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How does flying a biplane differ from flying a monoplane? Apart from the increased drag and lift, how does it handle differently, and what effect does that have on manoeuvring?

In particular, I'm interested in differences between a biplane and a hypothetical equivalent monoplane, not differences between vintage airplanes and modern ones, or differences between aerobatic and utility airplanes.

Dan Hulme
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    There are a lot of opinion-based answers and answers copied from other sources, but very little actual evidence. In some ways, this question is meaningless, unless someone can come up with two actual airplanes that have the same engine and airframe, but with different wings. – rbp Dec 04 '14 at 19:38
  • I thought the design choice between a biplane and single wing was a matter of wing area, and early plane designers used box designs because of the weight advantage over a long, heavy strong spar. – rbp Dec 04 '14 at 19:42
  • @rbp The answers tend to focus on design decisions, but I was thinking more from a pilot's point of view. When I fly my biplane, what am I doing differently from just a slow, old monoplane? – Dan Hulme Dec 04 '14 at 20:33
  • having flown both, the real answer is not much. you'll find a bigger difference between the gear styles, tailwheel and tricycle, than between a single wing and a biplane. – rbp Dec 04 '14 at 22:35
  • Better visibility with a monoplane. – Tony Ennis Dec 08 '14 at 22:09
  • What is the stall speed of the Tiger Moth? – Koyovis Jul 04 '17 at 06:39

5 Answers5

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Short answer:

For clarification: Equivalence between biplane and monoplane means than both have the same wing area and the same engine. Then the main differences in maneuvering are:

  • The biplane has better roll acceleration than an equivalent monoplane.
  • The biplane has a higher roll rate than an equivalent monoplane at the same speeds.
  • All biplane 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) hammerhead turns will end with less altitude gain.

Differences in handling: The biplane has

  • lower aileron forces for the same roll rate at the same speed
  • lighter control forces overall due to lower flight speed

Differences in performance:

  • shorter take-off and landing distances
  • a lower stall speed
  • a much lower maximum speed
  • a lower optimum cruise speed and range
  • a 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:

  • Smaller wing span at the same wing area, and
  • 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 are 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.

Peter Kämpf
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  • how much of these advantages can be attributed to the lighter weight of biplanes versus single-wing airplanes, for the same wing area? aside from the box kite construction, most biplanes have fabric wings, versus metal skin and wood or metal spars of single-wing planes, making them much lighter. – rbp Dec 05 '14 at 13:00
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    @rbp: Equivalence means that both should use the same materials and processes. With the lower dynamic pressure of biplanes, they have less problem with flutter and gust loads, so it is possible to build them of wood and fabric. Now both are not equivalent anymore, like with the gear retractability I used as an example. Optimizing each will mean to lose the basis for a fair comparison. When optimized designs are compared, the biplane will leverage the lightweight wing design to also save weight in all other parts. In the end, all this is really a consequence of the box kite construction. – Peter Kämpf Dec 05 '14 at 13:31
  • "the lower dynamic pressure of biplanes, they have less problem with flutter and gust loads" can you improve the answer to cite a reference? or maybe explain better why? – rbp Dec 05 '14 at 14:14
  • @rbp: Both speed and dynamic pressure are factors in flutter. The speed determines the frequency of aerodynamic oscillations, and the dynamic pressure determines how much energy is involved. Oscillations must not have similar frequencies than structural eigenmodes to avoid flutter, and a lower speed means less structural stiffness is needed to ensure this. Gust loads are again proportional to both, and higher flight speed produces steeper gradients of force changes, and the force change itself is proportional to dynamic pressure. – Peter Kämpf Dec 06 '14 at 08:34
  • Why has a biplane a lower stall speed than a monoplane with equal wing area? – Koyovis Jul 03 '17 at 06:08
  • @Koyovis: Interference between both wings. The high pressure below the upper wing is reduced by the suction on the lower wing. A clean single wing can produce more lift. – Peter Kämpf Jul 03 '17 at 17:12
  • @PeterKämpf The top wing still stalls though, doesn't it? – Koyovis Jul 03 '17 at 22:45
  • @Koyovis: Given enough angle of attack, both will stall. Staggered wings make the top wing behave a little bit like a slat and delays stall on the bottom wing, but only because the bottom wing sees a reduced angle of attack. – Peter Kämpf Jul 04 '17 at 05:02
  • If a monoplane used kingpost and wire bracing to mitigate its structural weakness, how much does that help it to catch up with the box kite? (assuming otherwise similar lightweight construction, engine power). Obviously both have wire drag in that case... – user_1818839 Feb 25 '18 at 13:02
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    @BrianDrummond: This helps a lot and makes the single wing structurally very similar to one biplane wing. – Peter Kämpf Feb 25 '18 at 17:31
  • @Koyovis Reading your question and my answer now makes me realize that I gave you a poor answer. The lower stall speed of the biplane is entirely due to its much lighter wing and lower maximum speed which in turn allows also the fuselage to be built lighter. So the biplane, despite its wings producing less lift per area than the monoplane, will stall at a lower speed due to its much lower wing loading. – Peter Kämpf Sep 13 '20 at 04:07
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In a nutshell, the advantages and disadvantages are:

Advantages

  • 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.
  • 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

  • Each wing negatively interferes with the aerodynamics of the other, requiring greater overall surface area to produce the same lift as the equivalent monoplane.
  • A biplane typically also produces more drag than a monoplane, especially as speed increases.

Comparison

Technological Advancements

In early days (1900-1930s), 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.

NASA has a historical discussion about these.

Farhan
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    What do you mean by the second advantage point? How does the box girder structure affect power (beyond increasing it because it has higher drag)? – Jan Hudec Dec 03 '14 at 19:12
  • @JanHudec I didn't paste the entire explanation from Wikipedia but have done it now. If you think it still doesn't fully clarifies the point, please let me know and I'll update it. – Farhan Dec 03 '14 at 19:27
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    It doesn't. Because it doesn't make sense on Wikipedia either. The box girder allows building it from weaker material. But it has larger drag, and therefore requires more engine power, at all speeds. – Jan Hudec Dec 03 '14 at 19:30
  • @JanHudec: The statement about power is correct, but only because the biplane can fly much more slowly. At the same speed it needs more power. – Peter Kämpf Dec 03 '14 at 20:50
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    @PeterKämpf : indeed, so slowly that an Antonov 2 reportedly doesn't have an official stall speed. If you fly slow enough, you'll just descend with it like with a parachute. – vsz Dec 03 '14 at 21:18
  • @vsz … only when the c.g. is all the way aft. Biplanes still obey the laws of physics, and this mushing stall is only possible close to neutral stability. – Peter Kämpf Dec 03 '14 at 21:58
  • @PeterKämpf: Biplane may be able to fly more slowly, but that does not follow from the use of box girder structure. And with worse L/D it's not clear it will need less power even at lower speed. – Jan Hudec Dec 03 '14 at 22:28
  • This is a good discussion of the advantages and disadvantages of each, but I was asking specifically about the differences in flying from the pilot's point-of-view. – Dan Hulme Dec 04 '14 at 09:40
  • Note that adding more planes will not always increase the mentioned advantages: http://images.slideplayer.de/1/666186/slides/slide_19.jpg – PlasmaHH Dec 04 '14 at 10:30
  • @JanHudec right, in fact, the L/D curve tells us that you need MORE power to fly slower, not less. – rbp Dec 05 '14 at 13:04
  • @JanHudec: Mostly wrong. Don't use slightly nonsensical designs like the Beech Staggerwing or DH89 Dragon Rapide for a comparison. A well designed biplane will have low wing loadings which no equivalent monoplane can match. Remember that the power requirement goes up with the cube of airspeed. A little less speed goes a long way in reducing power needs. – Peter Kämpf Dec 05 '14 at 13:55
  • @PeterKämpf: Cube of airspeed? Power = thrust * speed. And thrust only increases with square of speed above Vy. Biplane will have lower Vy (due to lower wing loading), but need higher thrust at it's respective Vy due to lower L/D. The two differences work against each other. – Jan Hudec Dec 05 '14 at 14:33
  • @JanHudec: Yes, and then drag ~ speed². Makes power ~ speed³ when thrust = drag (as it should be). A prop will give more thrust at lower speed, the airplane is slower AND lighter - I don't see how it would need MORE power. This discussion should not hide in the comments, but might spawn a good question in itself. – Peter Kämpf Dec 05 '14 at 15:36
  • @PeterKämpf: And what about induced drag?? That one is inversely proportional to speed, isn't it? And most aircraft don't cruise all that faster than Vy, the speed of minimum drag. – Jan Hudec Dec 05 '14 at 19:05
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    @JanHudec: Induced drag is also proportional to lift, and a lighter airplane requires less of it. Again, a small speed difference will already make a big difference in power. – Peter Kämpf Dec 05 '14 at 23:01
  • You may wish to mention visibility. An extra wing makes it a little bit harder to scan for traffic. – Dan Pichelman Dec 06 '14 at 01:53
  • I love this conversation -- I've learned a lot. But it seems to me that from a handling standpoint, it would be difficult to impossible for a blindfolded pilot (ok, one wearing a "view limiting device") to tell the difference between the two. And is there more difference between flying a EX300 and a SIAI-Marchetti, than there is between an EX300 and a Pitts? – rbp Dec 08 '14 at 15:28
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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 Fairey 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 Messerschmidt pilot dumb enough to follow them in this maneover would be flying their last mission.

Tyler Durden
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There is no principial 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 manoeuvrable.

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.

Jan Hudec
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Biplanes have typically higher wing areas for the same sized aircraft. I remember I biplane that I took a ride in crashed not on hour later because on short final he caught a wind gust and ended up upside down on the runway!

(Everyone was okay, but you never forget the tin can hanging off a bike sound of an airplane crash.)

Bassinator
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