Stall speeds

[SgtPappy]

I took another look at the various stall speeds (particularly on the F4U and the P-38) and they're starting to confuse me.

Widewing's post (scroll to reply #26) http://bbs.hitechcreations.com/smf/index.php/topic,210991.15.html states that the P-38J's stall speed; power on at 54"HG, 3000 rpm is 53 mph..?! Here's a test that supports this: http://www.wwiiaircraftperformance.org/p-38/p-38-67869.html. That's lower than the Spitfire V; stated at 58 mph http://www.spitfireperformance.com/aa873.html!

however, I haven't a clue if this Spitfire stall speed is at full power. But still a P-38J at 53 mph? The power off stall in the manual was 89 mph I think.

The Corsair's stall speed is an iffy one... i can't seem to find it but a chart from the F4U-4 manual someone posted stated it was around 72 mph calibrated. Unknown throttle and rpm. Which seems pretty low as the Mk. IX spitfire's (in manual) is stated that its stall speed (full fuel, no ammo, no ext. stores) was 75 - 79 mph IAS, power off.

May be proof that the F4U really does turn about as tight as a Spitfire IX, but regardless, what's determining stall speeds? And does this mean our P-38 is seriously off in terms of stall speed?

[beddog]

This will help explain it a little better than I could with a few short sentences.


Q:
   

7. What is the "stall" speed of a Cessna 150-152 ?

A:
   

The Short Answer: 48-55 MPH (42-48 knots) with and without flaps.

The Longer Answer:  Most pilots know that an airplane does not stall at any particular airspeed, but rather at a given "angle of attack", this means that an airplane can stall at any airspeed and at any attitude. (this is demonstrated very effectively by modern aerobatic pilots who stall and spin their airplanes at high speeds as part of their aerobatic routines.) Why then are student pilots taught that the airplane will stall if it slows down too much?

Here's why: In a relatively low performance airplane like a Cessna 150-152, the engine does not generate enough thrust to keep the airplane flying at a combined low speed / high angle of attack. When the airplane is flown at slow speeds, the pilot must raise the nose of the airplane to keep it flying. As the nose rises, the wing gets closer and closer to a critical angle of attack, (sometimes called the "stall angle of attack"). Pilots are taught to keep the airplane flying above a certain threshold speed where this is likely to happen.

Every fixed wing airplane has a performance "envelope" that varies with aircraft weight, attitude, and environmental factors.   In a Cessna 150-152 the low end of this envelope is about 40 miles per hour, (about 35 knots). At this speed and below, the wing is producing so little lift that altitude cannot be maintained without exceeding the critical angle of attack, even in level flight. Attempting to climb over obstacles (like trees) or steer around them makes a stall likely. For the Cessna 150-152 the official "stall" speed (with flaps) is 48 miles per hour (42 knots). Pilots are taught to keep a margin of speed above what is likely to result in a critical angle of attack. This is why the recommended landing approach speed with flaps for the Cessna 150-152 is 60-70 MPH (52-60 knots).
[ top ]

Bottom line is.. there are many different configurations and speeds that both the p-38 and the spit may be in when they stall.

Hope it helped pap. 

[SgtPappy]

Thanks! It does really help me understand.

Perhaps the P-38 was being flown at a much lower Alpha than the tail-draggers...?

[CAP1]

This will help explain it a little better than I could with a few short sentences.


Q:
   

7. What is the "stall" speed of a Cessna 150-152 ?

A:
   

The Short Answer: 48-55 MPH (42-48 knots) with and without flaps.

The Longer Answer:  Most pilots know that an airplane does not stall at any particular airspeed, but rather at a given "angle of attack", this means that an airplane can stall at any airspeed and at any attitude. (this is demonstrated very effectively by modern aerobatic pilots who stall and spin their airplanes at high speeds as part of their aerobatic routines.) Why then are student pilots taught that the airplane will stall if it slows down too much?

Here's why: In a relatively low performance airplane like a Cessna 150-152, the engine does not generate enough thrust to keep the airplane flying at a combined low speed / high angle of attack. When the airplane is flown at slow speeds, the pilot must raise the nose of the airplane to keep it flying. As the nose rises, the wing gets closer and closer to a critical angle of attack, (sometimes called the "stall angle of attack"). Pilots are taught to keep the airplane flying above a certain threshold speed where this is likely to happen.

Every fixed wing airplane has a performance "envelope" that varies with aircraft weight, attitude, and environmental factors.   In a Cessna 150-152 the low end of this envelope is about 40 miles per hour, (about 35 knots). At this speed and below, the wing is producing so little lift that altitude cannot be maintained without exceeding the critical angle of attack, even in level flight. Attempting to climb over obstacles (like trees) or steer around them makes a stall likely. For the Cessna 150-152 the official "stall" speed (with flaps) is 48 miles per hour (42 knots). Pilots are taught to keep a margin of speed above what is likely to result in a critical angle of attack. This is why the recommended landing approach speed with flaps for the Cessna 150-152 is 60-70 MPH (52-60 knots).
[ top ]

Bottom line is.. there are many different configurations and speeds that both the p-38 and the spit may be in when they stall.

Hope it helped pap. 

cessna172 power off, landing config advertised stall is i think 43KTIAS.......buzzer starts at 52 on our clubs 172's.......

[CAP1]

This will help explain it a little better than I could with a few short sentences.


Q:
   

7. What is the "stall" speed of a Cessna 150-152 ?

A:
   

The Short Answer: 48-55 MPH (42-48 knots) with and without flaps.

The Longer Answer:  Most pilots know that an airplane does not stall at any particular airspeed, but rather at a given "angle of attack", this means that an airplane can stall at any airspeed and at any attitude. (this is demonstrated very effectively by modern aerobatic pilots who stall and spin their airplanes at high speeds as part of their aerobatic routines.) Why then are student pilots taught that the airplane will stall if it slows down too much?

Here's why: In a relatively low performance airplane like a Cessna 150-152, the engine does not generate enough thrust to keep the airplane flying at a combined low speed / high angle of attack. When the airplane is flown at slow speeds, the pilot must raise the nose of the airplane to keep it flying. As the nose rises, the wing gets closer and closer to a critical angle of attack, (sometimes called the "stall angle of attack"). Pilots are taught to keep the airplane flying above a certain threshold speed where this is likely to happen.

Every fixed wing airplane has a performance "envelope" that varies with aircraft weight, attitude, and environmental factors.   In a Cessna 150-152 the low end of this envelope is about 40 miles per hour, (about 35 knots). At this speed and below, the wing is producing so little lift that altitude cannot be maintained without exceeding the critical angle of attack, even in level flight. Attempting to climb over obstacles (like trees) or steer around them makes a stall likely. For the Cessna 150-152 the official "stall" speed (with flaps) is 48 miles per hour (42 knots). Pilots are taught to keep a margin of speed above what is likely to result in a critical angle of attack. This is why the recommended landing approach speed with flaps for the Cessna 150-152 is 60-70 MPH (52-60 knots).
[ top ]

Bottom line is.. there are many different configurations and speeds that both the p-38 and the spit may be in when they stall.

Hope it helped pap. 

if he does a search also,m there's been multiple threads on the critical AOA. i think the 172..and most GA aircraft is 17 degrees? not sure what it was/is on our rides in here though. i do know that the stall buzzer sounds as you start approaching this angle in here.(also mentioned in a recent thread)

nice explanation sir!!!!
<<S>>

[beddog]

Those are indicated airspeed too which are probably higher than ground speed figures.  I know that when forcing a 150 to stall at full power the actual ground speed must be really slow. With full flaps it looks like you can get out and walk except for the fall....

[colmbo]

Those are indicated airspeed too which are probably higher than ground speed figures.  I know that when forcing a 150 to stall at full power the actual ground speed must be really slow. With full flaps it looks like you can get out and walk except for the fall....

Ground speed has nothing to do with the stall...and if you're going downwind doing the stall you may be moving quickly across the ground -- if across the wind you'll notice only drift.

[CAP1]

Those are indicated airspeed too which are probably higher than ground speed figures.  I know that when forcing a 150 to stall at full power the actual ground speed must be really slow. With full flaps it looks like you can get out and walk except for the fall....

the airspeed is the only thing that matters for the aircraft to stall.

<<S>>

[colmbo]

the airspeed is the only thing that matters for the aircraft to stall.

<<S>>

Technically it's he angle of attack. <G>

[beddog]

Technically it's he angle of attack. <G>

Correct...  Like the plaque I once saw hanging on a P-51 said...


      Stall speed   -    All speeds.......

[CAP1]

Technically it's he angle of attack. <G>

good point

[dtango]

What determines stall speed?
Assuming small thrust angle approximation the following is the equation for stall speed at 1g level flight (load_factor = 1, climb_angle=0) that I hope helps us understand the factors that influence stall speed:

Vstall = sqrt (2 * weight / wing_area * air_density * Clmax)

From this relationship we can see that in order to lower stall speed we can:

reduce weight,
increase wing area,
increase air density,
or increase maximum lift coefficient (Clmax).

For this discussion the factors most interesting to us are Clmax and to some extent wing_area.

Lift coefficient (Cl) is related to angle of attack of the wing and is bounded by Clmax.  Clmax occurs at the critical aoa for that aircraft.  The wing stalls when Clmax (critical aoa) is exceeded.  Pressure distributions on a wing are greatly influenced by wing geometry, thus wing geometry has a huge impact on determining aircraft Clmax.

So how can we increase Clmax while in flight to reduce stall speed?  High lift devices such as flaps and slats change the pressure distribution on the wing resulting in increased lift potential of the wing and thus Clmax.  Also, propwash induces airflow higher than freestream air velocity across parts of the wing.  This produces more lift on the portions of the wing in the propwash and has the effect of increasing aircraft Clmax.

Wing area can sometimes be dynamically increased in flight as well.  The P-38 fowler flaps are an example of this which extend out beyond the trailing edge of the wing increasing the wing area.

The key point is that stall speed is actually dynamic and varies with things like the effect of deploying high-lift devices and propwash.  Comparing relative differences between aircraft we can't assume that relative difference in when part of the flight envelope translates to the same relative difference in a different part of the flight envelope.  They don't call it aerodynamics for nothing

The differences between the Spit and P-38 as listed in the flight test reports referred highlights this fact.  With both aircraft clean, the Spit has a lower 1g stall speed than the P-38.  But in landing configuration with flaps fully deployed the P-38 has a lower 1g stall speed than the Spit.  Why is that?  The reasons are complex but are related to how Clmax changes under the influence of things like flap deployment and power-on propwash.

Hope that's useful!

Tango, XO
412th FS Braunco Mustangs

[SgtPappy]

It is! Thanks Dtango, once again.