Effect of Undercarriage on Vstall

[Q4NVS]

Was asked this question a while ago, but after interrogating a number of text books (including Handling the Big Jets etc), could find no technical explanation for/against.

Anyone with a solid technical explanation would be welcomed. Have thus far tried to formulate my own answer....

[Solid Rust Twotter]

Possibly a smidgen more induced lift on a prop job, due to extra thrust required to overcome extra drag?

Don't think a jet will be affected unless it's something like the AN72/74 with the fans running the exhaust over the wing and inboard flaps to add a little performance.

Just my 2c worth...

[Mad (Flt) Scientist]

In theory there isn't a thrust effect if we're talking about a certification-type stall (1kt/sec straight, for example) because the thrust is assumed to be zero and the demonstration will be usually conducted at idle power.

You may have a gear effect on stall speed due to a wing-mounted gear disrupting the flow on the wing - instead of a nice airfoil shape in that part of the wing, you've got a big hunk of undercarriage sticking out, and a big hole where the bay is. Similarly, the nose gear and doors may be disrupting forward fuselage flow and reducing lift from the body.

[Old Smokey]

and a big hole where the bay is

Just like the B737 with the gear UP.

Regards,

Old Smokey

[-IBLB-]

taking a wild shot here, let me know if i am barking up the wrong tree; here goes.

if the stallspeed formula = sqrt ((2 x W) / (S x rho x Clmax))

there is no Cd in there, W doesn't change nor do rho, and S. The only question is does Clmax change?

Assuming it doesn't, this means that there is no change of the stallspeed.

If it would have a negative influence on the Clmax, the stallspeed would increase.

[Mad (Flt) Scientist]

@Old Smokey : but at least the portion of the bay where the leg is is presumably covered by a door with the gear up (I've never looked that closely at a B737, but I assume it's the wheel portion of the bay with no door.) That would be the more critical part of the bay to cover, since it's more on the wing, the wheel being in the wing/fuselage fairing.

@IBLB it's not quite so simple; yes, there's no "CD" in the equation, but there are things that can affect the lift on the wing that are secondary effects of the drag, if you assume the power setting is adjusted to account for drag changes. Just as there's nothing directly for pitching moment, but that will impact tail lift and hence overall lift.

[safetypee]

I believe that the BAe146 / Avro RJ demonstrated a slight difference in stall speed with gear up/down (1-2 kts). With the gear doors extended they trap an aircushion under the centre fuselage and the horizontal parts of the doors act as small ‘bi-plane’ wings.
In addition, with a power-on level stall there was a small lift increase due to a recirculation effect over the wing/flap from the increased thrust.

[DFC]

There will be a change in the position of the C of G when the gear is lowered.

There will also be a change in the position of the drag line.

These can have effects on stalling characteristics especially at the outer edge of the envelope.

Regards,

DFC

[M.85]

both windshear and GPWS:if the gear is down...action is to leave it down...can only mean one thing...

M.85

[two speed prop 3]

M0.85

I think you will find that the reason you leave the gear down in a windshear/GPWS situation is in case you hit the ground during the recovery.

You are far better striking the groung with an U/C designed for absorbing huge forces rather than then skin around the tail.

[sanddancer]

I was lead to believe that you leave the gear down in windshear and terrain go-arounds (until clear of threat and climb established) as, particularly in large jets, the gear doors themselves are pretty large and they will increase drag significantly thus decreasing net climb gradient.

This always sounded a plausible answer to me (but then again I'm a bit of a thicky and most technical answers sound fairly plausible!)

[kaepa]

landing gear extended brings the CG forward , thus the Vs increase , it is the same also with an increased PA , thats why most aircraft have an altitiude limit to operate the landing gear ,
basically it is aerodynamics


regards

[BOAC]

landing gear extended brings the CG forward

- not so, kaepa. It depends on how the gear extends. Mainwheels tend to come out 'sideways' with minimal trim change and nosewheels either forwards or backwards. 737 gear extension moves c of g aft a tiny bit.

[safetypee]

Apart from the technical arguments for retracting or leaving the gear and flaps in position after a GPWS or windshear warning, there are practical and human factor issues.

With gear and full flap, a retraction procedure may require two sequenced operations and a revised speed schedule, but with intermediate flap and/or gear up, there are likely to be different procedures; complexity aids confusion.

The human issues are that even experienced pilots will encounter some surprise or shock after a warning, this affects the ability to recall drills or it may delay action; some manufacturers considered this, and their consensus was to focus on the important issue of pulling up quickly using the correct technique (FSF ALAR/CFIT Task Force). A quick review of CFIT accidents identifies that a large proportion occurred just below a ridge or hill top; 60-100 ft. This indicates that the most important time during an encounter is the first few seconds; here excess speed can be converted to altitude without concern of stalling or any longer term effect of drag.

[M.85]

sanddancer,thats the reason i remembered as well as the time and thinking it may take a crew to retract flaps/min maneuvering speed..when then should be focused at constantly trading airspeed for altitude and vice versa..

M.85