Biplane versus Monoplane

Hello All,
I hope this is the correct place to post this series of questions.
If not, then please move it to the proper area.

When working with a Biplane, certain characteristics such as Wing Area are fairly comparable to a Monoplane.
What about Wing Span, Aspect Ratio and Mean Aerodynamic Chord?
What I am looking for is some basic translations between Biplane characteristics and their Monoplane equivalents.

I have already found Max Munk's series of NACA reports but the math is at a level that I have not touched since my college days.

Thanks.
- Ivan.

Hey Ivan1GFP,

Is your query as to where the monoplane becomes more efficient than the biplane, and vice-versa? and why this transition occurs?

Ivan1GFP said:

Hello All,
I hope this is the correct place to post this series of questions.
If not, then please move it to the proper area.

When working with a Biplane, certain characteristics such as Wing Area are fairly comparable to a Monoplane.
What about Wing Span, Aspect Ratio and Mean Aerodynamic Chord?
What I am looking for is some basic translations between Biplane characteristics and their Monoplane equivalents.

I have already found Max Munk's series of NACA reports but the math is at a level that I have not touched since my college days.

Thanks.
- Ivan.

Click to expand...

Maybe my mind is getting tired, but I can't quite get what you are asking.

Biplane wing spans, in general are going to be shorter/smaller because with two wings you can get a lot of wing area in a short span.
Gloster Gladiator had 323 sq ft with a 32'3" span. but the aspect ratio and Mean Aerodynamic Chord seem to be pretty normal. Two 161 sq ft wings?

other biplanes used different sized wings but the aspect ratio and plane form aren't going to be very far from "ordinary" most of the time.

The Microsoft Flight sim did not model biplanes directly in their aerodynamic equations. I did a few biplanes using mostly an iterative approach, adjusting the flight characteristics till the closest approach was made to observed results.

Generally lift for a given airfoil shape was higher due to acceleration of flow between the wings, induced drag was higher because of the low aspect ratio and drag higher from all the hardware needed to support two wings. Strength and roll rates can be quite good!

When sufficient construction materials and design was available, monoplanes had better speed, giving the tactical advantage of engaging and disengaging.

fliger747 said:

The Microsoft Flight sim did not model biplanes directly in their aerodynamic equations. I did a few biplanes using mostly an iterative approach, adjusting the flight characteristics till the closest approach was made to observed results.

Generally lift for a given airfoil shape was higher due to acceleration of flow between the wings, induced drag was higher because of the low aspect ratio and drag higher from all the hardware needed to support two wings. Strength and roll rates can be quite good!

When sufficient construction materials and design was available, monoplanes had better speed, giving the tactical advantage of engaging and disengaging.

Click to expand...

Of course, if you were the VVS, and wanted fighters to escort your ground attack aircraft and bombers then biplanes such as the I-153 are okay as they are there just to drive the enemy away.

Aren't the aerodynamics of bi planes different anyway because the two wings interfere with each other?

Shortround6 said:

Maybe my mind is getting tired, but I can't quite get what you are asking.

Biplane wing spans, in general are going to be shorter/smaller because with two wings you can get a lot of wing area in a short span.
Gloster Gladiator had 323 sq ft with a 32'3" span. but the aspect ratio and Mean Aerodynamic Chord seem to be pretty normal. Two 161 sq ft wings?

other biplanes used different sized wings but the aspect ratio and plane form aren't going to be very far from "ordinary" most of the time.

Click to expand...

Hello Shortround6,
I thought I had posted a reply but apparently it got lost somewhere.
In your example of the Gloster Gladiator with 323 square feet of wing area over two wings but a total wingspan of 32 ft 3 in, how would one calculate the effective aspect ratio? Would it be with a 32.25 foot wingspan or twice that? Or would it more likely be something in between? How would that apply to the other factors and how does actual geometry get incorporated into the formulae?

fliger747 said:

The Microsoft Flight sim did not model biplanes directly in their aerodynamic equations. I did a few biplanes using mostly an iterative approach, adjusting the flight characteristics till the closest approach was made to observed results.

Generally lift for a given airfoil shape was higher due to acceleration of flow between the wings, induced drag was higher because of the low aspect ratio and drag higher from all the hardware needed to support two wings. Strength and roll rates can be quite good!

Click to expand...

Hello Fliger747,
Your idea would work if the flight characteristics and handling were well known beforehand, and I have used that method when there was no other information to go on, but in this case, I am trying predict differences in performance based on physical dimensions.

You comment that Lift would be higher, but other reading suggests the opposite to be the case because of interference which is lessened as the inter plane spacing is increased.
Your comment about low aspect ratio is also interesting. Are you simply taking the area of all the wings and dividing by the span of the longest wing to determine chord?

Thanks.
- Ivan.

The chord of the wing has nothing to do with the span really. The chord is the width of the wing. The upper wing in normal maneuvering is somewhat in "ground effect" with reduced induced drag. The flow does accelerate between the wings. This effect is well known in sail boats that use a large fore sail known as a Genoa or a "Jenny" where the flow acceleration between the Jenny and the main sail provides much greater propulsive power (lift). Perhaps one might make a first order approximation via taking the sum of the wing properties and applying some "fudge factor" for the proximity of the upper and lower wings.

One might look into the example of the development of the F4F which began it's development concept as a biplane.

I think that there are too many variables to arrive at a simple mathematical model.

Good luck!

I believe the aspect ratio of the wing would be calculated the same.
A Gloster Gladiator would have wings of about 6.4-6.5 aspect ratio. Aspect ratio helps determine things like spanwise airflow.

Unless the wings are close enough together that the airflow from one affects the airflow of the other Then the practical difference is going to be very small.

The Gladiator is easy because the wing are of equal (or nearly equal) size and shape and don't have much stagger.

When you get into sesquiplanes (lower wing is much smaller) things may change. I don't know.


ANd you had Biplanes with both forward and negative stagger.

Shortround6 said:

I believe the aspect ratio of the wing would be calculated the same.
A Gloster Gladiator would have wings of about 6.4-6.5 aspect ratio. Aspect ratio helps determine things like spanwise airflow.

Unless the wings are close enough together that the airflow from one affects the airflow of the other Then the practical difference is going to be very small.

The Gladiator is easy because the wing are of equal (or nearly equal) size and shape and don't have much stagger.

When you get into sesquiplanes (lower wing is much smaller) things may change. I don't know.
View attachment 534909

ANd you had Biplanes with both forward and negative stagger.
View attachment 534910

Click to expand...

I've never seen a SWB on floats before, nice find!

fliger747 said:

The chord of the wing has nothing to do with the span really. The chord is the width of the wing. The upper wing in normal maneuvering is somewhat in "ground effect" with reduced induced drag. The flow does accelerate between the wings. This effect is well known in sail boats that use a large fore sail known as a Genoa or a "Jenny" where the flow acceleration between the Jenny and the main sail provides much greater propulsive power (lift). Perhaps one might make a first order approximation via taking the sum of the wing properties and applying some "fudge factor" for the proximity of the upper and lower wings.

Click to expand...

Hello Fliger747,
Unless there is something I am seriously misunderstanding, you mentioned low aspect ratio earlier. If the Wing Span is known and the Wing Area is known, then the Chord would be the result of simple division, therefore the equivalent would be a long chord wing?
As I understand "Ground Effect", it is a result of air being trapped between an aircraft wing at low altitude and the ground which results in higher than ambient air pressure. The extra pressure works as additional lift to the wing but also creates and equal and opposite force on the ground. In this case, the Lower Wing is the "Ground" and the extra lift would be counteracted by reduced lift on the lower wing.
The Genoa sail sounds very much like the equivalent of a leading edge slat on an aircraft wing. I wonder if its area is as efficient as that of the main sail.

fliger747 said:

I think that there are too many variables to arrive at a simple mathematical model.

Click to expand...

I have already tried to read a few of Max Munk's NACA Reports from the 1920's. I find that I am getting lost in the math very close to the beginning. I suspect my children would understand the reports better than I do but they are busy with school and the last time I worked with this kind of math was when I was their age.

Shortround6 said:

I believe the aspect ratio of the wing would be calculated the same.
A Gloster Gladiator would have wings of about 6.4-6.5 aspect ratio. Aspect ratio helps determine things like spanwise airflow.

Unless the wings are close enough together that the airflow from one affects the airflow of the other Then the practical difference is going to be very small.

Click to expand...

Hello Shortround6,
From what little I have found in general "rules of thumb", 1.0 x Chord is considered a sufficiently wide gap and 0.7 is considered to be too narrow of a gap to be efficient, so for practical separation, there will be some interference and it will not be beneficial.
As you can imagine, the stagger is also going to affect how the flow of one wing affects the other.

Hello BiffF15,
There was a fellow named Cornelius Burmood who flew a couple of these float Staggerwings in China back in the 1930's in support of the Kuomintang against the Japanese. I am not sure what became of him.

- Ivan.

Biplanes have much more wing area per unit of span than monoplanes as a general rule. I have not ever looked into biplane aerodynamics because I originally started studying aero in the late 1960s as a 1st-year aero engineering student. I transferred to electrical when they finished the F-111 and laid off all the aeros.

I can tell you this from radio control models. If you fly a well set-up bipe, you will always have one in your stable of planes. They are too much fun. I have the good fortune to also be a full-scale pilot and biplane are WAY fun in real life as well as RC.

It would be interesting to see what a modern biplane design could be made to do. The Turbine Toucan comes to mind as a possible example of the breed:


Perhaps it isn't really modern since I have no idea what the airfoils are or what the design goal was or anything else about the design. But ... it certainly doesn't lack for power.

Glauert's "THE ELEMENTS OF AEROFOIL AND AIRSCREW THEORY" has a chapter on biplanes. He has plenty of maths, but also gives some typical values to correct aerofoil behaviour.

Interestingly enough, he was brash enough to state in his R&M 910 regarding thin aerofoils: "A treatment of this problem has been given by Munk in Report No. 142 of the National Advisory Committee for Aeronautics, but the analysis is not quite free from errors."
They were both brilliant in their field, so I am a little surprised that this barb was published.