Stall speed - gear up vs gear down

mandm

mandm

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How does gear down vs gear up affect the stall speed?
 
Gear down creates more drag, which needs more lift to be overcome, so gear down raises the stall speed
 
dans2992 said:
Doesn’t more drag require more thrust?
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Good point.

So, back to the OP, are you saying there’s no difference in stall speed for the same flap setting between gear down and gear up if you just add power (thrust) without changing airspeed?
 
Llewtrah381 said:
So, back to the OP, are you saying there’s no difference in stall speed for the same flap setting between gear down and gear up if you just add power (thrust) without changing airspeed?
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If gear extended decreases the lift coefficient at critical angle of attack the stall speed will increase. If gear extended increases the lift coefficient at critical angle of attack the stall speed will decrease. There are many examples of the first, I can't think of any of the second, but won't rule it out. There's also some small impact due to the effect of trim changes on lift coefficient, which can also go either way - and I have seen examples of both in this case.

If you're talking about secondary effects like thrust inclination then you have to talk about what power setting you're asking about. Full power and power off, little to no change in and of themselves because the thrust contributions are the same for both configurations. If setting power to trim for some particular airspeed, quite possibly lower gear extended due to higher power required for a given airspeed and the resultant vertical component of thrust supplementing lift.

Or TLDR: It depends.

Nauga,
rolling over
 
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My stall speed gear up and flaps up is 69mph. Gear down full flaps is 63mph. The POH does not say what the stall speed is for gear down flaps up. Since my plane has the automatic gear extension at low speeds, I would think this is an important number to know.
 
mandm said:
My stall speed gear up and flaps up is 69mph. Gear down full flaps is 63mph. The POH does not say what the stall speed is for gear down flaps up. Since my plane has the automatic gear extension at low speeds, I would think this is an important number to know.
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What aircraft are you flying?

I would expect the stall speeds to be given for the landing configuration.
A Piper Arrow II (also had automatic gear extension) POH specifically states "Stalling Speed (flaps and gear down)"
https://wayman.edu/files/Piper_Arrow_II_POH.pdf Page 11
 
guest user said:
What aircraft are you flying?

I would expect the stall speeds to be given for the landing configuration.
A Piper Arrow II (also had automatic gear extension) POH specifically states "Stalling Speed (flaps and gear down)"
https://wayman.edu/files/Piper_Arrow_II_POH.pdf Page 11
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Piper Arrow PA28R180
Yes I see flaps & gear down. But if you are slow coming in with flaps up, gear will come down automatically. What is the stall speed with gear down flaps up.
 
Well, that POH shows gear up / flaps up and gear down / flaps down but not your requested combination.

While there may be somebody who knows, why not just take it up and find out in your local practice area?
 
guest user said:
why not just take it up and find out in your local practice area?
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This....I think you'll find the difference is smaller than the precision of your ASI.

What it will do is massively increase your drag. Thankfully cherokees are very well mannered and will tend to pitch down to maintain airspeed all on their own.

There's pros & cons to the auto-extender, but on balance I'd have it removed from any plane I own. Ymmv.
 
I would expect gear down to raise the stall speed for two reasons.

First, gear down means big interruptions in the lower surface of the wing airfoil, which must interfere with airflow and decrease Cl, though I don't have any data to back it up.

Second, lowering the gear lowers the drag center of the aircraft and causes a pitch down tendency, requiring increased tail downforce to compensate, increasing the amount of lift the wing has to generate to offset it.

Whether either of these are significant enough to be noticed in flight I couldn't say.
 
Changing the CG changes the stall speed and extending the gear changes the CG. Low wing aircraft not so much, but the nose gear extension results in a small CG change. In the Cessna single engine retractable, the CG change is about 2”.

Open gear bays cause an increase in interference drag.
 
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Martin Pauly said:
Sorry, but no.

By definition, drag and lift are perpendicular to each other, so one cannot overcome the other.

- Martin
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True. Boneheaded answer on my part. Flaps increase both drag and lift, too.

Still hard not to think gear down increases stall speed somehow.
 
Llewtrah381 said:
Still hard not to think gear down increases stall speed somehow.
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And that's typically the case, yes - just for different reasons than the added drag. Intuitively, it makes sense that the disruption of neat, laminar airflow once the gear is lowered affects how much lift is created at a given airspeed.

Another factor, as @Dana wrote, that is easy to see is that the lowered landing gear creates a pitch down moment, which needs to be countered with an increased down force of the elevator - which in turn requires more lift from the wing so that the sum of the wing's lift and the elevator's downward force can carry the weight of the airplane.

- Martin
 
mandm said:
My stall speed gear up and flaps up is 69mph. Gear down full flaps is 63mph. The POH does not say what the stall speed is for gear down flaps up. Since my plane has the automatic gear extension at low speeds, I would think this is an important number to know.
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Sounds like a good excuse to go flying and try stalling the aircraft in the published configurations as well as the unpublished one(s) and see how your airplane compares to what Piper originally advertised. I usually do this when I'm getting checked out or checking someone out in a new type of airplane.
 
Checkout_my_Six said:
How does increased drag change the lift characteristics?
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As noted above, the thing which, in this case, increased the drag did so in a way which I would have imagined decreased the lift since, unlike a banner being towed behind, the extended gear is integral to the airfoil.

Again, I’m open to learning and realize my initial reply was wrong (or at best, misleadingly incomplete).
 
dmspilot said:
Thrust decreases stall speed.
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More drag requires more thrust to maintain the same stall speed.

Observe the liftoff of a rocket sometime.
 
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Jim K said:
This....I think you'll find the difference is smaller than the precision of your ASI.

What it will do is massively increase your drag. Thankfully cherokees are very well mannered and will tend to pitch down to maintain airspeed all on their own.

There's pros & cons to the auto-extender, but on balance I'd have it removed from any plane I own. Ymmv.
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You can’t inhibit the auto extension system in the cockpit?
 
Tarheelpilot said:
You can’t inhibit the auto extension system in the cockpit?
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You can, but on the rare occasion you need to, will you remember? I'd say it's easier to remember to put the gear down for every normal landing than to remember to disable the auto extender when your engine fails during climbout, or you're trying to ditch.
 
Jim K said:
You can, but on the rare occasion you need to, will you remember? I'd say it's easier to remember to put the gear down for every normal landing than to remember to disable the auto extender when your engine fails during climbout, or you're trying to ditch.
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So that’s my point. In both of the arrows I’ve flown I always inhibited the stupid gear extender for every flight.
 
Tarheelpilot said:
So that’s my point. In both of the arrows I’ve flown I always inhibited the stupid gear extender for every flight.
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Then you might as well have it removed if you own the plane....
 
MauleSkinner said:
I’m told it’s kind of awkward when the gear tries to extend at the top of a loop.:rolleyes:
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It’s also awkward when it extends during an emergency landing and you have decided to land gear up but forgot to inhibit the auto extension. It’s also mighty inconvenient if it dumps the gear at best glide during an engine out. Lots of scenarios where that system is more dangerous than the “problem” it fixed.
 
Tarheelpilot said:
Do rockets have a stall speed?
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Well when they lack enough thrust to keep climbing they tend to flip over and come down nose first at a very high rate of speed.

The point is, if you apply enough thrust you can make a brick fly, thus thrust counters increased drag.

Drag does not apply only to the airfoil it applies to the entire airframe which is why high speed/high performance aircraft designers tend to reduce drag as much as possible.

You'll also notice some incredible aerobatic aircraft that have enough thrust to continue climbing virtually straight up or as some of our more interesting late WWII era aircraft proved take off standing straight up on their tails. Yes there were several propeller driven aircraft in this category.

iu


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Tarheelpilot said:
It’s also awkward when it extends during an emergency landing and you have decided to land gear up but forgot to inhibit the auto extension. It’s also mighty inconvenient if it dumps the gear at best glide during an engine out. Lots of scenarios where that system is more dangerous than the “problem” it fixed.
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If the gear being down doesn't create more drag why would this be an issue?
 
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