Making the B-35 / B-49 Stable

[Madoc]

Folks,

What would it take to have made Northrop's B-35 / B-49 stable enough for operational service?

According to the test pilots who flew those planes, General Bob Cardenas being chief among them, those flying wings were "dynamically unstable" around all axis.

What would it have taken to have solved that - with 1940's technology and not today's advanced fly-by-wire avionics?

A tail? Winglets? Actual vertical stabilizers?

 

[Kryptid]

Dr. Daniel P. Raymer had this to say of the YB-49:

Flying wing fanatics are still arguing as to whether the several crashes of Northrop flying wings, including the YB-49, were caused by pitch instability or by more mundane causes such as structural or hydraulic system failures. Whatever the cause, the flight performance of the YB-49 was outstanding for its day. The only technical difficulty this author is aware of, a tendency to "hunt" in yaw making for a poor bombing platform, could have been solved a few years later with the active yaw damper developed for the B-47.

 

[Stargazer]

B-35/B-49, like the V-22, is a reminder that what works fine on small-scale flying articles does not necessarily translate so well or so easily on large-scale ones.

One thing that has puzzled me for a while is the fact that the N-9M sub-scale prototypes, like many Horten or Blohm und Voss designs, had drooping wing tips. Why was this configuration not adopted on the B-35/B-49, and if adopted, would it have changed anything to the aircraft's behavior in flight?

 

[Madoc]

So, droop the wingtips and stick a big honkin' rudder on the thing, right?

 

[sferrin]

It should be pointed out that Dr. Raymer was an actual aircraft designer.

 

[Sea Skimmer]

I recall reading somewhere that the B-35 was stable mainly thanks to its propellers, and that the loss of this stability is why the B-49 had small vertical fins added,. In the end it didn't matter, by the time the B-49 flew the far faster B-47 was only a month away from flying itself. Switching to jet power took away most of the range advantage of the flying wing over conventional designs, and bigger fins would only further reduce range.

 

[Sundog]

Based on all of my reading the only axis where the YB-49 truly had a stability problem was in yaw. It had a tendency to "wander." It isn't so much that it was unstable, it's that it wasn't stable enough. It was actually O.K. for just flying, the problem was when it came to dropping bombs, it's hard to be accurate if the aircraft won't track properly. Now I don't know if this was just the yaw, or if they were referring to dutch-roll stability, which is the relationship between yaw and roll. I lean toward the latter, as it's one of the most basic stability relationships of any aircraft and would explain the wandering.

Also, the problem in pitch, once again, isn't that it's unstable, it's that it is short coupled.

However, my understanding is that the YRB-49 had a flight control system computer designed by Sperry to fix all of these problems, but it was too late.

 

[Madoc]

Thanks for the replies folks.


The reason I'm curious about this is that I've recently picked up a second B-35 kit and want to do it up as an operational bird. Hence the interest in learning what changes would've been required to make it so.


I know that the Northrop bird, in its XB/YB-35 form really "wasn't ready for prime time." Those planes were too much the prototype and not enough the "operational type." Lotsa stuff goes into that. One thing I'm minded of was Bob Cardenas' report of flying the plane in which he commented on, after having taken off, of having to wait in making any maneuvers for the fuel in the tanks to stop sloshing back and forth. This, because Northrop hadn't thought to install the standard anti-sloshing baffles in the plane's fuel tanks. He also had nothing favorable to say about the co-pilot's position. The view from it was awful when compared to the pilots and there was no real way to coordinate the crew's actions due to the physical separation between the pilot and co-pilot.


What I've in mind there is perhaps a longer bubble canopy that is more reminiscent of a B-47's with the pilot and co-pilot in trail. That and put a nice big vertical stabilizer on the thing. Yeah, Jack Northrop would've had a cow over that as he was obsessed with the purity of form for his flying wings. Hence the "billows" flight controls on the XP-56. Those things didn't work worth a damn but they were more "elegant" in Jack's view.


Not too sure about drooping the wingtips though.


As to the B-49, one big thing working against it's being an efficient jet powered bomber was its wing form. Simply put, the airfoil section of that wing was simply too thick for the speeds it operated at as a jet. That meant excess drag. And that, coupled with the relative inefficiency of the early jet engines, cut the range of the thing to ribbons. At least so when compared to the prop version. As I recall however, the B-49 got a trophy for being the longest ranged jet engined bomber. This being in '47 or '48.

 

[Scott Kenny]

Thanks for the replies folks.


The reason I'm curious about this is that I've recently picked up a second B-35 kit and want to do it up as an operational bird. Hence the interest in learning what changes would've been required to make it so.


I know that the Northrop bird, in its XB/YB-35 form really "wasn't ready for prime time." Those planes were too much the prototype and not enough the "operational type." Lotsa stuff goes into that. One thing I'm minded of was Bob Cardenas' report of flying the plane in which he commented on, after having taken off, of having to wait in making any maneuvers for the fuel in the tanks to stop sloshing back and forth. This, because Northrop hadn't thought to install the standard anti-sloshing baffles in the plane's fuel tanks. He also had nothing favorable to say about the co-pilot's position. The view from it was awful when compared to the pilots and there was no real way to coordinate the crew's actions due to the physical separation between the pilot and co-pilot.


What I've in mind there is perhaps a longer bubble canopy that is more reminiscent of a B-47's with the pilot and co-pilot in trail. That and put a nice big vertical stabilizer on the thing. Yeah, Jack Northrop would've had a cow over that as he was obsessed with the purity of form for his flying wings. Hence the "billows" flight controls on the XP-56. Those things didn't work worth a damn but they were more "elegant" in Jack's view.


Not too sure about drooping the wingtips though.


As to the B-49, one big thing working against it's being an efficient jet powered bomber was its wing form. Simply put, the airfoil section of that wing was simply too thick for the speeds it operated at as a jet. That meant excess drag. And that, coupled with the relative inefficiency of the early jet engines, cut the range of the thing to ribbons. At least so when compared to the prop version. As I recall however, the B-49 got a trophy for being the longest ranged jet engined bomber. This being in '47 or '48.

Pardon a very old reply, but the short answer would be engines, props, and extension shafts that didn't have vibration problems. Maybe 3500hp turboshafts?

The B-35s were stable and had the range. The B-49s had heavy bomber load but medium bomber range due to how thirsty the early jets were.

 

[riggerrob]

… having to wait in making any maneuvers for the fuel in the tanks to stop sloshing back and forth. This, because Northrop hadn't thought to install the standard anti-sloshing baffles in the plane's fuel tanks. …

On smaller airplanes, anti-slosh baffles serve dual purpose as wing ribs with lightening holes.

Not too sure about drooping the wingtips though.

I would have increased the wing sweep angle, installed the center-of-gravity forward of the center-or-lift and installed fins on the wing-tips. Increased sweep would lengthen the tail moment arm to improve pitch and yaw stability.

As to the B-49, one big thing working against it's being an efficient jet powered bomber was its wing form. Simply put, the airfoil section of that wing was simply too thick for the speeds it operated at as a jet. That meant excess drag. And that, coupled with the relative inefficiency of the early jet engines, cut the range of the thing to ribbons. ….

Perhaps they needed deep wings to hold all the volume fuel burned by thirsty early jet engines.