Theoretical Ultimate Biplane

In My Humble Opinion
1

One thing that bothered me in aircraft development is the sudden jump from planes that would not look out of place in WWI to the BF-109 and similar fighters. A sudden switch from rotary-engine, fixed landing gear, open-cockpit, strut-braced wing, fabric-covered frame biplanes to inline (or V) engine, retractable landing gear, enclosed cockpit, cantilever-wing, monocoque-skin monoplanes. Why were there never any biplanes that tried to incorporate all these features, especially since many of the last biplanes experimented with one or two of these design elements?

I’d like to perform a thought experiment to design the ultimate biplane of 1936. Why 1936? Because the BF-109 starts flying in Spain that year, and I’d like to see if a biplane could have possibly challenged it.

Now, we know that the later BF-109s and Spitfires and Yak-9s will completely clean the clocks of any biplane, so this is not a “Biplane that can win WWII.” This is a “Biplane that was better than any biplane that really existed, and maybe challenge the BF-109A in Spain.” What would it look like if someone had really held onto the idea of two wings, but accepted every other aeronautical upgrade?

What sort of engine would this biplane use? How heavy would its armament be? How fast could it expect to go (and can we beat the 323mph of the CR.42db), and how would it perform in a dogfight vs the early BF-109s?

Can we beat the Polikarpov I-153, the Gloster Gladiator, the Avia B-534, and the CR.42 Falco? And could we beat the BF-109 of 1936, or at least give it a run for its money?

2

This is kind of a difficult theoretical to answer as we have the benefit of hindsight and a specific target to reach toward. But I’ll give it a whirl.
Engine: Rolls Royce “R” Rolls-Royce R - Wikipedia attached to a three bladed Rotol variable pitch prop.
Armament: 4 Browning M2 .50 cal 350 rds ea.
Full monocoque aluminum body, retractable gear sesquiplane. Bubble canopy.

This thing would be about the same size as a Typhoon but the 2800 Hp and big guns along with the low wing loading of the sesquiplane design (like the CR.42) would make it an interesting fight.

All of the technology would have been available but convincing the Air Staff to fund and build this beast would have been your biggest challenge.

3

Part of the bi-plane design was to make the wings strong enough, the wings were also truss structures which are incredibly strong, though it did have disadvantages, it did quite a bit of lift, but it did have lots of drag inefficiencies, so it was sacrificing speed for lift and strength. It was not just the struts between the wings that contributed to the drag, but the 2 wings themselves.

To go faster you needed to reduce drag (among other things), so in that eliminate the exposed landing gear, streamline (enclose) the cockpit and ditch the second (or 3rd) wing(s), though you needed to be able to make a wing strong enough to be able to do this.

So they go hand in hand, single wing, enclosed cockpit, retractable gear = faster plane. With a bi-wing it really didn’t matter that much that it had exposed landing gear or open cockpit as the wing structure caused a large amount of drag that the landing gear was not all that much by comparison.

So there was a divergence with speed vs maneuverability, but there was no reason to combine the designs as they were intended for 2 different tasks.

4

I’ve been considering the biplane interference drag equation. It seems like the biggest drag contributor (aside from all the struts and wires) is the distance between the two wings. I was considering inverse-gulling the lower wing, like the Levasseur PL.107/108.

5

An excellent point, but this is assuming the Air Staff would fund it and make at least a limited production run of this plane.

Why bubble canopy? I thought that “razorback” canopies with the side panels (like the P-40) were aerodynamically faster.

6

Ultimately biplanes had 2 wings because the early state of the art in wing design was pitifully aerodynamically efficient. So it took a comparatively vast wing area to generate enough lift at the very low speeds the weak engines could push those very inefficient wings. Lots of vicious circles at work there.

Meantime, the structural strength of the wooden structures they knew how to build light enough would not support a long enough span to give the wing area needed to generate the needed lift.

Applying all this to roughly fighter-sized aircraft …

Said more simply, a biplane was a way to fold an e.g. 50 foot wingspan into two 25-foot wingspans that were strong and light enough to not fall apart under load while also generating 50 span-feet worth of lift.

They did this at the expense of needing a lot of external bracing, struts, etc., all of which created drag.

Once by the early 1930s we had learned to design more efficient wings that could carry the same weight with, e.g. a 35 foot span and were also strong enough to hold together when that long, the whole reason for 2 wings evaporated. Which reduced drag which increased speed which improved unit lift per wing area, which … in a very virtuous feedback loop.

The need for 2 wings was the logjam that prevented that virtuous feedback. Once it was broken the need quickly evaporated.

In general longer wings are better for cruising. cf. sailplanes and long-haul airliners. While stubby wings are good for maneuverability, both to enhance roll rate and the G-load they can sustain before failure. cf. F-16s, F-18, and earlier fighters as well.

Applied to interwar fighter tech, things like the e.g. Brewster Buffalo could already roll faster than the pilot could smoothly control. Unboosted controls were more of a limiting factor than wing length. And those aircraft could pull more Gs than the engine could sustain energy through over combat-meaningful time intervals or that the G-suitless pilot could maintain consciousness through.

Said another way, adding a second wing would fix a problem they didn’t really have.

If I were trying to design a 1936 ultimate fighter biplane I’d be aiming at two very short stubby wings, boosted flight controls & G-suits. That’d create maneuverability that would eat a Zero’s lunch. If you could stuff a big enough engine into it.


Today there are efforts afoot to build quasi-biplanes as airliners and as pilotless urban air mobility vehicles.

In the former case it’s about making much much longer sail-plane like wings for cruise efficiency that can’t support themselves without external struts. The difference today is we have the aerodynamic design chops to make the strut also a lifting wing, not pure drag as they would have been in 1935. This is one example of several such research projects: Boeing Truss-Braced Wing - Wikipedia

For the urban mobility mission it’s about making the vehicle footprint smaller so the landing & parking areas can be smaller.

Once you can do the CFD work, there are also aerodynamic and structural advantages to two wings that are connected to each other at the tips. Here’s one configuration but there are many others being messed with now days. A Physics-Based Approach to Urban Air Mobility | Drone Below

Some attempts in these directions were made in the 1930s. But the materials science, structural science, and aerodynamic science just weren’t there yet.


I have a minor quibble about your premise. you overstate a bunch how quickly radial engines went out of fashion. Your other design points are well-founded, but radials were still being specified in new fighter designs as late as the middle of WWII. e.g. https://en.wikipedia.org/wiki/Grumman_F8F_Bearcat was a vague notion in 1942, serious design was late 1943, and production stated in 1944.

Had jet engines been 10 years farther into the future than they really turned out to be, I doubt the Bearcat would have been the very last piston fighter and may not even have been the last radial-engined fighter.

To be sure, 1944 is only a few years after your target date of 1936. But it internet years’-worth of aero-progress was made in that short time.

7

Also, I’ll add another quibble the OP’s premise: metal skins didn’t happen completely simultaneously with other advances, either. For instance, the most numerous British fighter in the Battle of Britain was fabric-skinned (Hawker Hurricane - Wikipedia).

8

An excellent point! I know advanced radials came back in vogue, but for a while at the end of the 30s, it seems like every advanced engine was an in-line or V engine. Is there a better radial in 1936 than the DB 600 series? I’d love to hear about it.

Another excellent point, but the Hurricane was out for less than a year before the BF-109 came out, and most people think the 109 was a better fighter plane on average. I suppose my question would then be, do you think the TUB (Theoretical Ultimate Biplane) would be fabric-skinned instead of monocoque?

9

Great post by @LSLGuy. Only thing I’d add is that in addition to airframe improvements there were huge advances in aero-engine technology in the 1930s.
In 1930, the RAF’s frontline fighter was the Bristol Bulldog (introduced 1929), with a 440hp engine. This was replaced in turn by the Gauntlet (1935, 645hp), Gladiator (1937, 830hp) and Hurricane (1938, 1030hp) - a >100% increase in power in less than 10 years.
As fighter speeds went from less than 200mph to over 300, drag became much more critical, hence the move to monocoque construction, enclosed cockpits, retracting undercarriage etc. I suspect that if engine power had stayed around 5-600hp, biplanes would still have been viable.

10

The current TUB’s are the sport racing bipes. http://www.biplaneracing.org.

They’re fabric. And generally built for pretty high G fighter-like maneuverability. They’re too small to carry meaningful weapons loads, but they could certainly be scaled up to do so. Subject again to that pesky area/volume = square/cube law that makes a mess out of so much aero design.

11

I’ve seen those! The Phantom in particular has the sort of wing plan I was considering. Well, that combined with a Beechcraft Staggerwing, where the top wing connects with the top of the cockpit to reduce interference drag between wing and fuselage. Give it a transparent wing panel, and visibility would not suffer too much. Beechcraft Model 17 Staggerwing - Wikipedia

However, all the racing biplanes have tiny engines, 125-150 horsepower or so. My guess is the TUB would have something closer to/just over 1000 HP, like the R engine [swampspruce] suggested.

It’s looking like fabric-skin might be best for 1936.

12

Not necessarily. The Northrup Gamma was all metal monocoque and there’s no reason to assume that you couldn’t do the same with our biplane. I thought about it a bit more and I would add two more concepts that would make this thing completely over the top.
I’ll address the bubble canopy first. Visibilty. One of the big drawbacks to the early Mustangs/Thunderbolts were rear visibility. This is minimized with a bubble at the expense of aero.

OK, here’s the wiggy bits; Our TUB is a pusher prop and the guns are located in the nose a la the P-38. Two reasons for this,1) The R engine is big, 37 liters big and 1640 lbs. We plonk that toward the back and counterbalance it with the guns up front. We lose some efficiency but we have 2800 hp at our disposal and it makes smoothing out the fuselage easier. It also gives you the added benefit of now being able to use tricycle gear and better frontal vis.

Technically, we could stagger the wings in such a fashion that the upper is much smaller (think a big ass canard) and the lower wing is the main lifting surface. Think of a larger K7W Kyushu with an upper wing.

13

Just about any biplane was more maneuverable than any monoplane, given that the early biplanes had weak wings that limited diving speeds. The Russians and the Italians took the biplane about as far as it could go, but was obvious that biplanes were coming up against speed limitations.

However, if you insist on building such a beast, and counting a sesquiplane as a biplane; monocoque body and aluminum wings, the wings being as far apart as possible and with no bracing or struts, retractable undercarriage, and a big engine. My guess is that even in the best case you would be lucky to achieve 400 mph.

The other way to look at it is to have two wings, but one behind the other instead of one of top of the other. This has been mooted, but I don’t know how effective it is.

14

“… do you think the TUB (Theoretical Ultimate Biplane) would be fabric-skinned instead of monocoque?”
No. The Hurricane gained quite a bit of speed from metal wings instead of fabric.

As for Hurricane vs Bf109, it depends on which model. Pre-Bf109E, the Hurricane is the winner, even with a two-blade prop and fabric wings. In the BoB they were fairty evenly matched, but it depended on the altitude and the pilot.

15

The slip-wing Hurricane may have been the TUB:

A ground up design using the concept could have produced a long range fighter (upper wing fuel tank) that jettisoned the upper wing in order to engage in combat. An empty upper wing would have provided a high rate of climb for interception and been jettisoned at altitude for combat.

Such an aircraft would have been an effective bomber escort early in the war. Monster fighters like the P-47 with drop tanks were a better solution, but they used technology that was not available in 1936.

16

I’m not sure what this is doing in CS? IMHO is probably a better fit.

17

I’ll add two more points to this thread that I think are germaine. 1) The Hurricane was designed as a hybrid build as an evolution of Sydney Camm’s earlier biplane designs. Sticking with a proven design is pretty typical for most aircraft manufacturers and the RAF at the time. You can read more about it here:
Hawker Hurricane The metal wing was needed to accommodate the retractable gear and the guns as noted in the article.
2) What we have been discussing would have been bleeding edge technology in 1936 and as LSLGuy notes most of the advantages of a biplane are limited by the increased drag. I still think a biplane could have been built that would have been able to take on a Bf109A, but like most of these things it also comes down to how well the pilot can utilize the airframe.

18

Does anybody know the hard performance numbers of the Bf109A? I can find info on the E-series easily enough, but aside from the engine horsepower, I don’t have much info on the 109As and Bs in Spain. Did they have 3 guns or just 2, and how fast could those Jumo 210 engines propel the 109A?

19

Good info here: Messerschmitt Bf 109 (Me 109) - History and Pictures of German WW2 Fighter Plane

20

A few points:

Radial engines stuck around long after WWII. The F4U Corsair saw combat in Korea, even shooting down MiGs, and the Douglas Skyraider wasn’t even in service until 1946, stayed in production until 1957, and stayed in the U.S. arsenal throughout the Vietnam war. The last country to use it retired it in 1985

A modern biplane could be efficient if the front wing acted like a canard, eliminating the need for an elevator. So, a staggered wing, fully cantilevered biplane/canard could possibly work.

The Rutan Quickie is such a plane, and it could cruise at 125 mph on 18 hp. A bigger 2-seat version could do 165 mph on 100 hp.

The problem, as others have mentioned, is that there’s no real reason to do this for a fighter.