiler secured to the top of the wing just forward of the flaps by two hinges and with a tip tank attached to the wing tip at two points.
Depending on the serial number Learjet 35's are fitted either with vortex generators or boundary layer energizers, whichever is used, they delay airflow separation allowing for lower stall and minimum control speeds, for shorter takeoff and landing rolls at high gross weights, and for higher mach number operation, mixing the higher kinetic energy from outside the boundary layer with the slower air of the boundary layer, overcoming the adverse pressure gradient resulting from the flow of turbolent air sweeping from the trailing edge forward as the airfoil reaches the critical mach number or the maximum coefficient of lift, and thus breaking the shockwave.
While Learjet 35's serial numbers from 002 through 278 use two rows of vortex generators, subsequent numbers and those retrofitted with the AKK 79-10 modification use the so called soft-flight modification which includes three rows of boundary layer energizers, if any are missing MMo is reduced to .78Mi; a full chord stall fence which delays airflow separation over the ailerons at high angles of attack as, because of the swept wing design, the shokwave tends to move from the wing root outwards; a stall strip attached to the inboard section of the leading edge and which at high angles of attack induces buffeting to inform the pilot of the impending stall by anticipating the airflow separation, and a gap seal along the leading edge of each aileron to use the high pressure air from underneath the wing to energize the boundary layer over the ailerons, increasing their efficiency.

The empennage of the Learjet 35 has a high T tail design ant it consists of a vertical stabilizer with the respecting rudder and of an horizontal stabilizer with the respective elevator.
The vertical stabilizer has a swept back design and it is constructed around 5 spars.
The horizontal stabilizer has a swept back design as well, it is formed by 5 spars and it is secured to the vertical stabilizer at two attaching points: the forward one is an electrically operated screwjack that provides pitch trim while the aft one is a heavy duty hinge.

As stated on a previous post, the Learjet 35 is certified under FAR Part 25 which governs the construction and performance of Transport Category aircrafts. In order to obtain this certification the Learjet had to demonstrate its capability of meeting certain requirements mainly dealing with climb performance during single engine operation as the airplane experiences an engine failure right after V1 and the pilot elects to continue on and go around capabilities.

For FAR Part 25 certification purposes it is assumed that the computed N1 is set prior to start the T/O roll, that the runway is paved and dry, that the airplane is rotated at Vr so as to accelerate to V2 at 35ft agl.
FARwise the takeoff profile is devided into 4 segments.
The first segment extends from 35ft agl, the so called reference zero since it is the point where the takeoff distance ends and where the takeoff climb begins, up to the point where the landing gear is completely retracted. During the firt segment the airplane is configured for take off, the flaps are in the T/O position, the operating engine is at the T/O power setting, the gear is down and the airplane is at the V2. The only change in configuration which takes place during this first segment is the gear retraction, even though during the test flight the gear was retracted at V2, it is assumed that in the event of a real engine failure scenario the gear would be retracted after apositive rate of climb is attained. To meet cerification criteria the aircraft must demonstate a positive rate of climb during the first segment.
The second or climb segment extends from the point where the first one ended up to 400ft agl. During the second segment the flaps are still in the T/O position, the operating engimne is at the T/O power setting, the airplane is at the V2 while the only change respect to the first segment is that the gear is retracted. To meet certification criteria the aircraft must demonstrate a minimum gos climb gradient of 2.4% meaning 2.4 feet of climb for every 100ft travelled horizontally. The second segment is the one limiting the maximum allowed takeoff weight of the aircraft.
The third or acceleration segment extends from the point where the aircraft has achieved 400ft agl up to where either the airplane accelerates through the Venr or sigle engine enroute climb speed or when 5 minutes have elapsed since the application of T/O power. During the third segment firstly the airplane is accelerated to the flap retraction speed, V2+10, and the flaps are retracted, then the airspeed is increased up to Venr, finally upon reaching Venr the power is reduced to maximum continuous power. To meet certification criteria during the third sement the aircraft has to demonstrate its capability of maintaining a positive rate of climb.
The fourth or final segment extend from the the point where the third one ended up to where the aircraft reaches 1500ft agl. To meet certification criteria the aircraft must demonstrate a minimum gross gradient of 1.2 percent. When the aircraft reaches 1500ft agl the takeoff manuever is considered to be over.

Landingwise FAR Part 25 certification requires the aircraft to demonstrate its capability to meet or exceed a minimum climb gradient of 2.1% if in a SE condition or of 3.2 if both engines are operative, at any point above the reference point assumed to be at 50ft agl above the runway threshold. During this segment, which is called approach climb segment, the engines or engine is at the go around setting, the flaps are set to the T/O position, the gear is retracted after attaining a positive rate of climb, and the airplane is accelerated to the Vref applicable to that specif weight.

Once again I hope you enjoyed my virtual speech, next time I will illustrate you the electrical system.


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RE: A System A Day Keeps The Troubles Away 4

Wed Nov 27, 2002 3:48 am

Excellent post Diego. Thanks for the information on the Lear 35.
Firings, if well done, are good for employee morale.
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

RE: A System A Day Keeps The Troubles Away 4

Wed Nov 27, 2002 12:40 pm

That's very informative, Diego. Thanks for taking the time to write it.

"In God we trust, everyone else bring data"

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