Four Lift Surface Aircraft

Conventional aircraft designs have settled on two lift surfaces for reasons of efficiency. The monoplane wing with either a canard or a tail plane provide the highest cruising efficiency for high speed flight with a very limited number of exceptions. This is the reason monoplanes began to replace biplanes in the 1930’s, and continues to this day. The advent of canard aircraft in the 1970’s improved the stall characteristics of the conventional tailplane aircraft, but generally at the expense of higher landing speeds and longer runway requirements.

Recently, a number of 3 surface aircraft have been developed and implemented to improve on the limitations of two surface aircraft for two different purposes:

  1. For STOL aircraft, reduce the tailplane down-thrust required during high-lift requirements on the main monoplane wing. The best example of this is the King Katmai aircraft.
  2. For executive transport aircraft, to provide less required wing area and a displaced main wing spar carry-through location to provide more cabin space. The best example of this is the Piaggio Avanti.

The three surface aircraft advantage comes from being able to replace the required down-thrust from the tailplane with lift from the canard during STOL operations, and the ability to move the main wing rearward since the tail can now be relied on for lift instead of down-thrust.

The three surface aircraft still utilizes the main monoplane wing as its primary lifting surface, with the other two surfaces providing primarily pitch control. For landing configurations, both types of three surface aircraft depend on the main monoplane wing remaining at an angle of attack less than the stall. Generally the canard is designed to stall first to prevent the main wing from stalling, with the monoplane wing at a lower angle of attack.
However, unlike canard/monoplane two surface aircraft, the three surface aircraft can allow the main wing to approach its critical angle of attack, since the tail plane is designed to be at an even lower angle of attack. Thus, with the three surface configuration, the main wing can be allowed to stall, which is something the two surface canard/monoplane wing can never allow to happen to avoid a tail-first stall.

One central fact remains for both the two surface and three surface aircraft configurations: these aircraft are not designed to operate with the main wing at an angle of attack well beyond the critical angle of attack.

It is instructive here to look at a graph which displays the lift coefficient of an airfoil versus its angle of attack. The advent of the stall is depicted by the drop in lift as the angle of attack of the airfoil exceeds the critical angle of attack( at the maximum lift coefficient). Almost all depictions of airfoil performance versus angle of attack stop depicting lift at a few degrees above the critical angle of attack. This is a result of not needing to show lift performance at conditions the aircraft are never designed to reach.

The critical angle of attack for this airfoil is at about 15°, which is typical for most modern airfoils. Much less common, and in fact hard to find, are plots which carry the lift coefficient out to 90°.

This graph, for the NACA-0015 airfoil, carried the lift curve out to 180°, where the airfoil is going backwards! While the typical range applies up to the critical angle of attack at around 13-14° and a lift coefficient of about 1.11, once the airfoil angle of attack reaches 45°, the lift coefficient peaks again at 1.05.

This characteristic becomes crucially important for one type of flying, and during one part of that type of flying. This is for VTOL flight, during approaches to VTOL landings.

When making a VTOL landing, it is often important to make very steep descents to land. For rotorcraft, controlled descents can be made while under engine power, with no contribution from wing-type lifting surfaces required. For aircraft which use wings for cruise flight or to assist with takeoffs, and engine power for VTOL landings, the monoplane wing pose a dilemma. During the near vertical descent, the wing will be fully stalled, with its lift characteristics varying with airspeed and angle of attack. Thus, as is demonstrated in the F-35, Harrier and the Osprey, the wing cannot be very large or it will affect aircraft stability at high angles of descent.

This means that in the event of a power failure or fuel exhaustion, these type of aircraft will make very high speed deadstick landings. This is unacceptable for routine civilian operation of an aircraft, and means these type of aircraft will not prove to be acceptable for general or commercial aviation use.

VTOL aircraft for these applications will require the ability to make near vertical, controlled descents safely to open space on the ground. While whole-aircraft parachutes are certainly feasible, the uncontrolled nature of the descent can still result in injuries, since once the parachute is deployed the landing is at the mercy of the wind.
An alternate has developed which involves the use of 4 surface aircraft.

The alternate 4 surface configuration involves a biplane with canard and tailplane. This configuration was the predominant configuration during the first five years of controlled flight by Voisin and the Wrights, but began to fall out of favor with Bleriot’s channel crossing, and was completely abandoned by the start of WWI.

The key advantage of the 4 surface configuration is the ability for the aircraft to descent vertically while under full control, with the ability to either drop the nose to gain airspeed, or to add power to regain forward momentum and lift. The aircraft enters a condition similar to a flat spin, but without the spin. For this to happen, the sizes and airfoils used in the canard, main wings and tail must be specifically designed. In particular, the biplane arrangement needs to provide significant negative stagger, with flaps on the bottom wing and ailerons on the top wing.

As the aircraft descent angle increases, the canard does not stall first. The main forward(lower) biplane wing stalls first to drop the nose, but the canard still provides lift and pitch control, as does the tailplane. Thus, instead of the nose dropping, the descent rate increases to put the forward, lower wing into the region of increasing lift. This is in the range of 20 to 45°, the lift of the lower, forward wing increases, stabilizing a relatively flat aircraft angle during the descent with the canard and tailplane providing trim.

As the top, rear wing stalls, the canard and tailplane still provide control control, but the descent angle again increases as both wings are now stalled.

Beyond the 45° angle depicted in the graph, one needs to understand that the graph above no longer applies. This graph shows lift relative to an airfoil angle of attack in a wind tunnel. At the 90° angle of attack, the airfoil is not producing any lift since it is sideways to the airflow. In a vertical descent, the airfoil acts more like a flat plate dropping flat, which has a drag coefficient, not a lift coefficient. At a 90° angle, the drag coefficient is close to 1.0, which is similar to the maximum lift coefficient. Thus, as the angle of descent increases from 45° up to 90°, the drag coefficient rising compensates for the lift coefficient dropping.

The specific conditions where vertical descent is stable and controllable are set by the following parameters:

  1. The canard moment arm, chord and span; the canard’s elevator surface angle and % of chord
  2. The tailplane moment arm, chord and span; the tailplane elevator surface angle and % of chord
  3. The arrangement of the two main wings.

The photos below indicate examples of the 4 surface aircraft which worked.

The conclusion from this is that it would be useful to have VTOL aircraft which can vertically descend under control when they are without power, instead of having to deploy an uncontrolled parachute.