The Pelikan tail was originally proposed for the X-32. Why was the Pelikan tail proposed rather than a V-tail? What advantages and disadvantages does the Pelikan tail have compared to a V-tail or a conventional tail?
A Pelican tail is a V-tail with the two tail surfaces set apart. Compare this to a straight V-tail as that of the Lockheed F-117:
Tail view of the F-117 (picture source)
The closest to a Pelican tail which has actually been flown is maybe that of the Northrop YF-23:
Tail view of the YF-23 (picture source)
Clearly, the distance between the two surfaces is dictated by the engines, but Northrop could have placed them on top of the nacelles. However, doing so would had been a poor choice for several reasons. A Pelican tail has those advantages:
- If the vertical part is too close to the wing strake or fuselage chines, the strake vortex could hit the tail surfaces and cause oscillatory loads. The F-18 vertical tails needed to be reinforced so they would not break off. A Pelican tail should easily avoid this.
- A pure V-tail has poor pitch damping, because the pitch-induced angle of attack change is reduced and the reaction force, too, by the V angle. Placing a horizontal strip in the middle improves pitch damping.
- Wikipedia has conflicting information on weight savings, but generally V- and Pelican tails should be more efficient than a conventional tail. However, for the best maneuverability a conventional tail is preferable. Again, placing a horizontal part between the two V surfaces improves pitch maneuverability, even when the control surfaces are only on the V part. Their symmetric deflection will induce a considerable lift change on the horizontal part.
- Placing the vertical tail outside and aft of the engine exhaust will shield the lateral IR radiation better.
The drawbacks are:
- Higher structural complexity compared to a true V-tail.
- Lower maneuverability and yaw damping than a conventional tail.
EDIT: I knew I had seen a Pelican tail before, but could not remember where. Now I found it:
This ist the German Fletter 282 V23 helicopter (picture source), and yes, this is a Pelican tail; however without control surfaces. It had two intermeshing rotors which made a tail rotor unnecessary. Anton Flettner had built the similar Flettner 265 in 1940 and introduced the 282 in 1942.
The X-32 was Boeing's entry for the JSF contest, which was supposed to serve USAF, USMC and USN. JSF called for an affordable VTOL capable aircraft operated by all three services. Boeing decided to go for a tailless single piece delta with a direct lift system, similar to the Harrier.
Image from militaryfactory.com
The adoption of direct lift system and other considerations meant that X-32 had weight issues (They actually had to strip off some parts for VTOL). However, midway through the program, USN changed its requirements, requiring improved performance and payload capabilities. This caused serious problems for Boeing as its present design was expected to fall short of the requirements.
In order to overcome this, Boeing decided to change the wing and tail design of X-32. For tail design, two designs were proposed- a conventional four poster tail (like Lockheed's X-35) and a Pelikan tail (it was conceived by Ralph Pelikan, who worked for McDonnell Douglas and now for Boeing after their merger). The main points put forward in favor of Pelikan tail were,
It caused less drag compared to conventional tail
Offer pitch control at high AoA.
It had better stealth characteristics (two tails would be better than four).
The design would look different from the Lockheed concept.
However, the problem was that nobody has flown with this design so far. After some discussion, Boeing decided to go with the Pelikan tail. However, after a few days, the decision was reversed as the design was found to be heavier by a considerable amount as the larger control surfaces required beefier hydraulics and Boeing decided to go with a conventional tail, which turned out to be around 1500 lb lighter compared to the original design.
However, the prototype was not actually modified due to lack of time. The modified aircraft would've looked like this:
Image from aerospaceweb.org
Saab is testing a (sub-scale) concept with Pelikan tail called the FS2020, which has Pelikan tail.
Image from gripen4canada.blogspot.in