Hawker 800A Fuel Questions (LJ60 hourly burns as well)

[GREAT LAKES]

Yeah, thats where I fly it. You know at 410 how annoying that rumble is.

Stealth,

Let the wings get below 900lbs each before going to gravity. No rumble.

 

[ultrarunner]

750 is better, in order to keep wing-fuel warm.

 

[Stealthh21]

Stealth,

Let the wings get below 900lbs each before going to gravity. No rumble.

I pretty sure I get the rumble at less then 900 lbs. but i am flying tomorrow so I will for sure try it!! Thanks for the tip!!

 

[ultrarunner]

It's all about weight vs. altitude vs. speed. And not about how much fuel is in the wings.

The 60 will fly a higher than optimal altitude because it's so over powered. And poorly designed, IMO. The 60 is simply a variation on a theme.

Anyway, you can easily see this when you look at the normal-cruise charts, in that at heavy weights and high temps the plane will operate an a higher inefficient altitude.

For instance, level off at 410 at isa at 23,000 lbs which essentially represents a gross wt. take off.

The card shows that 390 is most efficient, but 410 is also usable, but at a higher fuel flow. This is due to wing-ineffficiency at that wt./temp/Alt.

And when you push the wing, it yells back.

I have take off NUMEROUS times at gross weight, climbed to the appropriate altitude and set power for .775-.780 and everything is happy. No rumbling.

But because of the crappy design, this is a plane that requires some thought.

Had Marcel designed it, we'd climb right to 410 at any time and cruise at .84, but that's not the case with this home-build we call the learjet.

Oh well.

 

[LRvsH25B]

750 is better, in order to keep wing-fuel warm.

Ultra, would you mind going into a little more detail and explain what your theory is behind that. sounds like you'r eon to something and I'd like ot know more about it. Thanks and I look forward to hearing more.

 

[fly4kix]

Ultra, would you mind going into a little more detail and explain what your theory is behind that. sounds like you'r eon to something and I'd like ot know more about it. Thanks and I look forward to hearing more.

I think what he's getting at is this, you let the wings burn down, the outboard sections are going to cool faster than the relatively fatter and larger chord inboard section. So now you've only got fuel in the inboard 1/3 or perhaps abit more, now turn on gravity, the warmer(again relatively) fuselage fuel keeps the wing warmer...
I believe the AFM calls for that procedure if it's below -65 ambient??
oddly enough, I don't believe the 55 has that requirement, and I wish I knew if the 31 did as well
I too have heard letting the wings burn down does away with the rumble, but I've experienced it many times both heavy and light

 

[ultrarunner]

Ultra, would you mind going into a little more detail and explain what your theory is behind that. sounds like you'r eon to something and I'd like ot know more about it. Thanks and I look forward to hearing more.

LR, fly4kix pretty much summed it up. And I believe he is correct on the AFM reference. There is a touch more to it than that however. And that has to do with the fuel temp compenstating probe in the left wing.

If that probe get's too cold, all heck breaks loose on the fuel quantity guage, in that it pretty much becomes worthless. By burning wing fuel down to 700 or so before going to gravity, you insure a constant supply of warmer fuselage fuel flowing into the wing, thus keeping that little compensating probe happy. And the fuel quantity guage indicating properly.

There you have it.