Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Citation Design

Tech Log The very best in practical technical discussion on the web

Citation Design

Old 27th May 2012, 10:39
  #1 (permalink)  
Thread Starter
 
Join Date: Jun 2000
Location: Johannesburg, South Africa
Posts: 80
Citation Design

I have tried a search to no avail.

Can anyone tell me why the Citation 500 series has a flange on the trailing edge of the lower portion of the rudder?

Further, I am looking for an aerodynamic guru or website resource that can answer similar design characteristics on various types of aircaft.

Thanks
Actuator
The Actuator is offline  
Old 27th May 2012, 10:50
  #2 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 6,498
This website has a number of very experienced aerodynamicists .. so ask away.

A link to a photo would be a starting point.
john_tullamarine is online now  
Old 27th May 2012, 11:02
  #3 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
I have one each on the three hulls of my trimaran.

A "Wakestill" ?

A small apology to Mother Nature for disturbing her Ocean....

Last edited by Lyman; 27th May 2012 at 11:06.
Lyman is offline  
Old 27th May 2012, 11:15
  #4 (permalink)  
Thread Starter
 
Join Date: Jun 2000
Location: Johannesburg, South Africa
Posts: 80
Thanks John, I figured here is as good a place as any. I dont have a photo but I will post one tomorrow.

A wakestill is a term I have not heard before. I would be interested to know how this flange affects the airflow and why it was neccesary. Further are the wingfences on the outboard leading edge of the wings of any aerodynamic value?
The Actuator is offline  
Old 27th May 2012, 11:25
  #5 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
Hi

Anything in the airstream is there to increase/decrease drag. IMO. I am assuming it is not planted on the rudder for correction of lack of authority in high AoA, a follow on to distribute load at the lowest attach, etc. it resembles nothing more than the exact opposite of a vortex generator, whose function is to annoy the slipstream...Cessna has had problems with low Rudder damage at the flange torque tube join...(rib)

Last edited by Lyman; 27th May 2012 at 11:29.
Lyman is offline  
Old 27th May 2012, 15:45
  #6 (permalink)  
 
Join Date: Jan 2006
Location: Dorking
Posts: 470
Lyman is not an aerodynamicist.
boguing is offline  
Old 27th May 2012, 18:56
  #7 (permalink)  
 
Join Date: Jun 2010
Location: Canada
Age: 32
Posts: 382
I'm assuming you mean this little guy?! - http://i.imgur.com/7xsvU.png

They're called strake(s) and are for added directional stability, especially at high altitude (increasing yaw damper inop altitude or eliminating the need for a yaw damper); helps prevent Dutch roll; improved single-engine performance; improved climb and cruise performance; less drag. Generally, most aircraft will have dual aft strakes - Raisbeck is well known to modify King Air and Learjet models with dual strakes. On the Learjet 45 I've been told that they also eliminate the need for a stick pusher as the strakes (Learjet calls them "Delta fins") will pitch the nose down if the angle of attack gets too high - it doesn't prevent the aircraft from stalling but it helps to avoid a stall.

A single strake wouldn't provide the same benefits as dual strakes but it'd be there for similar reasons.

I suggest watching this video: Raisbeck Engineering: Dual Aft Body Strakes
italia458 is offline  
Old 27th May 2012, 19:09
  #8 (permalink)  
Thread Starter
 
Join Date: Jun 2000
Location: Johannesburg, South Africa
Posts: 80
No, I am familiar with strakes or ventral fins. It is on the actual rudder, say lower third, almost just a thickening of the trailing edge, unable to be seen in that picture.

Last edited by The Actuator; 27th May 2012 at 19:11.
The Actuator is offline  
Old 27th May 2012, 19:14
  #9 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
That is indeed a strake, and not what I had assumed you meant in the OP.

It is definitely for directional stability at high (ish) AoA. It is not on the Rudder, it is on the ventral aft fuselage.

imo.

You don't for silly sake mean the Rudder Trim Tab, do you?

Actuator, I ran across a Rudder AD that originated when a rudder was damaged by blast from another a/c on the ground. Corrosion was found, and a weakening of the riveted join to the torque tube. The area you point to may be an structural accomodation for the upgrade to the rib/tube join. A stiffener, say?

To be precise, the photo here shows damage at the Stab, but led to discovery of damage at the trailing edge of the rudder that warranted a complete rebuild of the damage area you see here, plus a stiffening of the entire lower rib area...to include an enhanced TE flange?

https://www.duncanaviation.aero/imag...ind-damage.jpg

Last edited by Lyman; 27th May 2012 at 19:32.
Lyman is offline  
Old 27th May 2012, 19:52
  #10 (permalink)  
Thread Starter
 
Join Date: Jun 2000
Location: Johannesburg, South Africa
Posts: 80
It may well be a stiffner, and an add on post design for whatever reason. I will post a picture tomorrow. Thanks.
The Actuator is offline  
Old 27th May 2012, 21:44
  #11 (permalink)  
 
Join Date: Mar 2006
Location: Ormond Beach
Age: 44
Posts: 71
Probably just because it looks nice.
flyboyike is online now  
Old 27th May 2012, 21:52
  #12 (permalink)  
 
Join Date: May 2003
Posts: 398
Not just citations....

the Lear 45 had just such a flange added to the rudder, a few years after it entered service. A service bulletion. I don't remember exactly, but I think it also extended onto a portion of the rudder tab as well, not sure. It was not along the full length of the rudder, maybe for half of it I'm guessing. IIRC, this allowed the aircraft to dispatch with the yaw damper inop. I do not know if there are additonal operating limitations if dispatched inop. so I assume the flange contributed to the yaw (directional) stability.

Sounds very similar. I can try get a picture.
hawk37 is offline  
Old 27th May 2012, 22:04
  #13 (permalink)  
 
Join Date: Aug 2000
Location: Near Stuttgart, Germany
Posts: 878
Hello!

To your first question I have no answer (wondering myself every time I do my walkaround!).

Further are the wingfences on the outboard leading edge of the wings of any aerodynamic value?
They only serve the purpose of preventing glare from the lights in the outer leading edges. Some 500 series Citations (e.g. the 560 Encore) have real boundary layer fences, but these are positioned about midwing and reach far deeper into the wing.

Regards, max
what next is offline  
Old 28th May 2012, 11:46
  #14 (permalink)  
 
Join Date: Jul 2001
Location: Gold Coast
Age: 54
Posts: 1,611
It's a device called a Gurney Flap, invented by race driver Dan Gurney in the late 60's early 70's and is one of the rare things that have transferred from motor racing up to aviation, instead of the other way round.
I'm really tired right now and it's probably not the technically correct description but I believe Gurney Flap 'fools' the airflow into making the aerofoil longer than what it really is, but not adding a lot of drag.
(probably wrong, it's all I've got sorry)
The other motor racing name for them is a wickerbill.
18-Wheeler is offline  
Old 28th May 2012, 13:30
  #15 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
From Flight Safety, August 14, 2002

The basic idea behind a gurney flap is to increase the lift of an airfoil at higher angles of attack. When the flap is on one side only (most applications) it is located on the high pressure side of the airfoil. As Nick said, it's positioned at the trailing edge, and stands up 90 degrees to the airfoil surface.

The flap creates a small vacuum behind it, and since the flap in on the high pressure side of the wing, it forms a dam to the airflow on that side, so that the only way to fill the vacuum is from the low pressure side of the wing. The benefit to high angles of attack, is that the vacuum pulls the airflow over the low pressure side of the wing back down unto the wing surface at higher angles of attack. This helps to control the boundary layer separation that occurres on the low pressure side at high angles of attack, and the loss of lift associated with it. The price for this however is more drag.

Up to a point, taller gurney flaps further enhance the high angle of attack performance of a wing. but always at increasing drag penalties. The size of the flap is usually referred to as a percentage of wing cord, the same way that Nick referred it.
Last edited by Flight Safety; 19th Aug 2002 at 11:26.



From AIAA

A. Effect of Gurney Flaps and T-Strips on Baseline Wing Lift Curve
The effect of Gurney flaps on the baseline wing lift curve is shown below in Figure 4. Figure 5 shows the effect due to trailing edge T-strips. The coefficient data presented in Figures 4 and 5 was taken at a Reynolds number of 1.95x106. The coefficient data shows that Gurney flaps produced a positive increment in lift coefficient, a negative shift in the zero-lift angle of attack, and an increase in the wing maximum lift coefficient. Larger Gurney flaps produced larger lift increments. T-strips produced an increase in the slope of the lift curve and an increase in maximum lift coefficient. However, T-strips produced no shift in the wing zero-lift angle of attack.

Cannot post the graph....

Most Rudders have a symmetrical airfoil, and need equivalent lift in both directions of deflection. So one Gurney flap is counter intuitive. The "T-Strip" makes sense in that it is effective in both sweeps. So why does Cessna add a "t-Strip"? Do they?

Last edited by Lyman; 28th May 2012 at 14:01.
Lyman is offline  
Old 28th May 2012, 16:45
  #16 (permalink)  
Thread Starter
 
Join Date: Jun 2000
Location: Johannesburg, South Africa
Posts: 80
Thanks. That sounds viable. Does anyone know if it was a mod? Do the other Citations have similar strips?
The Actuator is offline  
Old 29th May 2012, 00:13
  #17 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 6,498
Which is why a picture always helps at the start...

The following paper gives some detail.

As I recall, the original idea passed into the flying game via DAC and, because it turned out not to be patentable, has found a reasonably general application.

Further (if my recollection is correct - I'd left the organisation some years prior so I don't have any detail) the first Nomad fatal mishap (Avalon) in the late 70s (in which Dave and Stuart were killed and Patrick severely injured) involved the use of a similar modification in an attempt to work a fix for the flap 20 long stab problem on that aircraft .. so the idea goes back a ways. The suggestion was that the lack of research data probably didn't help the fix attempt .. If djpil sees this thread, he may be able to add some details.
john_tullamarine is online now  
Old 29th May 2012, 00:17
  #18 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
Can one get an STC w/o a patent? If so, would FAA have to develop its own specs for certifying each Approach to the add?

Once owned, can one supplier dominate the install?

Last edited by Lyman; 29th May 2012 at 00:38.
Lyman is offline  
Old 29th May 2012, 00:24
  #19 (permalink)  
 
Join Date: Jul 2001
Location: Gold Coast
Age: 54
Posts: 1,611
Thanks, Lyman, it had to be something like that.

FWIW it's common practice on racing cars to muck around with Gurney flaps to trim the rear-end grip. For a long time I couldn't get enough front-end grip on my racer but did some work on the nose and gradually got more and more out of it. The rear wing originally had a small Gurney on it but I took it off to improve the balance with the low-downforce nose. The Gurney was about 8 mm tall.
The final nose configuration I came up with had enough downforce that I had to refit the Gurney to the rear wing, extend the slots on the wing mounts to get more AoA, and also add some wind deflectors under the tail to try to control the airflow a bit better.
Made about 1.8 G's lateral at low speed, about 2.3 G's at high speed.
18-Wheeler is offline  
Old 29th May 2012, 00:27
  #20 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 6,498
Can one get an STC w/o a patent?

I'm not aware of any necessary link. The main problem would be exposure to litigation if one used a patented or registered design gadget without an appropriate licencing arrangement.

If so, would FAA have to develop its own specs for certifying each Approach to the add?

The applicant (ie the organisation applying for the STC) needs to show compliance with the Design Standards so the effort lies with the applicant rather than the regulator. The regulator's task is to ensure such compliance to minimise the risk of dodgy things getting into the mainstream .. experimental category is for doing one's own thing.
john_tullamarine is online now  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.