Bomber Tests

[Buzzbait]

S!

I took a B17 fully loaded with fuel and bombs in the Combat Theater.  I picked the Combat Theater because I wanted realistic fuel burn and wanted to avoid being killed.  (I got killed once by an overzealous 109 who followed me all over Central England)

Anyway here is what I found:

Ceiling for B17 with bombs:  33,300 ft

Ceiling for B17, no bombs:   35,750 ft

The figure for without bombs is very close to the historical one listed for the B17G.

Now it is possible that I could have gotten higher with both ceilings if I had loaded less fuel.  Because of the historical fuel burn in the CT, I still had 50% fuel when I quit trying to climb.

As I mentioned in an earlier post, in the MA I chased a B17G, minus bombs, up to around 38,000 in the MA.  But that was after 45 minutes of flying after I sighted the aircraft, and with a higher fuel burn rate.  So it was likely that plane had less fuel on board.

I tried some maneuvers both with and without bombs.

I was able to do 180 degree rolls at maximum altitude with both load configurations.  There was not a lot of altitude loss during these rolls.

I was able to do hard bank turns, (wings at close to 90 degree angle) without losing anymore than 500 feet.  In fact I would usually pick up a little speed with the altitude drop and be able to regain my height with a zoom.

Less heavily banked wings lost no altitude.

I almost did a loop at 32,000 feet with the bombless B17, but stalled at the top.

These don't sound like maneuvers the B17 should be able to do up this high.

[SOB]

That's great...you've now got half the data you need.  Now hows about some data regarding why a B17 should not be able to do what you did?


SOB

[Nifty]

Originally posted by SOB:
That's great...you've now got half the data you need.  Now hows about some data regarding why a B17 should not be able to do what you did?
SOB

I landed a B-17 on a CV during a snapshot event once!  I think I have a screenshot at home of it.  However, I don't think there are data on B17 carrier landings from WWII to confirm or deny the possibility of that happening.  LOL

[MiG Eater]

Originally posted by SOB:
Now hows about some data regarding why a B17 should not be able to do what you did?

SOB

Well,the remaining 50% of the fuel will move to the aft end of the tanks.  The crew and equipment would fall into the aft part of the airplane.  Both of these conditions would place the CG aft and much closer to the center of lift making the aircraft dynamically unstable in pitch.  The chances are very high that even slight control inputs would produce uncrontrollable oscillations in pitch, making a smooth climb in the first half of a loop nearly impossible.  When the airplane stalls with the CG far aft, recovery is extremely difficult and often impossible.  This is the primary reason P-51 pilots burned off the aux fuel located behind the pilot's seat before entering a maneuvering fight (even at low altitudes with the benefit of nice thick air to work against the control surfaces).  They tended to fall into uncontrollable accelerated stalls/spins, well above normal stall speed, with the aft-CG condition.  

The aft CG instability is basic physics that applies to every heavier-than-air airplane with a cruciform wing layout. (- some aircraft, like the modern fighters, use the triple and quadruple redundant computers to interpret the pilot's commands into effective control surface movements where the a/c's design features a CG that is on or aft of the CP)

The question is whether or not the fuel (or any other weight) moves forward and aft in the AH planes. If not, our aircraft are capable of maneuvers that would limit the actual airplane that the data was collected from.  

MiG

[ 09-21-2001: Message edited by: MiG Eater ]

[funkedup]

Good job Buzz.  How did you determine ceiling?  Service ceiling in the books is 100 fpm.

[Creamo]

The aft CG instability is basic physics that applies to every heavier-than-air airplane with a cruciform wing layout.

Well yeah, of course.  

You asked for it SOB. I hope the myriad of charts sure to come clogs your crappy cable modem to lock up.

And get those crew members some seat belts before they fly out the tail gunner position.

[funkedup]

I'll ask some of the B-17 pilots at the WB Con about this supposed instability in the B-17 with a small fuel load.

[hitech]

Why would the fuel or any thing else move aft?

HiTech

[hazed-]

even at the top of a 90degree vertical stall the men wouldnt be thrown to the bottom   lol
that only happens in cartoons....hey maybe ill try stepping off my plane as it hits the floor  

all inside b17 are at the same state of G or CFF.

hitech please tell us if the b17 should be able to do the manouvers it does.I seem to remember quite a while back that someone explained that in the very thin air at 30K+ the fact that the B17 has such a large wingsurface it would be able to manouver quite well.Have i remembered that right?

[Buzzbait]

S!

Just realized I should have said I did 360 degree rolls, not 180.

As far as ceiling is concerned, I let the aircraft climb on autopilot till it wouldn't go any higher.  Then I tried to climb it higher manually.

[hitech]

hazed: Other than power do to turbo chargers like the p38 and p47 there nothing special about the b17. A bigger wing does not effect hi alt perferformance any different than low alt performance all it does is change the stall speed. I don't rember the numbers exactly but even a 10mph differance in stall speed at that alt would make a very noticable difference in turning performance do to the very low air speeds at that alt.

I havn't done the math but it is very posible a b17 could out turn some of the fighters at hi alts once it was low and gas and was out of bombs. All it would take to out turn the fighters is for the b17 in that condition to have a lower stall speed than the fighters.

At lower alt's the b17 becomes very limited in turn performance do to stick forces and air frame g limiting forces. At hi alts your talking in the 2 g range max force on most planes do to low indicated air speed.

One other missunderstanding is how the rudders work when in the gun positions. They are no longer controling rudder but rather control the bank angle of the auto pilot. The normal auto stuff then controls elev and rudder to make a turn.

This might be the biggest factor in people beliving it performs better than normal because at that alt you just do not have the air speed to perform hi g turns. And any loss of alt or speed is extreamly hard to recover.


HiTech

[streakeagle]

High altitude performance depends primarily on the interaction of two parameters: Stall speed and specific excess power (sep).

Service ceiling is where the minimum speed required to generate 1 g of lift (stall speed) curve meets the maximum level speed curve with an sep of 100 ft/minute to spare.

Even with its lower stall speed, if the B-17 is near its service ceiling, any extra drag induced by increasing its g-loading to maneuver will cause it to either lose altitude or stall. A fighter that still has another 5 to 10,000 feet before it reaches its ceiling will be able to hold altitude and turn, or turn tighter while losing altitude. I would never expect an aircraft within 10% of its service ceiling to outperform in any way an aircraft that is only at 75% to 80% of its service ceiling. Any advantages due to wingloading, aspect ratio, and power are already reflected in the ceiling, so I would expect a slower B-17 at 30,000 feet or higher to turn in very wide circles compared to faster fighters with ceilings of 40,000 feet. I think history may even provide photos of contrails illustrating this fact. Anybody care to search for pics of B-17s trying to turn around while being intercepted at high altitude?

With regard to loops:
In general large aircraft do not have sufficient power-to-weight ratios to complete a loop before stalling since they cannot withstand the g-forces required to get over the top of the loop before they run out of speed. 1 g of deceleration (or a significant fraction thereof) slows down an airplane very quickly. Without excess power to compensate, the aircraft will stall well before it reaches 90 degrees.

Favorite quote: "The F-4 Phantom is God's proof that even a brick will fly if you give it a high enough thrust-to-weight ratio."

As for rolling a large aircraft, do some research on "Tex" Johnson and how he rolled the prototype Boeing 707 during a low pass in front of a large crowd on its public debut. He maintained 1 g throughout the maneuver. He also got fired for risking the company's future: if the plane had crashed they would never have sold so many. He got rehired when Boeing saw the orders come in: his little stunt impressed the hell out of the airline bean counters that decide whether a plane is a safe investment. Understand this: Tex was one of the best test pilots of all time and was at the limit of the aircraft's capabilitiy. I don't think the average airline (or bomber) pilot could safely achieve this. I do know there are some awesome B-52 pilots and the B-1 almost handles like a fighter.

Ask some WWII vets how they think a B-17 would handle a 360 deg roll or a loop. I know the waist gunners would not be happy. I have never read of a B-17 or B-24  being intentionally rolled inverted. Does anyone have a reference quoting a pilot saying that he successfully barrel rolled or looped a 4-engine bomber from WWII? When I was in elementary school, I read a book that claimed the Lancaster had sufficient power to complete a loop with a full bombload. While I doubt the accuracy of the claim, I do believe the Lancaster has the power to loop without a bombload (note: "believe" indicates an opinion since I am too lazy to calculate it on paper).

With respect to Aces High, the rudder turns possible with a B-17 just don't make sense. As large as the vertical stabilizer and rudder are, they are only a fraction of the size of the wing. Therefore they  should not generate enough force to actually change the velocity vector of the plane as well as the wing can. With the long distance from the center of gravity, they are able to drastically change the angle of the nose with respect to the direction of flight, but maintaining even a moderate slip angle causes tremendous losses in speed and lift. Even if the rudder was able to match the force of the wings (since it does not have to use any of its "lift" to fight gravity), it causes an assymetrical force in the roll axis. The plane would tend to roll hard in the opposite direction of the turn and the ailerons would not be able to generate enough force to compensate.

I have not recently done any math to support the above claims, but I have enough experience doing calculus level aerodynamic equations to make my guesses more accurate than most people's calculations. If any of you have the time and energy, feel free to prove me wrong and flame me. Personally, I don't have the time to do the math to prove myself right and without a concrete example or numbers to back them up, I have no reason to think someone elses opinion is more right than mine  

[ 09-22-2001: Message edited by: streakeagle ]