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

This website has a number of very experienced aerodynamicists .. so ask away.

A link to a photo would be a starting point.

I have one each on the three hulls of my trimaran.

A "Wakestill" ?

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

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?

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)

Lyman is not an aerodynamicist.

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 (http://www.raisbeck.com/ka/strakes.html)

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.

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/images/intelligence/content/citation/201003-rudder-wind-damage.jpg

It may well be a stiffner, and an add on post design for whatever reason. I will post a picture tomorrow. Thanks.

Probably just because it looks nice.

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.

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

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.

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?

Thanks. That sounds viable. Does anyone know if it was a mod? Do the other Citations have similar strips?

Which is why a picture always helps at the start...

The following paper (http://www.dept.aoe.vt.edu/~mason/Mason_f/AIAA2007-4175.pdf) 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.

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?

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.

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.

From John tullamarine:

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.

Who develops the design standards? For instance, having been through the Patent process, I found, surprisingly, it is not necessary for the patented device to function....it is only required that it be different from prior patented devices!

If found of no value, ( by anyone, or FAA) could the regulator prevent its installation?

When we talk about Design Standards, we are referring to documents such as FAR 23, FAR 25, etc. These are developed by the Regulator in accordance with obligations pursuant to ICAO decisions. In general, a particular Design Standard will comply with ICAO requirements other than in respect of differences notified formally by the relevant State.

I have never seen any interest on the part of aviation Regulators when discussion gets around to patents. The latter are a civil matter for litigation and design protection and, as such, are not within the ambit of the Regulator.

The Regulator will (should) act to ensure that the requirements of the Design Standards are met. If, for instance, a proposed gadget doesn't work then, so long as it doesn't create a problem, there is no reason why the applicant can't waste its money and effort on incorporating it. Problem belong applicant, not Regulator.

Sometimes, a Type may be found to have a problem some time after Certification and the TC basis may be reviewed. On occasion, additional or changed requirements may be imposed to fix such an after the event problem.

Actuator,

The attached picture is of a flange that has been added to portions of the trailing edge of the rudder and the electrical rudder tab on a Lear 45. As I said before, I was told this was done as a service bulleting to allow flight without the yaw damper.

Is this similar to what you see on the Citation?

Hawk

http://i41.photobucket.com/albums/e290/dpullan/IMG-20120529-00054.jpg

You can see the rudder tab at the bottom, with the rudder pretty well filling out the rest of the picture. The under portion of the right horizontal stab is on the right.

Lyman,

A Patent has nothing to do with application, permitting, or certification.

Figure this, a paint scheme on the aircraft requires certification. There are also many parts of the aircraft in the common domain, of which there is no patent, that require a cert.

While a thicken edge may work on that specific variant of that specific aircraft, it may not work on a variant of that same aircraft, and certainly not on any other aircraft.

Winglets would be a great example to follow.

Forgive the denseness, I'm trying to suss the similarities twixt USPATOFF and STC. The Attorney gets 335/hour, and you sound every bit as smart, so my question is: does there exist a proprietary interest in what can be described as the "value" of developing an STC.

Winglets are both trademarked and Patented, afaik, so how do they compare with a "Paint Scheme", which sounds more like a badging thing, or Trademark issue than STC, or Patent?

The work on the Lear looks, erm, kind of "homey". Non countersunk screws, and deformation of what looks like aluminum, uncovered. (Probably SS).

What is its purpose? Sorry, it replaces the Yaw Damper? I can see why that may not be subject to Patent, or STC. So of course it is approved, is there an STC, to read?

Lyman,

Perhaps a review of what the STC process is and how it is granted, or not, would be helpful. Don't confuse it with a patent application or patent issuance, they are two different subjects. An STC may often be not patentable but useful for installation on an aircraft. Conversely, a patented item or idea, although deemed valid, may not be determined useful enough to warrant installation. It is as simple as that, don't try to merge or confuse the two...

From the FAA:

STC process - major steps, as required
Applicant applies for STC
Familiarization and preliminary type certification board (TCB) meetings
FAA develops certification program plan
Establishment of certification basis by FAA
Applicant submits data for approval
FAA design evaluation
FAA and applicant hold specialists and interim type certification meetings, as required
FAA performs conformity inspections
Engineering compliance determinations
Pre-flight TCB Meeting
Applicant performs ground inspections, ground tests, and flight tests
FAA reviews manufacturer's flight test results and issues TIA
FAA performs conformity inspections, witnesses tests, performs official certification flight tests and flight standards evaluations
Functional and reliability testing
FAA approves flight manual supplement or supplemental flight manual and holds final TCB meeting
AEG completes continuing airworthiness determination
FAA issues STC
An STC will be issued only if:
the pertinent technical data have been examined and found satisfactory,
all necessary tests and compliance inspections have been completed, and
the alteration has been found to conform with the technical data.&
An STC will not be issued to:
approve minor changes, or for approval of identical replacement parts, unless the installation of such parts constitutes a major change to the type design
approve design changes to Technical Standard Order (TSO) approved articles unless the TSO is invalidated for the modified article. An STC which modifies a TSO article must provide for installation
combine two or more STCs without additional showing of compliance; or
manufacturers or applicants outside of the U.S., except as provided for in a bilateral agreement.

TD

Lyman,

I have several patents, and have been through the process and application. In the aviation world, you have to look at the history of aviation to understand just how many patents may be out there.

The paint scheme is a trademark, but each scheme or configuration needs to go through cert, simply due to weights on the paint. Sorry, but in the grand scheme of things, when the FAA certs are down to tens of pounds, the paint on a 747 can weigh a lot!

Thanks Hawk, my interweb prowess does not extend to posting a photo (yet). That is similar, the Citation does not have it on the tab though just the lower portion of the rudder and without the riviets behind. It extends about 1/4 cm 90 degrees to the rudder surface on both sides.

Lyman: I think I see what you're trying to get at with your questioning.

When an organisation applies for, and is granted, an STC by the FAA/EASA/etc, they effectively 'own' that modification. If anyone else would like to perform the same installation under the cover of that STC, they would need to be given the relevant design/installation/cert data by the initial organisation.

So, is there an inherent value to undertaking an STC? Yes, there is, if it's something that other aircraft owners want.

A good working example would be the recent regulatory changes which require 8.33 kHz spaced communication radios. Let's say I designed a modification for the 737 Classic to replace the existing radios with compliant radios under an STC. I could reasonably expect to be able to 'sell' the use of this STC to other 737 Classic owners, under which they could upgrade their radios.

An STC isn't exclusively a mechanism for protecting intellectual property, but it does permit you to own a particular modification which you can then sell to others.

Awesome, that is exactly what I wanted. My desire springs from a way to make money on a system I have developed, and the Patent Process is bizarre. I spent ten thousand dollars (unavoidable) to gain information that was useless. I should have directed it at development of the system, and networking with users.

Thank you, I am in your debt.

Actuator. That sounds like the T strip. But it also sounds like a stiffener at the corner of the Rudder, an area that takes great abuse at times. If, instead of driving the airstream, the corner is driven by jet blast, the stress is quite high, and in an unplanned for region, given its lightweight and the stop it has against a stronger structure, (the VStab).

The .25cm entry into the airstream would be interesting to compare, as percentage of chord, with the data John Tullamarine supplied. It sounds miniscule, but a week ago, I didn't know what a Tstrip was. Gotta love this place....

Lyman,

Another St Ivian is correct in what he says about STCs. But, an important fact is not addressed, that being liability. Take the case of modifying an aircraft with a devise that was not included as part of the original Type Certification by means of an STC. As holder and owner of the STC, you now assume liability for that devise, should it, for whatever reason, fail resulting in damage to the aircraft, personal injury or loss of life. The original aircraft manufacturer is absolved. In other words, STCs come with responsibilities in addition to ownership and or profit making opportunity. Just so you know...

TD

I am aware... If a new development passes the regulator, and its process, negligence is negligible, that leaves man defect or improper install.


My favorite pair of words: By Others. That and good insurance, presto a new sailboat....

One cannot get within twenty feet of the chocks w/o gaining exposure to suit.
That actually is not a bad thing, the more John Does, the more dilute the finding, and the less the award...

As in Engineering, in LAW it is always best to spread the stress...:ok:
Wanna lose your tail rotor?

Actually, that's where the whole affair becomes rather unpalatable as a business proposition.

Just because your modification passes the regulatory hurdles and gains approval, it doesn't mean the certifying authority is at any time underwriting your work. It's entirely possible that a civil lawsuit further down the line could find you guilty of negligence despite having received approval for your mod.

This sort of thing is exactly what killed many GA manufacturers in the last few decades. If you're interested, have a google around for some of the lawsuits held against Cessna, Lycoming et al.

I think a lot of innovative and important work is shelved, waiting for tort reform.
My best hope is to sell the system to a manufacturer, or, less probable, licensure. And the money is not huge. It's there, but the process is discouraging at times. Lawyers.... They're not people.

The Patent process itself, can be rather inexpensive for a private person.

The trick is to get the Patent in the door, with a provisional patent.
This allows you to market the device, to perhaps an OEM manufacturer, while being protected by the provisional patent.

That way, you can sell the patent, and let them finalize the process, or begin to license the patent, and pay the rest with those monies.

In reality, anything associated in aviation is a liability. Even though your innovation may not have been the cause, it will still be encumbered in the legal process, and you may be potentially dragged in.

That is why many of the patents are sold off to patent houses such as Intellectual Ventures.

Good luck.

Thank you for all your help, and another st ivian and TD. The most reward I have already, I have the prototype, and it is protected. As you must know, the key design is patience :ok:

I have no legal competence but I suggest that an STC provides negligible protection against having the design ideas pinched by others.

Protection is the province of patents, registered designs, and the like.

Concurrent pathways would be advisable. The patent would seem to be a priority, as the difficult and expensive part, the implementation, could be borne by others. Gurney was sol because it wasn't his idea in the first place.

Prior Art...

Actuator,

SB 45-55-6 for the lear 45 will "install straps of the trailing edge" of the rudder and rudder trim tab assembly. The "reason" was that "Learjet has received reports of yaw fluctuations (dutch roll) during flight. Accomplishment of this SB will reduce the dutch roll tendency of the aircraft during flight"

From the AFM; "aircraft not modified must have yaw damper engaged for all flight operations except take off and landing. On modified aircraft the yaw damper use is not required"

so the mod somehow negates the requirement for the use of the yaw damper.

Perhaps Cessna did the same?

Hawk

Perhaps a variation of aileron spades, which in null, provide very little drag, but power steering with input....

Strikes me it is a mechanical yaw damper, providing Rudder input from null, due added asym drag til returned to neutral... pretty slick. And from slop, w/o control input.

John Tullamarine

The "wags" of the day always said it was to prevent bird strikes from the rear by slow flying birds!

Tmb

Having done some work on Citations, that was always the obvious answer ..