Is the term “stall speed” proper for model airplanes?
11-09-2010, 02:12 PM
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RE: Is the term “stall speed” proper for model airplanes?
Quite. The extra G in the turn increases the stall speed.
So if you are flying downwind with low true airspeed - ie above but close to level stall speed and then turn, it's not decreasing airspeed but the increase in stall speed that'll get you.
Turns require more speed - simple as that.
So if you are flying downwind with low true airspeed - ie above but close to level stall speed and then turn, it's not decreasing airspeed but the increase in stall speed that'll get you.
Turns require more speed - simple as that.
11-09-2010, 06:08 PM
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RE: Is the term “stall speed” proper for model airplanes?
Stall speed is a correct term:
Wings actually "suck" the aircraft into the air vs actually lifting from the bottom.
A "Stall" occurs at the point were the air seperates from the wing or airfoil. ( the suction is broken)
This deprives the wing or airfoil of the ability to lift.
The "speed" at which this occurs varies with g load and is relative to the air 'speed" across the wiing or airfoil and..not the "ground speed" of the aircraft.
Normally "The stall speed" is considered that speed at which a wing or airfoil will stall(stop providing lift) at a 1 g load. in Straight and level flight with no wind in any direction.
The speed at which this happens can vary due to The G load being Induced.(like in a turn or simply increased weight)
A 'high speed stall" happens when the G load induced to the wing or airfoil exceeds that airfoils ablity to maintain airflow over the wing.The air is "ripped from the wing" (breaking the suction) even while traveling at a high speed.
The airfoil shape also has a factor in " how it stalls' or when it will stall".
Its better to understand the wheel ....rather than to try to invent a new one. Hope this helps.
Wings actually "suck" the aircraft into the air vs actually lifting from the bottom.
A "Stall" occurs at the point were the air seperates from the wing or airfoil. ( the suction is broken)
This deprives the wing or airfoil of the ability to lift.
The "speed" at which this occurs varies with g load and is relative to the air 'speed" across the wiing or airfoil and..not the "ground speed" of the aircraft.
Normally "The stall speed" is considered that speed at which a wing or airfoil will stall(stop providing lift) at a 1 g load. in Straight and level flight with no wind in any direction.
The speed at which this happens can vary due to The G load being Induced.(like in a turn or simply increased weight)
A 'high speed stall" happens when the G load induced to the wing or airfoil exceeds that airfoils ablity to maintain airflow over the wing.The air is "ripped from the wing" (breaking the suction) even while traveling at a high speed.
The airfoil shape also has a factor in " how it stalls' or when it will stall".
Its better to understand the wheel ....rather than to try to invent a new one. Hope this helps.
11-09-2010, 11:19 PM
#29
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RE: Is the term “stall speed” proper for model airplanes?
Yes, our models don’t have instruments that inform us about the airspeed and AOA, and our eyes are not good at estimating those values either.
I also believe that the response to control inputs and the deflection of the elevator stick in the transmitter are better indicators of how close our planes are to a stall.
Hence, the actual stall speed of any model does not have much practical value.
However, the term is useful as a comparison of performance of one same model.
According to the late Andy Lennon, the formula for stall speed at sea level is:
Stall speed [mph] = Square root { (Weight [ounces] x 3519) / (CL max x Wing area [sq.in.] ) }
This means that any change in the weight that the wing “feels” will induce a change in the stall speed or the air speed below which the airplane cannot create enough lift to sustain its weight in level (1g) flight just below the critical AOA (which is a constant value for any particular wing-Re).
Note that the relationship is not linear but exponential; e.g., certain change in weight will induce a relatively smaller change in the stall speed (square root).
The wings feel more weight in banked turns, coming out of dives, in loops, due to extra load and due to advanced CG locations.
Staying below the critical AOA in order to avoid a wing stall, those maneuvers require more air speed than the minimum required to sustain level (or circular) flight.
The animation shown in the link of post #1 is great showing how it happens.
In the same way, models that are overloaded (cargo competition) and models that have high wing loads (scale, warbirds) need to fly relatively faster than lighter models.
That is the reason for which nose heavy planes show a faster landing approach and touchdown.
Note as well that the only other way to reduce the stall speed in flight is by increasing the CL max. This is possible by deploying flaps, which are able to almost double the CL max of any airfoil in the wing span that they cover.
That is not an option for models without flaperons or flaps.
The point that I try to make (and understand myself) more clearly is that reaching the stall speed isn’t necessarily followed by a stall of the wing (as the term may suggest or induce to believe).
According to the Merriam-Webster dictionary, stall is the condition of an airfoil or aircraft in which excessive angle of attack causes disruption of airflow with attendant loss of lift.
1) Hence, there is no stall if there is no disruption or substantial separation of the airflow over the wing.
2) Below the stall speed, if the pilot keeps the AOA below the critical value, there is no stall but the plane looses altitude without changes in pitch or loss of control.
3) During the condition described in 2) above, if the pilot feeds elevator up, trying to regain altitude, a stall will follow. The same will happen during straight or circular flight paths.
Any comments, discussions and criticisms are very welcome.
I also believe that the response to control inputs and the deflection of the elevator stick in the transmitter are better indicators of how close our planes are to a stall.
Hence, the actual stall speed of any model does not have much practical value.
However, the term is useful as a comparison of performance of one same model.
According to the late Andy Lennon, the formula for stall speed at sea level is:
Stall speed [mph] = Square root { (Weight [ounces] x 3519) / (CL max x Wing area [sq.in.] ) }
This means that any change in the weight that the wing “feels” will induce a change in the stall speed or the air speed below which the airplane cannot create enough lift to sustain its weight in level (1g) flight just below the critical AOA (which is a constant value for any particular wing-Re).
Note that the relationship is not linear but exponential; e.g., certain change in weight will induce a relatively smaller change in the stall speed (square root).
The wings feel more weight in banked turns, coming out of dives, in loops, due to extra load and due to advanced CG locations.
Staying below the critical AOA in order to avoid a wing stall, those maneuvers require more air speed than the minimum required to sustain level (or circular) flight.
The animation shown in the link of post #1 is great showing how it happens.
In the same way, models that are overloaded (cargo competition) and models that have high wing loads (scale, warbirds) need to fly relatively faster than lighter models.
That is the reason for which nose heavy planes show a faster landing approach and touchdown.
Note as well that the only other way to reduce the stall speed in flight is by increasing the CL max. This is possible by deploying flaps, which are able to almost double the CL max of any airfoil in the wing span that they cover.
That is not an option for models without flaperons or flaps.
The point that I try to make (and understand myself) more clearly is that reaching the stall speed isn’t necessarily followed by a stall of the wing (as the term may suggest or induce to believe).
According to the Merriam-Webster dictionary, stall is the condition of an airfoil or aircraft in which excessive angle of attack causes disruption of airflow with attendant loss of lift.
1) Hence, there is no stall if there is no disruption or substantial separation of the airflow over the wing.
2) Below the stall speed, if the pilot keeps the AOA below the critical value, there is no stall but the plane looses altitude without changes in pitch or loss of control.
3) During the condition described in 2) above, if the pilot feeds elevator up, trying to regain altitude, a stall will follow. The same will happen during straight or circular flight paths.
Any comments, discussions and criticisms are very welcome.
11-10-2010, 03:46 AM
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RE: Is the term “stall speed” proper for model airplanes?
Lnewqban,
Yeah I'd agree with most of that - the formula shows that increasing G or mass will increase the stall speed just as increasing CL_max or wing area will reduce the stall speed.
A couple of points:
The CG position (partly) determines the amount of download on the tail, which has an effect on the CL_max of the whole model (not the wing). If the tail has a download then this negative lift is subtracted from the overall lift of the model. The wing will reach the same CL_max but the overall CL_max of the model will be lower - resulting in an increased stall speed.
If the CG is back far enough for the tail to have a no load then the wings full CL will be used for sustaining height and the stall speed will be lower. Taking this even further - a lifting tail will be able to fly slower still (ignoring problems with control and drag!)
The actual CG position for no load on the tail is also related to the pitching moment of the airfoil - so I would say that your statement "The wings feel more weight ...due to advanced CG locations." is a slight over-simplification... In fact, with a symmetrical airfoil section the best place for CG (in terms of stall speed) would be at 25% chord, whereas the same model with a highly cambered airfoil would have a tail download with the CG at 25%.
It would perhaps be more accurate to say that the models maximum CL is reduced by the tails having any significant download. This is as much the case in high G turns as it is in level flight.
The other point is that at model Reynolds numbers, stall and seperation is less predictable than in full size aircraft. This can be seen in the statistical scatter present in low speed wind tunnel tests. Sometimes the wing hangs on and stalls at a greater aoa and sometimes it gives up at a low aoa. This can even be affected by accoustic noise levels! Hysteresis and seperation bubbles can play a part and more often than not often there is seperated flow on the airfoil at way above what we would think of as stall speed.
Add to this the changing effects of air density as a result of temperature and humidity (Andy Lennon used the correction factor σ (sigma) which is 1 at sea level and standard conditions) and you can see that pinning down the 'stall speed' becomes very difficult!
Another factor is the stall characteristics of the airfoil used - some will gently descend as the lift drops off at CL_max - others will push the nose down firmly, some will snap. In practice too, there is the designed application of the airfoil and the wing characteristics - lift distribution, washout etc will all play a part in how the aircraft reacts.
Then in the real world any yaw or gust will have an effect on the way a model stalls.
It becomes obvious that 'stall speed' can only ever be something used as a rough guide for RC pilots and something to aim at for designers. The most important thing, I suppose, is to become familiar with the models near-stall behaviour, understand the principles behind it and learn how real world conditions affect your perception of speed.
And keep the airspeed up on finals!
Yeah I'd agree with most of that - the formula shows that increasing G or mass will increase the stall speed just as increasing CL_max or wing area will reduce the stall speed.
A couple of points:
The CG position (partly) determines the amount of download on the tail, which has an effect on the CL_max of the whole model (not the wing). If the tail has a download then this negative lift is subtracted from the overall lift of the model. The wing will reach the same CL_max but the overall CL_max of the model will be lower - resulting in an increased stall speed.
If the CG is back far enough for the tail to have a no load then the wings full CL will be used for sustaining height and the stall speed will be lower. Taking this even further - a lifting tail will be able to fly slower still (ignoring problems with control and drag!)
The actual CG position for no load on the tail is also related to the pitching moment of the airfoil - so I would say that your statement "The wings feel more weight ...due to advanced CG locations." is a slight over-simplification...
In fact, with a symmetrical airfoil section the best place for CG (in terms of stall speed) would be at 25% chord, whereas the same model with a highly cambered airfoil would have a tail download with the CG at 25%.It would perhaps be more accurate to say that the models maximum CL is reduced by the tails having any significant download. This is as much the case in high G turns as it is in level flight.
The other point is that at model Reynolds numbers, stall and seperation is less predictable than in full size aircraft. This can be seen in the statistical scatter present in low speed wind tunnel tests. Sometimes the wing hangs on and stalls at a greater aoa and sometimes it gives up at a low aoa. This can even be affected by accoustic noise levels! Hysteresis and seperation bubbles can play a part and more often than not often there is seperated flow on the airfoil at way above what we would think of as stall speed.
Add to this the changing effects of air density as a result of temperature and humidity (Andy Lennon used the correction factor σ (sigma) which is 1 at sea level and standard conditions) and you can see that pinning down the 'stall speed' becomes very difficult!
Another factor is the stall characteristics of the airfoil used - some will gently descend as the lift drops off at CL_max - others will push the nose down firmly, some will snap. In practice too, there is the designed application of the airfoil and the wing characteristics - lift distribution, washout etc will all play a part in how the aircraft reacts.
Then in the real world any yaw or gust will have an effect on the way a model stalls.
It becomes obvious that 'stall speed' can only ever be something used as a rough guide for RC pilots and something to aim at for designers. The most important thing, I suppose, is to become familiar with the models near-stall behaviour, understand the principles behind it and learn how real world conditions affect your perception of speed.
And keep the airspeed up on finals!
11-11-2010, 07:10 AM
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RE: Is the term “stall speed” proper for model airplanes?
"It becomes obvious that 'stall speed' can only ever be something used as a rough guide for RC pilots and something to aim at for designers. The most important thing, I suppose, is to become familiar with the models near-stall behaviour, understand the principles behind it and learn how real world conditions affect your perception of speed.
And keep the airspeed up on finals! "
I'd agree with all of that. As I mentioned, flying an RC airplane is not really an acedemic exercise, its more seat of the pants.
Understanding the acedemics behind the model wil help the pilot look for tell tail signs of pending doom. Experience will largely do the same.
And keep the airspeed up on finals! "
I'd agree with all of that. As I mentioned, flying an RC airplane is not really an acedemic exercise, its more seat of the pants.
Understanding the acedemics behind the model wil help the pilot look for tell tail signs of pending doom. Experience will largely do the same.
04-12-2013, 07:45 AM
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RE: Is the term “stall speed” proper for model airplanes?
From what I've seen working with pneumatic transport systems and "vacuum" pumps, suction or vacuum simply does not exist. It is just air pressure trying to equalize.
The carrier in a pneumatic system, like those you see at banks, is propelled by atmospheric pressure (close to 15 psi) pushing into a lower pressure (what ever the fan or pump on the unit can generate) area. Simply air pressure trying to equalize.
The carrier in a pneumatic system, like those you see at banks, is propelled by atmospheric pressure (close to 15 psi) pushing into a lower pressure (what ever the fan or pump on the unit can generate) area. Simply air pressure trying to equalize.
