Stall speed question

fbm2009

fbm2009

Well-Known Member
Here's one for those of you who like Aerodynamics. I've been trying to wrap my head around this for a few weeks now, and I have some ideas as to why, but I'd like to hear what some other trained professionals think.

Typically, the text books will teach that an increase in flap will decrease the aircraft's stall speed. And until now I've found this to be true. But on a rainy day in the office, I decided that for "entertainment" I'd check out some of the lesser examined performance charts for our trainers.

At idle power, the text books hold true, Flap 0 is fastest, and flap 60 is slowest. Fair enough, thats what we would all expect.

20150613_103344.jpg


Where it gets a little strange to me, is the stall speeds at max power. Yes, the usual effect of getting the bird to be a little slower with the power on holds up, but for some reason flap 60 will stall before clean, and well ahead of flap 15.

I found myself a brave student, and we tried it out. She gave us the most violent wing drop I've ever encountered, but the charts are right, flap 60 full power stalled at around 44, and flap 15 full power was around 40.

Like I say, I've got my own ideas as to whats happening and I'll add them later, along with a couple of other views from guys in the office, but I'd be very curious to see what the wider world has to say. Why are we getting a higher stall speed with full flap, when we have the power on?
 
The chart is still showing the lowest stall speed occurring with flaps 60 with full power. Are you saying that your indicated stall speed was higher than published?

Think about what affects the stall speed... How much load are you inducing at what AOA? If you're stalling higher than published the airplane is either over gross, or you are introducing more than 1G during the maneuver. Next time keep track of how smoothly you pull back on the stick. In the condition you are flying the airplane into, any slight release in elevator backpressure followed by a sharp jerk to get the attitude back will result in an increase in load factor. At this point, you may be so close to the critical AOA that small load increase may be all it takes. Turbulence can also play a part.

You did mention a wing drop, so you could have been uncoordinated or had a little aileron mixed in as well. The slightly downward deflected aileron would have caused the AOA of that wing tip to be exceeded before the wing root completely stalled. The affect is exacerbated since the wing tips are flying at a higher than normal AOA since the accelerated slip stream from the max power setting is delaying airflow separation over the wing root.

Try it again on a smooth morning and pay special attention to the control inputs.
 
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Adding flaps decreases the critical angle of attack. So the more flaps you add the lower the angle of attack is in which you will stall.
Slats are the opposite.
Airspeed is just a super crappy way of getting second hand AOA. Flaps 60 is a LOT of flaps for a small airplane. So you are decreasing the critical AOA a lot. Remember to think about it in terms of Angle of attack and not airspeed, and it makes perfect sense. Remember you can stall at any airspeed mostly because airspeed is irrelevant to a stall, but you will only ever stall at the critical angle of attack.

If you want to actually see it, get yourself a cheapish AOA meter.
Also, if I haven't emphasized it enough. Angle of attack.
 
TwoTwoLeft said:
The chart is still showing the lowest stall speed occurring with flaps 60 with full power. Are you saying that your indicated stall speed was higher than published?

Think about what affects the stall speed... How much load are you inducing at what AOA? If you're stalling higher than published the airplane is either over gross, or you are introducing more than 1G during the maneuver. Next time keep track of how smoothly you pull back on the stick. In the condition you are flying the airplane into, any slight release in elevator backpressure followed by a sharp jerk to get the attitude back will result in an increase in load factor. At this point, you may be so close to the critical AOA that small load increase may be all it takes. Turbulence can also play a part.

You did mention a wing drop, so you could have been uncoordinated or had a little aileron mixed in as well. The slightly downward deflected aileron would have caused the AOA of that wing tip to be exceeded before the wing root completely stalled. The affect is exacerbated since the wing tips are flying at a higher than normal AOA since the accelerated slip stream from the max power setting is delaying airflow separation over the wing root.

Try it again on a smooth morning and pay special attention to the control inputs.
Click to expand...

The top chart of the 2 is with power on, and the 60 speed is higher than 0 and 15. 60's aprox speed is 44, 0 is 43 and 15 is all the way down at 40. Like I say, normally you would expect the "order" to go 0 - 15 - 60. And it does with power off, but the power on condition is changing that.

As for the wing drop, our little bird has interconnected rudder and aileron, so we were likely habitually holding some rudder to correct for the yaw from full power, giving some unintentional aileron.
 
fbm2009 said:
Here's one for those of you who like Aerodynamics. I've been trying to wrap my head around this for a few weeks now, and I have some ideas as to why, but I'd like to hear what some other trained professionals think.

Typically, the text books will teach that an increase in flap will decrease the aircraft's stall speed. And until now I've found this to be true. But on a rainy day in the office, I decided that for "entertainment" I'd check out some of the lesser examined performance charts for our trainers.

At idle power, the text books hold true, Flap 0 is fastest, and flap 60 is slowest. Fair enough, thats what we would all expect.

View attachment 31845

Where it gets a little strange to me, is the stall speeds at max power. Yes, the usual effect of getting the bird to be a little slower with the power on holds up, but for some reason flap 60 will stall before clean, and well ahead of flap 15.

I found myself a brave student, and we tried it out. She gave us the most violent wing drop I've ever encountered, but the charts are right, flap 60 full power stalled at around 44, and flap 15 full power was around 40.

Like I say, I've got my own ideas as to whats happening and I'll add them later, along with a couple of other views from guys in the office, but I'd be very curious to see what the wider world has to say. Why are we getting a higher stall speed with full flap, when we have the power on?
Click to expand...
Could it be that with flaps 60 and the engine at a high power the air over the root of the wing separates earlier due to the angle of change required for the streamlines? The air can't turn as fast as the 60 degrees of flaps is asking it to?
 
fbm2009 said:
The top chart of the 2 is with power on, and the 60 speed is higher than 0 and 15. 60's aprox speed is 44, 0 is 43 and 15 is all the way down at 40. Like I say, normally you would expect the "order" to go 0 - 15 - 60. And it does with power off, but the power on condition is changing that.

As for the wing drop, our little bird has interconnected rudder and aileron, so we were likely habitually holding some rudder to correct for the yaw from full power, giving some unintentional aileron.
Click to expand...

I see the graph better now. Not looking at it on an iPhone helps!

I think what you are running into is a thrust vs drag issue. Density altitude will eventually be a factor too. As z987k mentioned, 60 degrees of flap is a lot. At some point you are going to reach a point of diminishing return as far as lift goes and drag will take over. According to the graph, it looks like that may be 15 degrees. Not sure what other settings are there, but the engineers (or lawyers) made sure to include 15 for a reason...

But for a given load, the 1G stall speed is still lower with power on vs power off when flaps are set at 60 for reasons addressed in earlier posts.
 
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"Weirdnesses" could potentially also have to do with accelerated slip stream buffet and/or blocking over the tail. Perhaps.
 
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