Aft-strakes/Back-porches

[Kryptid]

I've been interested in this particular aircraft tail design recently. However, it seems that I can't find much information about them. In Daniel P. Raymer's book "Aircraft Design: A Conceptual Approach", there is the following short passage about them:

The "back-porch" or "aft-strake" is a horizontal control surface that is incorporated into a faired extension of the wing or fuselage. This device, seen on the X-29, is mostly used to prevent pitchup but can also serve as a primary pitch control surface in some cases.

The only two aircraft I can think of at the moment with aft-strakes are the X-29 and MiG 1.44. With these aircraft, they seem to be secondary control surfaces with the canards playing the primary role for pitch control. Dr. Raymer seems to be suggesting that there are aircraft that use them as the primary pitching surfaces. Would anyone here know of any such aircraft? Even if they are unbuilt projects?

Also, I'm wondering what the relative advantages and disadvantages of aft-strakes are (without canards)? I can possibly see them as being less draggy and stealthier than a conventional, all-moving tail since it doesn't have a leading edge (since it blends into the rear of the wing). It might also serve as a place to store extra fuel (I believe I once heard that extra fuel is stored in the strakes of some upgraded variant of the MiG-29). I see a potential disadvantage in pitch authority versus conventional stabilators, however, particularly at high angles of attack.

I do kind of wonder why they aren't seen more often.

 

[Grey Havoc]

At least two configurations of the MDC Model 265 seem to have used aft-strakes.

EDIT: Woops, looks like I was wrong. Sorry about that!

 

[pathology_doc]

They do seem like a good way to provide for large control deflections without too many aeroelastic bending issues (thanks to their extremely low local aspect ratio), and to provide or supplement either pitch or roll authority that was being provided by other surfaces (ailerons/flaperons or spoilers on the planform you have posted; canards on other aircraft).


Interesting what you say about the MiG-29 storing fuel in the strakes; reminds me that the English Electric Lightning was so short-legged they even put fuel tanks in the flaps to squeeze as many Imp. Gals as they could in.

 

[ScrutorAudax]

Kryptid,

Other advantages include an increase in wing area and therefore lift. This is achieved similarly to the way the center fuselages of aircraft with widely spaced engines, such as Su-27, F-14, Fairchild Republic F-15 proposal, and the preliminary Rockwell ATF proposal generate lift on their fuselage, but instead of the wing sections being between the engines, they are on the sides of the fuselage. This configuration allows for an increase in roll rate when compared to the aforementioned aircraft due to a lower longitudinal rotational inertia, but less lift because the closely-spaced engines do not provide an endplate effect.

The airframe is also stronger than a similarly sized conventional tail (not all moving) due to the structural bracing from the wing.

Major disadvantages include low control surface efficiency and control authority because the surfaces are typically small and extend to the strake tip, producing a vortex (this is typically why ailerons do not extend all the way to the tips of the wing).

 

[ScrutorAudax]

This is what Dan Raymer says in "Aircraft Design: A Conceptual Approach".

 

[overscan (PaulMM)]

The tail surfaces on the Su-47 are half-way between an aft-strake and a tail.

Aft strake controls are conceptually the same as a beaver tail elevator. The McDonnell-Douglas Model 265 configuration has widely spaced engines, and the elevator between. With closely spaced engines, the elevator is split each side as aft strakes.

 

[overscan (PaulMM)]

Steve Pace's book on the X-29 gives a little more detail on the use of aft strake controls. On the X-29 they were especially used in low speed flight to augment the canards in pitch control.

The close-coupled canards on the X-29 are relatively close to the centre of lift so their pitch authority is less than it could be.

 

[kcran567]

Is it also for trim?

 

[red admiral]

Its a nice way of increasing lifting area and blending between wing and body. Probably really only of use for pitch control due to size and location. Might be relatively free to implement by having a small, low rate actuator. Use seems to be a secondary pitch control i.e. mostly trimming, rather than primary attitude changes.

F-16 has split devices in this location that act as airbrakes

 

[overscan (PaulMM)]

X-29 aft strake controls only deflected +- 30 deg. They were used throughout the flight envelope automatically by the flight control system, but especially at low speeds.