757-300 Buffet

[stilton]

I fly the -300 once every three months or so and have noticed, amongs it's differences from the -200 a pronounced buffet, perceived by most of us as aerodynamic, below 250 kias.

This is strictly a low speed phenomenon as it disappears soon after accelerating above 250kts.

Anyone else fly this aircraft that has noticed this? any ideas would be welcome, (nothing forthcoming from our company)

[Old Smokey]

stilton,

It might help prospective respondants if you can provide the typical Thrust setting when you notice the 'below 250 Knots' buffet. Is it during climb, cruise, holding, idle descent etc.

As a 'knee jerk' response I would say that if this buffet occurs over the entire useable thrust range that it is aerodynamic. If, on the other hand, it's during typical holding thrust settings for example, then it's an engine phenomena, not unusual in the mid-thrust range for high bypass engines.

Regards,

Old Smokey

[FLCH]

I flew one a couple a days ago out of Vegas, but the turbulence was masking any percieved vibration, could it be because of the long body? I usually fly the 764 and that seems to rattle a lot down low, the 762 on the other hand is a pretty good ride..Just my 2 cents.....

[stilton]

757-300

It is most apparent during climb (low IAS high thrust) and is not engine related, it feels ver much like aerodynamic turbulence around the (much longer) forward fuselage and flight deck.

[MkVIII]

Is it more a yawing type instability sensation, or all axes?

Sounds like a slight lateral instabilty caused by higher yaw moments of the longer fuselage - the longer distance from the datum in the -300 compared to the -200 is going to amplify the apparent degree of yaw for the same amount of instabilty. A simple diagramme would help me here, but I think you get what I mean - the distance disturbed from the centreline is going to be more apparent in a longer fuselage than in the shorter fuselage -200, for the same given parameters.

I do not fly the 757 (WORSE LUCK!), but I assume the fin and wing dimensions are the same for both types?

[stilton]

757-300

No, there is no yaw or other instability associated with it, the best way I could describe it is that you still have slat/flap extended, close to or beyond their limit speeds, despite the fact the aircraft is clean.

And yes the wing and all tail surfaces are unchanged in size on the -300.

[MkVIII]

So, just a general "rumbling buffet" kind of disturbance?

Sounds like a boundary layer issue perhaps?

[stilton]

757-300

That's correct, a phenomenon not experienced on the -200.

[MkVIII]

OK then, it sounds like a harmonic.

It could be an aerodynamic harmonic, or an engine related harmonic - try varying the thrust setting just SLIGHTLY to see if it dampens the disturbance. (should rectify either aerodynamic or engine harmonics)

My best WAG.

[stilton]

757-300

Thanks for the input, don't think its thrust related, as it's still there
during an intermediate level off (less than 250 kias) i'm sure its aerodynamic.

[MkVIII]

Yes, it probably is, what I am saying is try a slight alteration in either airspeed, or thrust setting. Check the engine vibration levels too.

I mean SUBTLE incremental adjustments in speed and/or thrust, not major deviations!