tgrayson
New Member
Er, that doesn't make sense. By definition, the weight ahead of the CG is the same as the weight behind the CG. That's why they call it a "Center" of Gravity.
Learjet design question.
- Thread starter Holocene
- Start date
Er, that doesn't make sense. By definition, the weight ahead of the CG is the same as the weight behind the CG. That's why they call it a "Center" of Gravity.
TFaudree_ERAU
Mashin' dem buttons
Right you are. I shouldn't type and watch TV at the same time. Doh!
I have vague recollections of some talk about the airflow over the tail being partially blanked by the wings at high angles of attack and the elevator losing some effectiveness. Like jwp says maybe the fins help pitch the nose down during stall recovery.
ZapBrannigan
If it ain’t a Boeing, I’m not going. No choice.
Yep, after the 35 the newer Lears don't have pushers or "nudgers" -- just shakers. The delta fins fill that role.
The tail being partially blocked by the wings in t-tail airplanes is that "deep stall" we were discussing and supposedly the delta fins prevent the aircraft from entering that flight regime. I hope I never find out if they work.
The tail being partially blocked by the wings in t-tail airplanes is that "deep stall" we were discussing and supposedly the delta fins prevent the aircraft from entering that flight regime. I hope I never find out if they work.
tgrayson
New Member
Don't really see how that would work. If elevators lose effectiveness, the AOA will decrease, since it's the elevators that are producing the AOA. Now, the vertical stab is probably blanketed by the wings, and the ventral fins will be descending into clean air as the AOA is increased.
I'm not aware of any direct role of ventral fins for anything other than directional stability.
ZapBrannigan
If it ain’t a Boeing, I’m not going. No choice.
Tgrayson,
Not saying you're wrong, but FSI disagrees with you.
Not saying you're wrong, but FSI disagrees with you.
rausda27
Well-Known Member
Delta fins are there for directional stability. As many have mentioned it decreases dependency on the yaw dampener. It also assists in dampening dutch roll tendencies. The addition of delta fins also eliminates the need for the stick pusher, hence the Lear 45 only has a shaker. Aircraft were further modified by a service bulletin that added a small strip of metal to the trailing edge of the rudder and rudder trim tab that further reduce yaw oscillations.
"...The delta shaped ventral strakes on the Avanti are most likely stall prevention devices, though they do provide some needed yaw stability. Such strakes were first used on the Lear Jet. At low angles of attack, the drooped strakes provide no lift; but at high angles of attack, they develop lift and raise the tail. The delta shape is key to this behavior...."
http://www.eng-tips.com/viewthread.cfm?qid=57055&page=12
"Here is the AIRCRAFT GENERAL DESCRIPTION from the Learjet 60 Pilot's Manual: ...Two inverted "V" ventral fins (delta fins) are fitted to the aft section of the tailcone to provide the aircraft with favorable stall recovery characteristics and additional lateral/directional stability."
http://www.anav8r.com/page01.htm
I'm not saying anything about the veracity of the above information. I'm just a Googler.
bdhill1979
Gone West
I counted to a googleplex once
"...The delta shaped ventral strakes on the Avanti are most likely stall prevention devices, though they do provide some needed yaw stability. Such strakes were first used on the Lear Jet. At low angles of attack, the drooped strakes provide no lift; but at high angles of attack, they develop lift and raise the tail. The delta shape is key to this behavior...."
http://www.eng-tips.com/viewthread.cfm?qid=57055&page=12
"Here is the AIRCRAFT GENERAL DESCRIPTION from the Learjet 60 Pilot's Manual: ...Two inverted "V" ventral fins (delta fins) are fitted to the aft section of the tailcone to provide the aircraft with favorable stall recovery characteristics and additional lateral/directional stability."
http://www.anav8r.com/page01.htm
I'm not saying anything about the veracity of the above information. I'm just a Googler.
jwp_145
GhostRider in the Sky
I read that the Citation Mustang has the Delta fins for that exact reason... it helps out the stall characteristics and assists in lowering the nose of the airplane during a high alpha situation.
tgrayson
New Member
That's a bit vague. I can see that since the strakes have some orientation in the vertical direction, that once they assume an AOA with respect to the airflow, they would have a tendency to raise the tail or at least oppose the effort of the pilot to lower the tail. The way I would interpret this behavior is an effective increase in longitudinal static stability, which is measured by the effort it takes to move the aircraft from its trimmed AOA.
It's possible this effect could be large enough to prevent the max elevator deflection from producing a stall AOA, which is an effective reduction in elevator authority. But I suppose that the stick pusher has a similar effect.
I've made a general rule to try to be as vague as possible when I'm completely out of my league.
Orange Anchor
New Member
Back when... I had the privilege to fly the Lear when the delta fins were new. I had seen them in an 'experimental Lear' on an earlier visit to ICT and had been asked to not mention them or write about them. I agreed.
Later when I flew with Pete Reynolds, then Chief Pilot at Lear, we took the airplane to around FL490 and Pete said, "Throttles idle and bring the stick full aft." figuring Pete knew what he was talking about I did as Pete said although everything I had read about T-tails and deep stalls at high altitude suggested Pete was preparing me for a wild ride.
Not so. We slowed, I hit the full aft stick position, we got the shaker and the nose *gently* fell through. It may as well have been a C-150. Reduce angle of attack, add power and recover. Simple.
ZapBrannigan
If it ain’t a Boeing, I’m not going. No choice.
Well there ya go. Somebody who has seen it first hand.
