Bowlus "flying fuel tank" - 1941

[Schneiderman]

A brief article in the October 1941 issue of Aeroplane mentions a proposal put forward by the US sailplane designer William Hawley Bowlus for a glider loaded with 500+ gallons of fuel to be towed into the air by a bomber and the fuel transferred to the bomber once in the air, basically a 'flying fuel tank'. The British Air Commission are reported to have requested specifications. There is a very brief description of the system in the article but does anyone know whether any drawings of details have survived?

 

[jstar]

Nothing to offer on the Bowlus, sorry. Including this in case you may not have seen it. Very similar concept, I imagine:

https://www.airforcemag.com/PDF/MagazineArchive/Magazine%20Documents%2F1996%2FJune%201996%2F0696flashback.pdf

 

[hesham]

Maybe it was based on CG-7 ?.

 

[riggerrob]

Hawleny Bowlus designed a wide variety of sailplanes. He also started the whole aluminum, Airstream trailer line because he needed a place to sleep while testing gliders in the Mojave Desert.

 

[Schneiderman]

Maybe it was based on CG-7 ?.

Doubtful. 30 troops occupy a far larger volume than 500gals of fuel and weigh quite a bit more too.

 

[iverson]

I suspect that nothing came of these fuel glider ideas because the glider increased wing area (and drag) but not power. The tow plane still had to get the load off the ground and keep it in the air. So the engines had to operate at higher power, consuming more fuel and reducing engine life.

Compare the above scenario with aerial refueling: the airplane takes off light, climbs to altitude, takes on the extra fuel, and flies on. Airframe drag is no greater. The engines may have to put out a little more power to deal with the extra weight. But they do not have to get the load off the ground.

 

[Schneiderman]

I suspect that nothing came of these fuel glider ideas because the glider increased wing area (and drag) but not power. The tow plane still had to get the load off the ground and keep it in the air. So the engines had to operate at higher power, consuming more fuel and reducing engine life.

Compare the above scenario with aerial refueling: the airplane takes off light, climbs to altitude, takes on the extra fuel, and flies on. Airframe drag is no greater. The engines may have to put out a little more power to deal with the extra weight. But they do not have to get the load off the ground.

Absolutely, the idea is fundamentally flawed

 

[riggerrob]

George Wilbur Cornelius formed Cornelius Corp. in 1930.
His first forward-swept airplane was the PW-1 Fre-Wing, two-seater, parasol monoplane (registered NX-182w) with unique lateral control via wings that pivoted in pitch. Initially it used servo tabs - behind the wings' trailing edge - but those were later eliminated.
Cornelius second Fre-Wing was a two-seater, low-wing, cantilever monoplane.

His third forward-swept wing was the Cornelius Mallard, a two-seater, powered plane (registered NX-34212) which resembled a Fauvel AV-10, AV-14 or Av-41, albeit with leading edges swept forward slightly and major forward sweep on trailing edges. Mallard was too heavy and under-powered to be practical.

Then they applied forward-swept wings to the Cornelius ZFG-1 fuel glider (tail number 428049).

NACA wind tunnel tests revealed stability problems when a sub-scale model XFG-1 "flew" faster than 68 mph. It flew fine at low airspeeds - with spoilers deployed - but ossccillated in pitch and yaw at faster speeds. I doubt if a towing bomber could climb when the fuel glider extended spoilers.

It was not until the 1970s that Jim Marske tamed stability and control on gliders with forward swept wings. His Pioneer prototype first flew in 1968.

The primary reason for sweeping wings on tail-less gliders is to increase the distance (moment arm) between the center of lift and pitch control surfaces. Those surfaces are called elevators (forward swept) or elevons (aft swept). Forward swept wings have the advantage of near conventional controls because their ailerons are near the aerodynamic center with elevators at the extreme rear.
All swept wings suffer problems with span-wise airflow. MiG solved the problem by installing fences on their MiG-15 jet fighter.

Forward swept wings suffer from turbulence migrating aft and inwards along the wing, concentrating stalled, turbulent airflow on the center tail ... where elevators are positioned. In the worst case scenario, the farthest aft part of the wing stalls, pitching the nose up. This can create un-recoverable stalls and spins.
Aft swept airplanes have suffered similar stalled, pitching problems. Both Curtiss Ascender and Velocity canards installed wing fences - on their aft-swept main wings to limit spanwise flow. Velocity was more successful.

Finally, forward-swept wings suffer structural problems with divergence. As lift tries to raise forward-swept wing tips, the wing structure tries to twist leading edge up. This twist only gets worse with increasing airspeeds. The simplest solution is building stronger, stiffer wings, but those stiffer wings are heavier. Jim Marske invented carbon pull-trusions to stiffen the wings in his later sailplanes.

 

[Justo Miranda]

Cornelius XFG-1

 

[dan_inbox]

Compare the above scenario with aerial refueling

Yes, a towed glider is less efficient than IFR, but IFR was not operational during WW2. So even with the drawbacks that you mention, a towed tank-glider could extend the range of a bomber or transport when that extra range is crucial. And you can release the glider when empty. (can think of it as a bomber-sized jettisonable drop tank).
AFAIK the nazis investigated the same idea (with bomb-schlepper and fuel-schlepper IIRC)

Here are 2 photos of the Cornelius XFG-1 proof-of-concept article:

 

[Schneiderman]

WH Bowlus formed Cornelius Corp. to bid on gov't contracts during WW2.

Are you sure? As far as I can see George Wilbur Cornelius established Cornelius Aircraft Corporation in 1931.

 

[Schneiderman]

Bowlus then applied forward-swept wings to the Cornelius ZFG-1 fuel glider.

Again, this does not appear to be likely. Cornelius filed a patent for forward-swept designs in 1942 with no mention of Bowlus. Patent US2417189
Images from the patent on this forum here

https://www.secretprojects.co.uk/threads/cornelius-fsw-powered-transport-and-unmanned-glider-aircraft.11416/

and here

https://www.secretprojects.co.uk/threads/cornelius-mallard.30226/

 

[Schneiderman]

....but IFR was not operational during WW2.

Very nearly though. Work was underway by Flight Refuelling Ltd to convert Lancasters for IFR, both as recipients and tankers, for use in the Pacific campaign.

 

[dan_inbox]

....but IFR was not operational during WW2.

Very nearly though. Work was underway by Flight Refuelling Ltd to convert Lancasters for IFR, both as recipients and tankers, for use in the Pacific campaign.

Hope you'll forgive me for taking exception with "very nearly". IMO it is rather in the eye of the beholder.
Operational isn't doing it with ONE airplane and a handpicked demo/sales crew. It is sending a few hundred bombers blast the bad guy.
While IFR was performed in 1929 (Curtiss Robin, etc), and while ONE B-24D refuelled ONE B-17E in 1943, the first "operational" KB-29M squadron was activated in June 1948. And that was with the loop-hose system, which was barely practicable.
The really operational systems, Probe-and-drogue and Flying-boom, were developed even later.

So yes, investigating jettisonable tanker-gliders in '44-45 was a reasonable endeavor. That Cornelius chose a forward-swept design suffering aerodynamic problems doomed that attempt, but exploring the glider option made some sense. YMMV.

 

[Dilandu]

Essentially the idea of towed fuel glider is the idea of drop tank generating some of its own lifting power. The traditional external drop tank generates only drag & additional weight for the plane to carry. The glider (which could be optimized for plane cruising speed) could at least generate its own lift, thus lessening the problem.

 

[Schneiderman]

....but IFR was not operational during WW2.

Very nearly though. Work was underway by Flight Refuelling Ltd to convert Lancasters for IFR, both as recipients and tankers, for use in the Pacific campaign.

Hope you'll forgive me for taking exception with "very nearly". IMO it is rather in the eye of the beholder.
Operational isn't doing it with ONE airplane and a handpicked demo/sales crew. It is sending a few hundred bombers blast the bad guy.
While IFR was performed in 1929 (Curtiss Robin, etc), and while ONE B-24D refuelled ONE B-17E in 1943, the first "operational" KB-29M squadron was activated in June 1948. And that was with the loop-hose system, which was barely practicable.
The really operational systems, Probe-and-drogue and Flying-boom, were developed even later.

So yes, investigating jettisonable tanker-gliders in '44-45 was a reasonable endeavor. That Cornelius chose a forward-swept design suffering aerodynamic problems doomed that attempt, but exploring the glider option made some sense. YMMV.

That's a bit harsh. True, the loop hose method was less elegant than their later drogue system but it had been tested many times and proven both safe and practical. FRL had received a contract to provide a full squadron of converted bombers and tankers, "Tiger Force", and had the work well in hand in 1944 when the rate of progress retaking islands in the Pacific made the project unnecessary.

 

[iverson]

Essentially the idea of towed fuel glider is the idea of drop tank generating some of its own lifting power. The traditional external drop tank generates only drag & additional weight for the plane to carry. The glider (which could be optimized for plane cruising speed) could at least generate its own lift, thus lessening the problem.

True, but only up to a point. A glider fuel tank could reduce the structural loads associated with the extra weight of underwing drop tanks. But it would not reduce the extra loads on the engines during takeoff and cruise. This was a problem even with relatively short-range tows of airborne-infantry gliders in WW2. Experiments with using gliders to augment the carrying capacity of transports were abandoned largely for this reason: the modest load/volume improvement was not worth the wear and tear on the engines. Glider towing also reduced aircraft performance, restricted the flight regime in which the tow plane could operate, increased fuel consumption, and caused handling problems. Experiments with the Cornelius fuel glider also showed that static buildup on the tow line could ignite fuel vapor in the hose during fuel transfer.

That said, in my opinion, history is the strongest argument against practicality of the fuel glider: the Allies and the Germans experimented with the idea (the US spent a quarter million dollars on it), it seems obviously beneficial, yet no one put it into practice. Troop-carrying gliders were used despite the problems mentioned above, but only as long as they remained the only way to land heavy equipment and cohesive infantry units in combat--a unique capability justified wearing out engines and tow planes. But once transports were large and powerful enough to reliably drop artillery and vehicles and once the helicopter was available, armies and air forces abandoned their transport gliders.

Flight Refueling's contemporaneous systems may have been less than ideal. But the idea was good enough to be adopted by the RAF during the war, retained post-war by all major air forces, and developed and improved continuously up until the present day.

 

[Dilandu]

That said, in my opinion, history is the strongest argument against practicality of the fuel glider:

Essentially yes, and I would not argue with it; I just clarified why this idea was so popular in WW2 and why efforts were made to test it.

 

[Schneiderman]

So.......back on track, it appears nobody has drawings or information on the Bowlus fuel glider proposal.
The same article mentions a related proposal, also Bowlus, for a glider on a short towline that would take-off at 30mph and then provide 3000lb of additional lift to get the towing bomber airborne. A somewhat riskier version of the slip-wing method considered by many, patented in the UK by Stieger in the late 1930s and tested by Hillson during the war.