Biplanes

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     Advantages

         1. Biplanes (or triplanes) can usually lift up to 20% more than can a similarly sized monoplane of similar wingspan. A biplane will therefore typically have a shorter wingspan than the equivalent monoplane, which tends to afford greater maneuverability.

          2. The struts and wire bracing of a typical biplane form a box girder. Particularly when divided into bays, this permits a very light but strong and rigid wing structure. This allows a biplane to fly with very little power…

     Disadvantages

          1.Each wing negatively interferes with the aerodynamics of the other, requiring greater overall surface area to produce the same lift as the equivalent monoplane.

          2. A biplane typically also produces more drag than a monoplane, especially as speed increases.

       Technological Advancements

       In early days (1900-1930), the biggest advantage of biplanes was having twice the surface area and a rigid structure to support the wings. But at present, high strength carbon fiber reinforced plastics made it possible to build very high aspect ratio wings without any (or little) external support. With the advent of steel, then aluminum air frames, the previous considerations were mute, and monoplanes have become more common than biplanes. For clarification: Equivalence between biplane and monoplane means than both have the same wing area and the same engine.

     Differences in manouvering are:

         1.The biplane has better roll acceleration than an equivalent monoplane.

         2. The biplane has a higher roll rate than an equivalent monoplane at the same speeds.

         3. All biplanes flying takes place at lower speeds, resulting in a lower space requirement for all maneuvers. This also means that inertial effects are less pronounced: When pulling up, there is less kinetic energy available for climbing, so (for example) turns will end with less altitude gain.

    Differences in handling: The biplane has

         1. Lower aileron forces for the same roll rate at the same speed

         2. Lighter control forces overall due to lower flight speed

    Differences in performance:

         1. Shorter take-off and landing distances 

         2. Lower stall speed

         3. Much lower maximum speed

         4. Lower optimum cruise speed and range

         5. Lower power requirement due to the lower flying speeds, or if both use the same engine, a better power-to-weight ratio when compared to an equivalent monoplane. These differences are most pronounced if the airplane carries just the pilot and not much payload.

     Flying techniques are the same as for monoplanes. Indirectly, differences are likely due to differences in design. Example: Few biplanes profit from having a retractable landing gear while gear retraction makes sense for monoplanes with higher power loading (installed power relative to wing area).

     Explanation

     Biplanes have two major differences:

         1. Smaller wing span at the same wing area, and

         2. Wire bracing results in very lightweight biplane wings.

    The smaller span reduces roll damping and roll inertia, so a biplane will accelerate into a roll more quickly than an equivalent monoplane and will reach a higher roll rate. This is the main difference in maneuvering. The smaller wing span results in more induced drag if both have the same mass and the same speed. With wire bracing, this condition is unrealistic, and an equivalent biplane will be much lighter. If the structure is a substantial part of the aircraft’s mass (this is typical for aerobatic airplanes), the result can easily be less induced drag, despite the lower span, and also lower wing loading. This in turn means that both fly at different speeds: The biplane will be able to fly much slower, but the aerodynamic drag of the bracing will restrict it to low speeds.

      This also means inertial effects are less pronounced: The lower mass and lower speed of the biplane combine for a marked difference to the equivalent monoplane. For aerobatic displays this is ideal: All action takes place close to the audience, and the biplane will need a much smaller area for all maneuvers than an equivalent, but heavier monoplane. The downside is low maximum speed and low range. Another difference in performance are much shorter take-off and landing distances due to the lower wing loading, which results in a lower stall speed. The optimum endurance and optimum range speeds are lower than those of an equivalent monoplane as well, so all biplane flying happens at lower speeds, which is beneficial for training aircraft. Since control forces are proportional to dynamic pressure, a biplane will have lower control forces than an equivalent monoplane. Here equivalence also means that the relative chord of all control surfaces is the same.

      In reality, a good designer will select a higher relative chord for the biplane’s control surfaces to ensure that control forces are above their required minimum. The heavy, unreliable engines of the early years made biplanes the ideal way of taking to the air. Once engines became more powerful and allowed higher payloads, the monoplane became better suited to carrying passengers and freight at higher speeds and over longer distances. Biplanes handle like bricks. They lose altitude more quickly at low speed, and they are much easier to stall. They have more drag and will enter a spin more readily. They are much more susceptible to cross winds than monoplanes and therefore can only take off and land in relatively moderate conditions, since they will flip over easily. The advantage they have is that they can turn much more quickly than a monoplane, so they are nowadays mainly used for acrobatics, like Pitts Specials.

     During the war pilots flying Fairy Swordfishes

and Fireflies

were in great danger from the much faster Bf109s.

      They would escape by plummeting to the ocean in a virtual free fall then pulling out at the last moment. Any Messerschmitt pilot dumb enough to follow them in this maneuver would be flying their last mission. There is no principal difference in handling. Both have the same set of control surfaces and stability is achieved using the same methods, so the piloting techniques are the same. There will usually be difference in performance. Biplanes loose speed faster, but are slightly more maneuverable. The main advantage of biplane is that it can be built with less strong materials, because the wings are shorter and the box girder bracing structure distributes the loads very well. This often meant lower weight which offset the increased drag. The main disadvantage of biplane is that shorter wingspan means lower aspect ratio and therefore higher induced drag, which dominates at low speed, and the larger frontal area and surface area (interference between the wings means they are less efficient than they would be independently) mean higher form drag, which dominates at high speed.

     So a biplane needs a stronger engine for the same weight and will not glide as far should the engine fail. Before cantilever (without bracing) wings were developed, the bracing didn’t allow monoplanes to achieve that much lower drag and their higher weight cancelled the little aerodynamic advantage they had, so biplanes dominated. With cantilever wings the aerodynamic advantage of monoplanes became more important and biplanes almost disappeared. 

      A few aerobatic biplanes (like the famous Pitts SC1) remained, most likely because with shorter wingspan they have lower moment of inertia in roll and therefore roll slightly more easily.

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