Hummingbird Aerobatics: the Wespe

Hummingbird and the Heinkel Wespe

In November 1998, long after Hummingbird’s conceptual design was complete, a reader drew my attention to the Heinkel Wespe, a VTOL design from Nazi Germany (and later the Lerche). While one assumes that this airplane was never built, I have never seen a design with a closer resemblance to Hummingbird; consequently it provides an opportunity to further clarify the theoretical basis for Hummingbird’s configuration.

Duct

The most obvious similarity between the two aircraft is the duct. In both cases the reasoning is that we want to use a duct to enhance static thrust (for vertical takeoff and landing, in the case of the Wespe), and since the duct is large and draggy, we elect to place it near the CG and integrate it with the lifting system so that some of that drag can be recovered.

The Wespe has a bizarre angular duct, which sacrifices some aerodynamic efficiency probably to ease manufacturing. Nevertheless the result is the same: the duct becomes a powerful lifting surface (annular wing), thus greatly reducing (planar) wing area.

Wing

The Wespe’s wing is much smaller than Hummingbird’s, relative to duct area. Duct diameter and chord are set by propulsive, structural, and propeller clearance constraints; planar wing area is then added to achieve the required wing loading. The Wespe, being a VTOL interceptor, could accept a high wing loading, and therefore a very small wing. Perhaps the Wespe could have got by with no outboard wing whatsoever, but then what to do about ailerons?

Duct Brace Geometry

Like Hummingbird, the Wespe’s fuselage loads appear to be taken around the propellers, through the duct.

Since the Wespe’s wing is so small, there is less concern about wing loads distorting the duct. Thus (likely for drag reduction reasons) the Wespe designers elected to incorporate the minimum practical number of surfaces bracing the duct to forward and aft fuselages—three forward and three aft. Since the wing already constitutes two surfaces forward, they are angled down 30 degrees, while an upper fin completes the structural triangle. The aft surfaces are of the same number and orientation. Consequently, the Wespe’s duct is braced at just three arc angles around the duct, which effectively renders the duct too flexible to pick up bending loads from the wing.

While the Wespe’s duct is braced every 120 degrees of arc, Hummingbird’s duct is braced every 45 degrees of arc. Through the addition of just two small surfaces—one fin forward and one stator aft—and rotating the aft surfaces (stators) 45 degrees, Hummingbird’s duct becomes the heart of an extremely rigid offset octagonal truss. The chief consequence is that Hummingbird’s structural system is stiffer and allowable tip clearances are smaller, thus improving propulsive efficiency.

Miscellaneous

The Wespe uses only one engine, and we don’t know how they proposed to neutralize torque. This arrangement may have had CG problems as well (tail heavy).

The Wespe appears to have just three tail surfaces. Probably this is a further attempt to negate some of the drag hit from the duct, though it would have increased instability on the ground over four tail surfaces.

A VTOL Hummingbird?

I have considered this VTOL variation of the Hummingbird configuration for some time, in response to the question: “Well, if it can hover, why not make it vertical takeoff!” In the case of Hummingbird, this would involve simply shortening the tail (increasing its area in the process) and placing a landing gear there. Voila, a personal VTOL airplane!

While feasible, the idea does have its problems. The configuration would be very unstable on the ground—a high wind might topple you over. Landing ass-backwards would be a trick; you would have to learn to use mirrors or be prepared for a big crick in your neck. And as for climbing in. . .

In short, other configurations are probably better for VTOL. Let’s just let Hummingbird takeoff and land on a runway and leave its tail alone.

Philip Carter
23 November 1998