Biplane vs Monoplane: Which has Greater Drag & Why?

[Cashlover123]

I thought that biplane has more wetted area due to the two wings and produce more drag. However, it is supposed to have half of the drag produced by the monoplane of the same wingspan, according to what i read in a book. Can anybody help me about with this? Which has the greater drag and why?

 

[Cyrus]

Which book did you read that says this, I'm having a hard time believing it. Biplanes are supposed to have less structural weight, but higher drag.

 

[mugaliens]

One of the most revered aircraft of all time is the Beechcraft Staggerwing (Beech Model D17S). With a staggering :) 40% payload fraction, quite high for aircraft designed in 1930, retractable gear, and a cruise speed of 176 kts, it's a testament to exceptional engineering of the period.

It was also designed such that the lower, forward wing stalls first, which causes a gentle pitch down to restore proper flight.

Even so, it carried only four people and required a 450 hp engine to achieve that 176 kts, a feat easily exceeded by numerous 200 hp monowing designs over the last half century.

Bottom line: A biplane of wingspan x has approximately twice the drage of a monowing of the same wingspan.

However, that's not the whole story, for if it has the same weight, it would have half the wing loading, as well, and that's counter to good design. Thus, if you were to modify a biplane into a monowing, you would need a larger wing!

 

[Cashlover123]

Which book did you read that says this, I'm having a hard time believing it. Biplanes are supposed to have less structural weight, but higher drag.

I read this in the book called "Aircraft Design: A Conceptual Approach, 4th Edition" by Daniel P. Raymer in page 71-73.

 

[Cyrus]

I read this in the book called "Aircraft Design: A Conceptual Approach, 4th Edition" by Daniel P. Raymer in page 71-73.

I have that book, it's a great one. Let me check those pages for you and report back.

 

[mugaliens]

I have that book, it's a great one. Let me check those pages for you and report back.

What did you find?

 

[Cyrus]

What did you find?

Sorry, I got busy flying over the weekend and forgot to look. I'll scan the page and post it in a day or two. promise.

 

[dtango]

Sorry, I got busy flying over the weekend and forgot to look. I'll scan the page and post it in a day or two. promise.

The OP doesn't quote Raymer completely. Raymer mentions that theoretically a biplane could reduce induced drag up to a 1/2. Raymer goes on to state however in practice it's not possible due to interference affects due to combined circulation affects betweeen the wings and because of this practically speaking bi-planes have higher induced drag compared to an equivalent mono-wing. There's more detail but that's the gist of it.

 

[Cyrus]

Thanks dtango.

 

[dtango]

Thanks dtango.

Your welcome! You should review it anyway though in case you want to elaborate. I left out all the juicy stuff in my foggy cliff notes version after I glanced at it yesterday!

 

[mugaliens]

The OP doesn't quote Raymer completely. Raymer mentions that theoretically a biplane could reduce induced drag up to a 1/2. Raymer goes on to state however in practice it's not possible due to interference affects due to combined circulation affects betweeen the wings and because of this practically speaking bi-planes have higher induced drag compared to an equivalent mono-wing. There's more detail but that's the gist of it.

From what I recall of aero engineering, the only reason biplanes existed in the first place had to do with the following three:

1. Materials strengths at the time didn't allow for highly maneuverable monoplanes.

2. Materials strength to weight ratios favored a box structure for those heavy, under-horsepowered engines with respect to the airframes which had to haul and maneuver them all over the sky.

3. With the advent of steel, then aluminum airframes, the previous considerations were mute, and monoplanes have ruled the air ever since. :)

Really, folks, it's this simple...

 

[Cashlover123]

how much of drag do you think is produced compared to aircraft's total drag if a bump of a size of a tennis ball is created on the lower surface of fuselage?? Assuming aircraft fuselage diameter is tad less than the tennis ball. Does it affect aircraft's lift much, because fuselage don't create much lift??

 

[boneh3ad]

Wait, you mean a bump that is slightly larger than the total diameter of the fuselage? I am just trying to make sure I understand you correctly.

 

[Cashlover123]

Yes. I was wondering if you had an RC aircraft smaller in fuselage diameter than a regular tennis ball and you want to carry the balls...

 

[dtango]

how much of drag do you think is produced compared to aircraft's total drag if a bump of a size of a tennis ball is created on the lower surface of fuselage?? Assuming aircraft fuselage diameter is tad less than the tennis ball. Does it affect aircraft's lift much, because fuselage don't create much lift??

I don't know of an easy way of evaluating that not requiring some fancy maths or wind-tunnel tests to know for sure. In general though, purely spherical objects tend to be pretty draggy.

[PLAIN]http://thetongsweb.net/images/Cd_v_NR.jpg

image source: http://www.me.utexas.edu/~dsclab/labs/lvsimulation/sphere/Cd_v_NR.jpg

 

[AlephZero]

From what I recall of aero engineering, the only reason biplanes existed in the first place had to do with the following three:

1. Materials strengths at the time didn't allow for highly maneuverable monoplanes.

2. Materials strength to weight ratios favored a box structure for those heavy, under-horsepowered engines with respect to the airframes which had to haul and maneuver them all over the sky.

3. With the advent of steel, then aluminum airframes, the previous considerations were mute, and monoplanes have ruled the air ever since. :)

Really, folks, it's this simple...

The basic reason was that a two wings and a set of bracing wires form a much stiffer structure than one. Effectively the wings and bracing structure form a single I-beam structure.

Flexible wings can fly perfectly well (look at any species of bird) but the big problem with early aviation was getting enough force from control surfaces. If you try to put a control surface on a flexible wing, you just bend and twist the wing instead of getting a control force.

As soon as it was possible to design efficient airfoil shapes with enough thickness to contain a wing spar that was big enough to resist bending and torsion, then goodbye biplanes.

 

[Cashlover123]

Thanks all for your interest in this thread.
Another question was for calculations, what AR is to be considered? The AR due to the single span of the total span of the wings?
Also, is there any method to calculate lift due to fuselage? or is it better off neglected?

 

[Phrak]

I thought that biplane has more wetted area due to the two wings and produce more drag. However, it is supposed to have half of the drag produced by the monoplane of the same wingspan, according to what i read in a book. Can anybody help me about with this? Which has the greater drag and why?

If this question was answered, I missed it.

We have to make a lot of simplifying assumptions. Assume the two airfoils don't interact. Each wing of the biplane is the same shape and aspect ratio of the monoplane. Neglect form drag and drag from external structure, and on and on.

Using the oversimplified model of lift and drag (no low-drag pocket) each wing of the biplane will carry half the load, the lift is linear with angle of attack and the induced drag increases as the square of the angle of attack. [I think this also assumes a symmetrical wing.]

The induced drag of each biplane wing is 1/4 the induced drag of the monoplane wing. The two, combined, have 1/2 the induced drag of the monoplane wing.