Spoilers? (Ventral Fins)

[AMM]

Hey all,

I'm looking for a bit of information regarding the spoilers (I believe they're spoilers) on a Learjet 45. The roughly triangular surfaces extending out of the bottom tail at about 45deg from the vertical.

Are these control surfaces or are they fixed? What are they used for? How are they modulated? I'm under the impression that they act as aerodynamic breaks. If so, why opt for this method versus the traditional wing mounted spoilers?

Also, I've noticed similar(ish) surfaces on the Q400; further forward on the bottom of the fuselage.

I'm pretty sure these are fixed. I'm guessing these are just for stability purposes. But why not just increase the size of the vertical/horizontal stabilizers?

And here is one more version of what appears to to be same attachments.


Any information on these design features would be appreciated.

[GilletteNorth]

Here's what I think. No, they aren't 'spoilers' (if they were on the wings they might be). I think they are called ventral fins. They are fixed. They are used to improve the airflow along the aft fuselage. Ventral fins are extensively utilized in the aircraft industry to provide directional stability for high-speed aircraft

[paydaymayday]

Not sure why you think they are spoilers, they aren't designed to disrupt lift in any way, nor are they moveable.

They are ventral fins.

They are designed to increase stability. If you look at a 1900, for example, you'll see they took sticky glue and slapped on a bunch of ventrals, horsals, and tailets to make it more stable.

[AMM]

Ventral fins eh? Thanks! That opens up allot of info.

[ea306]

I am no Aeronautical Engineer...however from past reading so long ago I can no longer remember the source this is my understanding:

You will notice that each of the aircraft examples you mention are all T- tail designs.

Those Ventral Fins (Strakes) will produce lift at a high angle of attack that will help avoid a "Deep Stall" scenario where the disturbed airflow from the main wing causes the tail wing to stall. Another benefit I recall from past reading was improved lateral stability and diminished dutch roll tendencies at High Mach speeds.

Look forward to seeing what further responses you get to this question.

[AMM]

For the record, here's an interesting patent for the use of ventral fins as speed breaks.
http://www.freepatentsonline.com/3610557.pdf

Also they're used to minimize fuselage upsweep drag on certain models. This thread (http://www.airliners.net/aviation-forum ... in/161627/) has a bunch of info/pictures.

[5400AirportRdSouth]

I thought those were Ventral Strakes, used for reducing adverse yaw and improving stability?

[sidestick stirrer]

I thought they were to aid in pushing the nose down when the angle of attack gets near a stall, to avoid having to go to the expense of installing and certifying a stickpusher...

[Ray-Ban]

Dual Aft Body Strakes...

Wing/body vortices normally disrupt airflow under the aft fuselage, creating drag. Dual Aft Body Strakes eliminate this separation, channeling the vortices and accelerating the air -- in effect, pushing the airplane through the air.

Also...

They streamline the wing and body vortices under aft fuselage, reducing drag, increasing climb and cruise performance, and improving directional stability. This allows for a quieter cabin, smoother ride, increased yaw damper limits and a reduced minimum control speed.

[middle marker]

Nailed it RayBan
mm

[. ._]

Good thread!

Now THIS is one reason why Avcanada is a good site. You want some info, but don't know where to start? Make a thread!

I learned something today.

-istp

[C-GPFG]

I agree with Ray-ban with the point about reducing drag by reducing back-fill under the tail...that it why some aircraft just have one fin under there....

[snoman]

The old King Air's can be upgraded to have a dual strake from the single, such as the one C-GPFG has shown us here. Raisbeck makes a kit for those planes for a cool $22 grand or so. It enhances your directional stability so much so that a yaw dampner is no longer required equipment above 17000' when you have the duals.

The story I have heard about how they work is that they capture the votices that come off of the wing root and channel them through providing a lot more stability.

An interesting tidbit of info from a course I had taken on the 1900... I asked about all the junk hanging off of the tail and why didn't they just design a new one. They had just tacked on a B200 tail to it when they built it and it did OK, but they had a timeline to keep for competition reasons and designing a new tail would have added a pile of time to the process and weight to the plane, and in the end, with the extra horizontal fins out of the fuselage, it actually gave better lift and a broader CofG range than if they had actually designed a larger tail. Same Idea for the little stabilizers out of the tips of the horizontal stab.

[rigpiggy]

Most ventral fins are symmetrical airfoils, and add little/no drag/lift when "med strom" or with stream if I remember my latin. When exposed to a significant angle of attack, ie: during high aoa, or with significant yaw they tend to lift away from the displacement and reduce the yaw, and add up force to the tail section reducing the stall tendencies. they also tend to reduce dutch roll as the torsion forces are more equal above/below the rolling axis. Just my understanding.

[200hr Wonder]

An interesting tidbit of info from a course I had taken on the 1900... I asked about all the junk hanging off of the tail and why didn't they just design a new one. They had just tacked on a B200 tail to it when they built it and it did OK, but they had a timeline to keep for competition reasons and designing a new tail would have added a pile of time to the process and weight to the plane, and in the end, with the extra horizontal fins out of the fuselage, it actually gave better lift and a broader CofG range than if they had actually designed a larger tail. Same Idea for the little stabilizers out of the tips of the horizontal stab.

Not quite what I was told. This came from a Beech factory pilot so I assume it is pretty darn accurate. The BE 300, BE B300 and BE 1900 all where built under what was know as SFAR 41a which allowed for what falls now into the commuter class of airplane. IE the plane was allowed to go over 12,500 but not required to meet all the performance requirements of the airliners which was the next size. There where however stipulations that you could not change the control surfaces. This is why the KingAir 300, KingAir 350, and BE 1900 all have the same wing and tail feathers. The problem with the 1900 was that it did not provide the required stability but they where not allowed to redesign the surfaces per SFAR 41a so they had to tack on all the extra googas and doodads, including the dual strakes.

As for the King Air 300/350, the earlier S/N had single strakes, however then raisbeck came along with the dual strakes and they where so good they are now included right off the line. This has happened with several riasbeck mods. Also note on the 350 the strakes allow the yaw damp to U/S up to 5000' on the singles but FL190 on the duals due to the stability increase. This can be the difference between getting home and being stuck in upper rubber boot

[xsbank]

For the record, there are speed brakes and there are 'spoilers.' Speed brakes are just that - they add drag and do not kill lift. Spoilers, the kind that usually deploy from the tops of the wings add drag and also kill some lift.

Where this would be most evident would be in a dive - speed brakes will help control your speed and should not affect your trajectory as you avoid hitting the ground. Spoilers will add drag but will affect your trajectory - you might not avoid the ground if they are deployed because of the spoiled lift, but you might hit the ground 50 knots slower than you otherwise might! This then becomes the dilemma, deploy the spoilers and keep the aircraft from a large overspeed, or keep 'em stowed, accept the overspeed but avoid the ground? At which point do you decide?

Just like you should never stall unintentionally, you should also avoid jet upset and this will never be an issue.

[loopa]

I believe the ventral fins are used to compensate for an aircraft design which has a centre of ROTATION FORWARD of the C of G.

It's often times misunderstood that the CENTRE of which an aircraft moves (C of R), is not ALWAYS going through the centre of gravity (C of G). Hence our longitudinal, lateral, and vertical axies do not always go through the C of G. They some times go through an imaginary point forward or aft of an aircraft.

Ex: Take a pencil and put it on a table.

Put your index finder on the right, and your thumb on the left at the center. Try to rotate the pencil. You will notice that the HEAD of the PENCIL rotates as much as the TAIL rotates.

Move your fingers forward of the center gravity of the pencil. Try to rotate it, you will notice that the head of the plane goes through a smaller range of motion in comparison to the tail. With a center of rotation that is forward of C of G, we have more yawing tendency as more of the fuselage (body of the pen) is susceptible to movement.

A way to reduce yawing moment can be done by installing ventral fins aft of the center of rotation so that the adverse affects of a forward Center of Rotation is minimized.


So if an airplane has a ventral find placed aft of the wings, it means that the design feature of this airplane has resulted in a C of R being ahead of C of G. If you notice that the Ventral Fin is placed at the centre of the fuselage, it means that C of R and C of G is relatively close.


This is my understanding of it; I'll let the more engineer speculated people quote me on it

[McPhoo]

I think Ray-Ban got it. I will add just one thing. I flew a B200/1300 for a few years. They were never certified with autopilots and would have been limited too 17K without a yaw damper. The 1300 model had extra large strakes which eliminated the need for the yaw damper and was fairly stable up to it's certified ceiling of FL250.

[GilletteNorth]

Is this a case of potatoes/potatos? Some people call them dual aft body strakes, some just dual strakes, some call them ventral fins? Is it the position on the fuselage that determines the designation?

[AMM]

Is this a case of potatoes/potatos? Some people call them dual aft body strakes, some just dual strakes, some call them ventral fins? Is it the position on the fuselage that determines the designation?

Seems to me that the ventral/dorsal designations come from fish anatomy.



Meanwhile the strake designation came from ships (which doesn't discriminate between upper/lower mounting positions; just that its longitudinally aligned.)

In aviation, a strake (also known as a leading edge extension when positioned forward of the wing) is an aerodynamic surface generally mounted on the fuselage of an aircraft to fine-tune the airflow. Aircraft designers choose the location, angle and shape of the strake to produce the desired interaction.
The aircraft industry expanded very rapidly at the time of World War I. The skills required both in woodwork and in designing were similar to those of boatbuilding. The aircraft firms like Sopwith eagerly sought boat builders, especially those accustomed to work with such as rowing club best boats, Thames A Class Raters and the pre-war racing power boats. At that time, a good deal of boatbuilding thinking and terminology transferred with the men. It is therefore not surprising to find a longitudinal plate resembling a rubbing strake, though with a different purpose, being called a strake.

[GilletteNorth]

I have to ask again because according to Wiki, strakes seem to refer to extensions on the wings, rather than to the objects you were asking about.

This vortex lift effect can be increased by the addition of an extension of the leading edge of the wing at its root, the juncture with the fuselage, known as a strake. The strakes act as a sort of additional slender, elongated, short-span, triangular wing running from the actual wing root to a point further forward on the fuselage. Blended fillet-like into the fuselage, including along with the wing root, the strake generates a high-speed vortex that remains attached to the top of the wing as the angle of attack increases, thereby generating additional lift. This allows the aircraft to achieve angles of attack beyond the point at which it would normally stall. The use of strakes also permits the use of a smaller, lower-aspect-ratio wing, which in turn increases roll rates and directional stability, while decreasing aircraft weight. The resulting deeper wingroots also increase structural strength and rigidity, reduce structural weight, and increase internal fuel volume.[40] As a result, the F-16’s high fuel fraction of 0.31 gives it a longer range than other fighter aircraft of similar size and configuration.[34]

In fact, other Wiki articles specifically call the objects ventral fins.

The F-16 design employs a cropped-delta planform incorporating wing-fuselage blending and forebody vortex-control strakes; a fixed-geometry, underslung air intake inlet supplying airflow to the single turbofan jet engine; a conventional tri-plane empennage arrangement with all-moving horizontal “stabilator” tailplanes; a pair of ventral fins beneath the fuselage aft of the wing’s trailing edge;

[sky's the limit]

I love aviation.

Show a hundred pilots the same thing, and you'll get 95 different interpretations..... No wonder CARS is so confusing.

stl

[GilletteNorth]

Battle of the wiki references. Somebody should go on wiki and remove all references to strakes... quick.

[AMM]

Hmm,

All ventral fins are strakes.
Not all strakes are ventral fins (as is the case with the leading edge extensions).

Sound right?

Either way they're not spoilers!

[T/A for life]

On the Q400, I've heard those fins on the back called Ditching Damns. I think they are on there for some certification reasons, say if it was to land on the water. They help keep the plane floating. I'm not 100% on that but, hopefuly this will spark up someone else's facts and not just speculation like mine.