For those of you in the know, how do the pilots(or computers) make up for the transfer in weight, the B2 getting heavier and potentially going down and the KC 135 getting lighter and elevating? Secondly, it looks pretty hilly/mountainous, does the change in updrafts/downdrafts make the refueling more challenging versus over ocean, desert, etc? thanks
Kind of a funky receiver door, instead of just doors. I wonder if it really adds to the stealth. Nice video.
That's one solid assembly, moving one unit 180 degrees to a clean closure. I think that is a pretty good way of doing it to make a smooth and fair surface.
Trim. The tanker has the autopilot on, unless its broken or doing AP off for currency, which automatically trims the plane out to hold altitude. The receiver pilot manually flies and trims off pressure to stay in the contact position, sometimes for up to 15 minutes if the jet is taking on a lot of gas (ie. >100k lbs, which is ~1/3rd fuel load in a C-5. I'm not sure how much gas a B-2 typically gets during operational AR's but I would imagine it is comparable).
With some aircraft, getting heavier could call for the tanker to do what is called a tobaggan, where the tanker descends in altitude during the refueling to give the receiver aircraft a little more throttle to play with. Different aircraft also refuel better at lower altitudes, like the A-10, and, in the olden days, the F-111. With a heavy load of bombs, F-111As would run out of throttle authority and would back out, light one engine in min AB, and refuel using the other engine for throttle inputs. Not an issue with the much higher thrust to weight current fighters with much lower wing loading. During refueling, the angle of attack on the receiver aircraft did increase as the fuel load increased, but on modern fighters, not enough for concern. Not much turbulence from the mountains at the altitude the B-2 was refueling. During refueling, it took pretty good turbulence to bother the refueling aircraft. Refueling was done in a tanker track normally, unless on an accompanied overseas deployment, and the receiver aircraft just continued to fly in formation during turns with the tanker while the boom operator maintained contact. Director lights on the bottom of the tanker give the pilot instructions on where to position the aircraft in the x (forward/aft) and z (up/down) axes. The basic thing about refueling is it is just flying formation with the tanker aircraft. The aircraft climbs and moves forward from the pre-contact position to the contact position using the director lights and the boom operator then flies the boom into the air refueling receptacle. This is different from probe and drogue refueling like the Navy uses, where the pilot has to see his probe and fly it into the drogue, but flow rates are much, much higher using a boom. The newest tankers can flow fuel at about twice the rate of the old KC-135s, but some receiver aircraft cannot accept fuel at those accelerated fuel flows.
Long story short, the auto pilot via trim compensates for the additional/decreased fuel. The auto throttles pull back or increase thrust based on what is in the speed window.
I'm sure it varies from jet to jet, but receiving in the Viper (F-16) requires no trim change as all I do is fly formation with the tanker, essentially, until I get what I need. The director lights are used for one thing only when I do it...to tell me when to stop moving forward when changing from pre contact to contact. As I move forward from pre contact, the boom operator will keep the boom at center/center and I literally fly right at the boom tip like I want to get hit in the face with it. This effectively sets my height. This requires a metric crap tonne of trust in the boom operator, because right before it cracks my canopy, he or she will fly the boom around the right side of my cranium and plug into the AR receptacle right as the lights tell me I'm where I need to be front to back. Right then, I take a brain snap shot of what things look like and just keep flying the jet to keep it the same...that's formation flying. The lights and colored boom length markings are just a few of a million cues that constantly get cross referenced to backup the picture I took at the beginning of what a good position is. The real challenges are when nature starts putting her fingers into the mix...night, turbulence, clouds and other storm phenomena. It goes from fun at the left of the list to exhausting at the right. Sucking fumes while in a rotation waiting your turn can also be painfully stressful. This was a very cool video. B-2 really is an amazing machine. Thanks for posting it.
In the KC-10 (what I fly) we dont use autothrottles for Tanker A/R when refueling heavies. We use C-130's as the cutoff... so C130 or larger no Autothrottles, fighters are ok. Has to do with the bow wave off the heavier receiver affecting our tanker.. the autothrottles would surge back and forth too much
In working on the KC-135 in 1954-55 I never thought about the bow wave since the initial receiver was B-47 and B-52 and the bow wave was never considered as far as I know. The B-52and B-47 don't produce as much as a C-130, I'm sure. In the early 50's I lived in a home north about 10 miles north east of Boeing Field. One evening just after dinner I heard some big engines at high power coming in fast and I ran out front to see what it was. Coming right at us at low altitude was a KC-97 with a B-47 hooked up to the boom. The KC-97 sounded like it was at full throttle, the B-47 was slightly nose up with flaps maybe at 20 as they roared over the house. I never heard what the problem was but at least there was no news of an incident at Boeing Field. Those days were far different than they are now. A lot of B-52 flight test was done at Boeing Field and I know of several near misses and some of them I witnessed. They even had an engine test stand in the Renton Plant parking that sometimes was running when we got off work. Ears hurt, Bunkey ?
Awesome video. Also? Bet I'm not the only one who loves the purity of form of the B-2. That is one FINE looking aircraft!
Not to stray too far from the thread content, but I've always admired Jack Northrop. Even after he was no longer a part of the company I think his vision inspired many Northrop Corp designs besides the B-2, including the YF-23 and YF-17 (from which the F-18 is derived). Just beautiful lines on these aircraft. Like other great designers he was not quite as successful in the political aspects of business. Jack Northrop - Ask.com Encyclopedia The flying wing and the pursuit of low drag high lift designs were Northrop's passion, and its failure to be selected as the next generation bomber platform after World War II, and the subsequent destruction of all prototypes and incomplete YB-49s ordered by the federal government were a blow from which he never recovered, causing his association with Northrop Aviation to become almost non-existent for the next 30 years. In an interview for the Discovery Channel's documentary The Wing Will Fly, his son John Northrop Jr. recounted his father's devastation and lifelong suspicion that his flying wing project had been sabotaged by political influence and backroom dealing between rival Convair and high-ranking officials in the Air Force. Northrop dabbled in real estate and lost much of his personal fortune. In 1976, with his health failing, he felt compelled to communicate to NASA his belief in the low drag high lift concept inherent in the flying wing. NASA replied that the idea had technological merit comforting Northrop that his flying wing concepts hadn't been completely abandoned. By the late 1970s a variety of illnesses had left him unable to walk or speak. Shortly before his death in 1981, he was given clearance to see designs and hold a scale model of the Northrop Grumman B-2 Spirit which shared many of the design features of his YB-35 and YB-49 designs.[7] Northrop was reported to have written on a sheet of paper "Now I know why God has kept me alive for 25 years". In the Wing Will Fly documentary, B-2 project designer John Cashen says: "As he held this model in his shaking hands, it was as if you could see his entire history with the flying wing passing through his mind."| Jack Northrop died 10 months later knowing his life's passion would be incorporated in the country's most technologically advanced Cold War weapon system.
135 doesn't have auto throttles. Also the auto pilot isn't that good so the tanker guys have to help it out and be prepared in case it disconnects which can be a common occurrence depending on receiver type and turbulence. Perhaps the worst aircraft to refuel is the C-17 it has the biggest bow wave and moves the 135 around a lot. In rough air a good tanker pilot can do a better job than the auto pilot. When offloading big fuel amounts to a heavy the transfer rate can be up to 6000 lbs per minute and requires constantly pulling tiny bits of power to maintain AR speed. The 135 has 10 fuel tanks and only the forward and aft body tanks can off load gas, they each have two hydraulically driven fuel pumps and that's why the transfer rates are so high. Fuel from the center body can be gravity drained into the forward body or feed the engines via two electric pumps. The four wing tanks can gravity drain to the aft body or feed their respective engines (there are two additional wing tanks that can drain into #1 wing tanks, and one additional aft body upper deck tank that can drain into the aft main body). Since the forward and aft body tanks "only" hold around 55k of fuel a busy AR mission requires constant draining fuel tanks and monitoring the CG of the 135. In addition during AR when the autopilot is used it is not hooked up to the navigation system of the 135. A couple of FYI things. The 135 has an empty weigh of around 122,000 lbs and has a max fuel load of 200,000 lbs of gas for a max T/O weight of 322,000 lbs.
This is one of the better descriptions Of receiver A/R that I've heard from a fighter guy. You seem to get it. F-16's are not initially easy to refuel for us boom operators. They're the smallest boom receiver we see and can easily ruin your day if you're not paying attention. I had quite a few "oh ****!" moments when I was instructing a boom student in the seat, and had a B-course student from Luke in the F-16 on the boom. By the way, we are taught to not move the boom from centerline until we get close to the canopy so you have a good centerline/elevation reference.
