Learjet design question.

Holocene

Well-Known Member
What is the purpose of the 'fins' mounted to the fuselage directly below the vertical stabilizer? Additional vertical stability?

 

bdhill1979

Gone West
They provide a weather vane tenancy (similar to the feathers on an arrow) and damp out Dutch Roll

Ie: better stability
 

AMH

Well-Known Member
What is the purpose of the 'fins' mounted to the fuselage directly below the vertical stabilizer? Additional vertical stability?

Delta fins. They help improve the stall characteristics and directional stability of the aircraft.
 

WMostellar

Well-Known Member
It all boils down to Dutch Roll.

The 25 and 35 require 2 yaw dampers to be active and 1 to be engaged at all times with exceptions for single engine situations and partial flap landings.

The dorsal fin modification (as shown in the picture, standard on 31 and later series, optional on 25 and 35) changed the requirement to 1 yaw damper...a big savings in maintenance and certification.

I only flew the Lear in the USAF (3 years operational, 5 years at the Schoolhouse), so my experience is limited (and 10 years old). If someone has more accurate and current knowledge, please post it.

Doug knows I only knows what I nose.

William
Delta ATL 765B
 

AMH

Well-Known Member
It all boils down to Dutch Roll.

The 25 and 35 require 2 yaw dampers to be active and 1 to be engaged at all times with exceptions for single engine situations and partial flap landings.

The dorsal fin modification (as shown in the picture, standard on 31 and later series, optional on 25 and 35) changed the requirement to 1 yaw damper...a big savings in maintenance and certification.

They also eliminate the need for a stick pusher.
 

BCTAv8r

Well-Known Member
Does anyone know why some of the older lears have a bunch of little "tabs" sticking up in different angles on both wings?
 

ZapBrannigan

Old School
Directs high energy airflow into the boundary layer, usually at high angles of attack.

Just like any other vortex generator, vortilon, boundary layer energizer, etc... engineers do funny stuff to try and manipulate airflow.
 

TFaudree_ERAU

Mashin' dem buttons
Does anyone know why some of the older lears have a bunch of little "tabs" sticking up in different angles on both wings?
Same reason my brand new Hawker has them...because of the reasons Zap mentioned. I have 15 on each wing, 6 on the bottom of the horizontal stab and a couple on top, from what I'm told. There is also the vortilon beneath each wing that'll stab you in the thigh if you're not careful.

Any tab, stall fence, BLE, vortilon, etc is basically indicative of a "poor" wing design; i.e. one that does high speed better than low, or vice versa. In the case of the old Lear, was designed before the technology was available to design a more efficient wing. In the case of the new Hawker, its because they haven't changed the wing in 46 years.
 

roundout

Bus Driver
How many Lears are left with the old VGs? Seems like at least most of the 35s have been converted to Century III Softflite.
 

jwp_145

GhostRider in the Sky
They provide a weather vane tenancy (similar to the feathers on an arrow) and damp out Dutch Roll

Ie: better stability
Really? I thought they were there to assist in pushing the nose over during a stall situation. Why would you want to add something that causes an airplane to weathervane?
 

bdhill1979

Gone West
Really? I thought they were there to assist in pushing the nose over during a stall situation. Why would you want to add something that causes an airplane to weathervane?
That is just the wording I read somewhere about them, so I am not sure. But my understanding was that that damps out dutch roll tendencies.
 

Number1atNumber2

Tries to keep it fun.
That is just the wording I read somewhere about them, so I am not sure. But my understanding was that that damps out dutch roll tendencies.

Yes it helps with Dutch roll and gives it a lot more stability. I noticed quite a huge difference with the 31 on final vs the 35.
 

TFaudree_ERAU

Mashin' dem buttons
Really? I thought they were there to assist in pushing the nose over during a stall situation. Why would you want to add something that causes an airplane to weathervane?
The weather vane tendency is a direct function of air flowing over the fins. Relating back to the arrow with the feathers; it is only when you have sufficient air flowing over the feathers, ie shooting it from a bow, that they become an effective means of stabilization. Same with an airplane. They are pretty much dead weight at low airspeed and become more effective as airspeed increases. Secondly, the nose is going to drop, one way or another. Either the pilot is going to push the yoke forward when it starts shaking violently, or that fancy thing called a pusher will do it for him.
 

tgrayson

New Member
Why would you want to add something that causes an airplane to weathervane?
Why would you not? You can't fly an airplane that doesn't have such tendencies, which is why we have a vertical stabilizer in the first place. Sometimes, however, the vertical stabilizer is weak, or relatively so:

  1. Swept-wing airplanes at high angles of attack. This increases the dihedral effect (lateral stability), which is the root cause of dutch roll.
  2. Any airplane at high altitudes. Damping in yaw is reduced due to the lower density of air, which can lead to dutch roll.
I don't know why this would be associated with a stick pusher. Perhaps the stall characteristics are very poor when the directional stability is weak.
 
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