What are the disadvantages of a delta wing biplane design?

Question

The idea of a delta wing as I understand it is to reduce stress on the wing and drag on the leading edge. Now using two delta wings on separate planes, on one frame would presumably allow for a reduced wing size, and therefore less drag. I imagine it could also lead to a nice compromise with better control in slow flight than delta wing designs?

However, clearly there are some major issues with this design, as a quick search uncovers no results for this design shape. therefore I pose the question: Why is it that designers have rejected this possibility; what are its major (dis)advantages?

Answer 1

The main benefit of biplanes is that the lift can be distributed across both wings. This provides more lift with less wingspan and places less load on each wing. Less load means the materials can have lower strength. This was very important in the early days of aviation, when engine power, materials, and weight were much more limited.

The major downside of a biplane is aerodynamics. The wings interfere with each other, causing drag. While structural bracing between the wings can reduce weight, it also adds more drag. This means that a single wing will almost always perform better, and modern materials and designs should make that possible.

A biplane configuration does not make much sense for delta wings. The main drawback of a biplane is drag, but drag increases with the square of velocity, and delta wings are best in high-speed flight. While delta wings have lower wave drag, they have higher viscous drag due to the large surface area. A second delta wing will add more area, increasing both wave drag (from the cross section) and viscous drag (from the surface).

Configuration of the aircraft also becomes problematic. Most aircraft with delta wings tend to be fairly flat, allowing lower area and therefore lower drag. The closer the wings are, the more they will interfere. The further apart they are, the more structure is needed, either in a larger fuselage or in structural braces, which increases weight and drag.

Answer 2

Lift is created by accelerating air downwards. If you use two wings stacked on top of each other to accelerate basically the same amount of air, your efficiency goes down, not up. The more air is involved in the creation of lift, the better for efficiency. Reducing wingspan will drive up the lift-dependent part of drag, because the span loading (unit of mass per unit of wingspan to carry) goes up.

Biplanes make most sense if the requirement calls for

• high roll rates
• minimum structural mass and
• low airspeed,

which was generally why early airplane designers chose this configuration to take to the air. When engines became more powerful and the aircraft had to earn their money by transporting payloads, the monoplane won.

A delta wing makes most sense for trans- and supersonic speed. Its capability for vortex lift allows low landing speeds even with thin airfoils without powerful flaps, and its high leading edge sweep enables a subsonic leading edge even at supersonic flight speed.

It might be interesting to mention one idea for a supersonic biplane here: This could be used to avoid shockwaves, but only at one particular Mach number when the shocks of the top and bottom wing cancel each other. No airplane has ever used this concept, mostly because in order to work no lift must be created.

Answer 3

Besides allowing to be built of materials with less strength (which made biplanes widespread in the early days of aviation), the sole advantage of a biplane is better handling at very low speeds with a shorter wingspan. There are still some biplanes in use today, but more in the "bush plane" category, as their shorter wingspans and low stall speeds allow them to land on rough and narrow fields. The famous An-2 doesn't even has a stall speed, it lands almost like a parachute if you turn off the engine and pull back the stick. So, the main reason one would chose a modern biplane is to be able to fly it at extremely low speeds or in rough environments. Delta wings are designed for high speeds, the polar opposite of where biplanes are good at. You can't optimize an aircraft for both very low and very high speeds, as it would perform worse in both categories.