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Gillbilly
Topic Author
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C-5 Off Center Aerial Refueling

Sun Jan 19, 2020 12:50 am

Why is the C-5 aerial refueling port off center (to the right)?
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Sun Jan 19, 2020 3:04 pm

No idea, probably due to the plumbing, but more receptacles and just about all probes are off center—not an issue. Never a thought of it while refueling. There’s an increase in wind noise as the boom approaches the receptacle, silence when connected.

GF
 
Galaxy5007
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Re: C-5 Off Center Aerial Refueling

Mon Jan 20, 2020 4:04 am

From what I was told, its offset due to the buffeting of the T-tail during wind tunnel testing. It was designed to avoid future problems. Amazing that little of a shift caused that much of a difference.
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Mon Jan 20, 2020 4:11 am

Considering the fatigue damage done to the t-tail during AAR due to wake turbulence and jet blast, I’d be amazed the boom position has any effect. The stab is right in the jet blast. Ever tank behind a KC-10? The center engine pushes you around a lot.

The tie box and torque deck cracking were AAR related.
 
timh4000
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Re: C-5 Off Center Aerial Refueling

Mon Jan 20, 2020 5:24 am

Is this during approach the buffeting around? Once connected there's definitely a difference with the two planes as far as one being significantly higher than the other, which I've always assumed was A, gravity of tanker to a/c being fuled AND for any safety issues, I figured there were numerous.
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Mon Jan 20, 2020 3:08 pm

During contact, the tail of the C-5 is right in the wash and buffets considerably. Here’s a side view in pre-contact, the receiver has about 5’ up to go. Remember the AoA of the tanker produces a downward component of the wash off the wing and engines.

https://nara.getarchive.net/media/an-ai ... ?zoom=true
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Mon Jan 20, 2020 3:40 pm

Here’s another view it’s a KC-46

https://www.amc.af.mil/News/Article-Dis ... er-galaxy/
 
JohnM
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Re: C-5 Off Center Aerial Refueling

Mon Jan 20, 2020 4:46 pm

It isn't offset very much. When crawling around up there, I really didn't notice the offset. I was more concerned about not sliding off the airplane and finding out it the restraint harness really worked. Lots of stuff get crammed into that area of the airplane. I'm not privy to the aerodynamic reasons, but the aerial refuel manifold does take up quite a bit of real estate, and it helps to route it down the right side of the fuselage by it's location. I know that the air refuel plumbing failed at least once that I remember (in flight during refuel), a bad situation of course. The FE was soaked.
 
meecrob
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Re: C-5 Off Center Aerial Refueling

Mon Jan 20, 2020 6:52 pm

JohnM wrote:
I'm not privy to the aerodynamic reasons, but the aerial refuel manifold does take up quite a bit of real estate, and it helps to route it down the right side of the fuselage by it's location.


Sometimes there are no aerodynamic reasons, rather simply "it fits here." Forward-tilting main gear bogies are a good example of this.
 
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Spacepope
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Re: C-5 Off Center Aerial Refueling

Tue Jan 21, 2020 3:25 am

GalaxyFlyer wrote:
During contact, the tail of the C-5 is right in the wash and buffets considerably. Here’s a side view in pre-contact, the receiver has about 5’ up to go. Remember the AoA of the tanker produces a downward component of the wash off the wing and engines.

https://nara.getarchive.net/media/an-ai ... ?zoom=true


Loving both the RF-4C and the paint schemes!
The last of the famous international playboys
 
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N14AZ
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Re: C-5 Off Center Aerial Refueling

Tue Jan 21, 2020 2:14 pm

Gillbilly wrote:
Why is the C-5 aerial refueling port off center (to the right)?

That's what I love about a.net. You learn something new almost every day (...even after reading it for 20 years...). I didn't know it's off-center:
Image

Spacepope wrote:
GalaxyFlyer wrote:
During contact, the tail of the C-5 is right in the wash and buffets considerably. Here’s a side view in pre-contact, the receiver has about 5’ up to go. Remember the AoA of the tanker produces a downward component of the wash off the wing and engines.

https://nara.getarchive.net/media/an-ai ... ?zoom=true


Loving both the RF-4C and the paint schemes!

Yes, it's a great picture. I guess it wouldn't be accepted here in a-net's picture database due to back light but it definitely has something...
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Tue Jan 21, 2020 6:00 pm

Funny, after 18 years in it, I never saw that picture or saw that is was offset. You notice from the front seats.
 
dfwjim1
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Re: C-5 Off Center Aerial Refueling

Tue Jan 21, 2020 6:23 pm

Slightly off topic but during flight training in the USAF how is it determined if a future pilot has the flying ability to handle air-to-air refueling?
 
SuperiorPilotMe
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Re: C-5 Off Center Aerial Refueling

Tue Jan 21, 2020 9:16 pm

GalaxyFlyer wrote:
Considering the fatigue damage done to the t-tail during AAR due to wake turbulence and jet blast, I’d be amazed the boom position has any effect.


Not the boom, the plane attached to that boom.
Stop the stupids!- Claus Kellerman
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Tue Jan 21, 2020 10:50 pm

dfwjim1 wrote:
Slightly off topic but during flight training in the USAF how is it determined if a future pilot has the flying ability to handle air-to-air refueling?


Surprisingly, not a whole lot. Fighters, it’s assumed because so much formation flying. Pilots for heavies like the C-5, C-17, KC-10, KC-46, refuelable tankers stream from the T-1. The T-1 syllabus has some trail formation imitating tanking, but that’s it.

GF
 
WKTaylor
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Re: C-5 Off Center Aerial Refueling

Wed Jan 29, 2020 9:49 pm

As I understand it, the bow-wake from the wide body aircraft [C-5, C-17, KC-10, etc] moving into position behind the tanker has a noticeable stability effect on the tanker when the flying-boom and probe system is coming in/out of contact.

To a degree, this is why hose/drogue/prove system is popular with USA, USN and other air forces… the tanker and receiver have greater separation hence fewer aerodynamic interferences and more 'wiggle room'.

The compromise is rate of fuel transfer. The flying-boom/probe can transfer fuel at a MUCH higher [2X-to-3X] rate than hose/drogue/probe. Just imagine the time-in connection [close-formation] when the rate of transfer is slow and many jets need fuel... or one big 'bird' needs fuel on-load MUCH faster that they burn fuel!
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Wed Jan 29, 2020 11:27 pm

Yes, the bow wave from the C-5 pushes the tanker’s tail up, forcing the tanker pilot to trim nose up. In opposition, the flow off the tanker impinging on the T-tail requires nose down trim—a lot! With the receptacle door open, the C-5 nose down trim envelope expands about 6 degrees, IIRC. It’s funny, you can be behind the tanker with a normal amount (depending CG, maybe +1 degree), close in for the hook and it’s steady nose down trim to about 2-3 degrees nose down.
 
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LyleLanley
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Re: C-5 Off Center Aerial Refueling

Thu Jan 30, 2020 1:29 am

WKTaylor wrote:
As I understand it, the bow-wake from the wide body aircraft [C-5, C-17, KC-10, etc] moving into position behind the tanker has a noticeable stability effect on the tanker when the flying-boom and probe system is coming in/out of contact.

To a degree, this is why hose/drogue/prove system is popular with USA, USN and other air forces… the tanker and receiver have greater separation hence fewer aerodynamic interferences and more 'wiggle room'.

The compromise is rate of fuel transfer. The flying-boom/probe can transfer fuel at a MUCH higher [2X-to-3X] rate than hose/drogue/probe. Just imagine the time-in connection [close-formation] when the rate of transfer is slow and many jets need fuel... or one big 'bird' needs fuel on-load MUCH faster that they burn fuel!


Hi WK!

It makes sense why you'd think that way, but you're a little off... In practice, the greater USAF at large doesn't really care about the bow-wave effect on the tanker. The tanker crews know what they're doing and they simply adjust for the receiver's bow wave. Occasionally, things get dicey... Just ask anyone who has had the pleasure of a KC-135's autopilot clicking off at night! But overall, it's not really anything to fret over. The pilots do a few things differently (auto throttles to off on the KC-10, for instance), but overall it's a non-event. The KC-135 boom operator does a few things differently with regards to boom-trim settings and his right arm will be tired much more quickly, but he/she will be fine. In the KC-10, our switch positions don't change at all! We just have to be cognizant that the boom will rise a little as the heavy receiver closes. A little flick of the wrist though, and the boom will be right where it should be. So as you can see, the receiver bow wave doesn't really affect the tanker all that much, at least in ways that truly matter.

Ironically enough, the bow wave effect is actually a STRENGTH of boom AR over the probe/drogue method favored by the USN/USMC and many foreign air forces! Imagine for a moment, a C-5 being fitted with a probe instead of a receptacle. Not only does the receiver pilot now have to maneuver his 747-sized aircraft to where he can insert a 3 inch diameter probe into a basket ~ 2 feet in diameter -- very difficult on any day, but insert turbulence and you'll have to change your destination -- but he'd now have his own bow wave PUSHING THE BASKET UP, UP, and UP as he closes into the contact position! As you can imagine, that would not be very safe at all! "Back in the day" F-14s tanking off the centerline drogue would inadvertently put on a show, as their VERY large nose would push the basket away as they approached contact; it was always an amazing sight to see the Tomcat pilot kick in a bunch of right rudder along with a surge in power, at the last minute, to finally catch the basket those last few feet!

Edit: I found a video to demonstrate. Not all Tomcat refueling were like this, but fairly sporty contacts like this were normal. https://youtu.be/c5PQ9obNSYo
"I've sold monorails to Brockway, Ogdenville, and North Haverbrook, and, by gum, it put them on the map!"
 
GalaxyFlyer
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Re: C-5 Off Center Aerial Refueling

Thu Jan 30, 2020 6:11 pm

I’ve done both and have absolutely no interest in trying probe refueling in a C-5. It was trying enough in the Hun.
In the Tomcat video, I’d bet he was looking at the basket, hence the gyrations. Now, I never refueling off a hose reel, just the pony length on the KC-135, but I, we, just fly near the basket, stabilized, checked alignment between probe and basket, added a touch of power to move forward to hopefully plug. Looking at the basket was a certain way to PIO. Also, the basket can’t guide the probe into position, it’d have push the whole receiver around. The probe has to be pretty close to center of the basket to plug in.

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