Is the term “stall speed” proper for model airplanes?
#1
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Is the term “stall speed” proper for model airplanes?
Is the term “stall speed” proper for model airplanes?
Stalls of airplane wings depend only on critical angle of attack (AOA), regardless the air speed.
However, lacking AOA indicators, full scale pilots have used the speed indicators as an indirect way to avoid reaching the critical angle of attack, beyond which the wing stalls.
That critical AOA is associated to a minimum air speed for each plane and wing configuration.
Since model airplanes have no speed indicator, is the term “stall speed” proper for model airplanes?
In addition, I believe the term produces certain confusion among RC pilots.
In many RCU threads I read “stall speed” in reference to wing stalls induced by trespassing that mysterious minimum speed or simply by flying too slow.
If the wing of the model airplane remains below the critical AOA, the wing will produce a lift force.
Just a hair below the critical AOA, the minimum air speed required to produce a lift force equal to the weight of the plane could be call “stall speed”.
Below that speed, the plane will start to descend slowly and controlled because the lift force will be less that the weight, not because the wing will stall.
Copied from http://en.wikipedia.org/wiki/Stall_s...speed.E2.80.9D
“Stalls depend only on angle of attack, not airspeed.
Because a correlation with airspeed exists, however, a "stall speed" is usually used in practice.
It is the speed below which the airplane cannot create enough lift to sustain its weight in 1g flight.
In steady, level flight (1g), the faster an airplane goes, the less angle of attack it needs to hold the airplane up (i.e., to produce lift equal to weight).
As the airplane slows down, it needs to increase angle of attack to create the same lift (equal to weight).
As the speed slows further, at some point the angle of attack will be equal to the critical (stall) angle of attack.
This speed is called the "stall speed".
The angle of attack cannot be increased to get more lift at this point and so slowing below the stall speed will result in a descent.
And so, airspeed is often used as an indirect indicator of approaching stall conditions.
The stall speed will vary depending on the airplane's weight and configuration (flap setting, etc.).”
Stalls of airplane wings depend only on critical angle of attack (AOA), regardless the air speed.
However, lacking AOA indicators, full scale pilots have used the speed indicators as an indirect way to avoid reaching the critical angle of attack, beyond which the wing stalls.
That critical AOA is associated to a minimum air speed for each plane and wing configuration.
Since model airplanes have no speed indicator, is the term “stall speed” proper for model airplanes?
In addition, I believe the term produces certain confusion among RC pilots.
In many RCU threads I read “stall speed” in reference to wing stalls induced by trespassing that mysterious minimum speed or simply by flying too slow.
If the wing of the model airplane remains below the critical AOA, the wing will produce a lift force.
Just a hair below the critical AOA, the minimum air speed required to produce a lift force equal to the weight of the plane could be call “stall speed”.
Below that speed, the plane will start to descend slowly and controlled because the lift force will be less that the weight, not because the wing will stall.
Copied from http://en.wikipedia.org/wiki/Stall_s...speed.E2.80.9D
“Stalls depend only on angle of attack, not airspeed.
Because a correlation with airspeed exists, however, a "stall speed" is usually used in practice.
It is the speed below which the airplane cannot create enough lift to sustain its weight in 1g flight.
In steady, level flight (1g), the faster an airplane goes, the less angle of attack it needs to hold the airplane up (i.e., to produce lift equal to weight).
As the airplane slows down, it needs to increase angle of attack to create the same lift (equal to weight).
As the speed slows further, at some point the angle of attack will be equal to the critical (stall) angle of attack.
This speed is called the "stall speed".
The angle of attack cannot be increased to get more lift at this point and so slowing below the stall speed will result in a descent.
And so, airspeed is often used as an indirect indicator of approaching stall conditions.
The stall speed will vary depending on the airplane's weight and configuration (flap setting, etc.).”
#2
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RE: Is the term “stall speed” proper for model airplanes?
It is as correct to use it with models as it is to use it with full scale. So, no it's not perfectly correct for models or fullscale.
But since it's in such common usage, it is as sensible to expect it to be understood as the term "tip stall" is understood.
The real problem with using stall speed to talk about models is the fact that almost none of us can judge a model's speed worth spit. Heck, can judge it at all. And worse, we can't judge the model's AOA either.
But since it's in such common usage, it is as sensible to expect it to be understood as the term "tip stall" is understood.
The real problem with using stall speed to talk about models is the fact that almost none of us can judge a model's speed worth spit. Heck, can judge it at all. And worse, we can't judge the model's AOA either.
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RE: Is the term “stall speed” proper for model airplanes?
I think the term is valid, although it helps if the underlying reasoning is understood.
In full scale aviation the V speeds are also somewhat arbitrary as they depend on payload and other factors.
I would define stall speed in a model as the level fight speed where handling of the aircraft is noticeably affected by the wing beginning to stall. Because we don't know airspeed or alpha it has to be the model's behaviour that defines it.
Of course actually it is the angle of attack where CL max is reached that is important and this can occur at higher speeds in a turn with increased G.
But in the same way that Vstall speed provides a 'danger area' to warn pilots when straying near the stall, knowing what the 'stall speed' of your model is (and being able to relate it to how the model looks in flight) should at least alert you to the possibility of a stall.
In full scale aviation the V speeds are also somewhat arbitrary as they depend on payload and other factors.
I would define stall speed in a model as the level fight speed where handling of the aircraft is noticeably affected by the wing beginning to stall. Because we don't know airspeed or alpha it has to be the model's behaviour that defines it.
Of course actually it is the angle of attack where CL max is reached that is important and this can occur at higher speeds in a turn with increased G.
But in the same way that Vstall speed provides a 'danger area' to warn pilots when straying near the stall, knowing what the 'stall speed' of your model is (and being able to relate it to how the model looks in flight) should at least alert you to the possibility of a stall.
#4
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RE: Is the term “stall speed” proper for model airplanes?
Unfortunately, none of us can tell the difference between 10mph ground speed and 20mph ground speed much less airspeed.
And of course, if there is any wind we can't see that to correct for it.
Think about your own flying. During landing, do you remember to feel what the wind on your neck is doing? Do you even make a rough estimate of how much of the underside of the model you can see? How often have you watched an "experienced" modeler land down wind. And often as not, then complain about how hard it was to get his bloody model to slow down or flare.
And of course, if there is any wind we can't see that to correct for it.
Think about your own flying. During landing, do you remember to feel what the wind on your neck is doing? Do you even make a rough estimate of how much of the underside of the model you can see? How often have you watched an "experienced" modeler land down wind. And often as not, then complain about how hard it was to get his bloody model to slow down or flare.
#5
RE: Is the term “stall speed” proper for model airplanes?
ORIGINAL: Lnewqban
Is the term “stall speed” proper for model airplanes?
Stalls of airplane wings depend only on critical angle of attack (AOA), regardless the air speed.
However, lacking AOA indicators, full scale pilots have used the speed indicators as an indirect way to avoid reaching the critical angle of attack, beyond which the wing stalls.
Since model airplanes have no speed indicator, is the term “stall speed” proper for model airplanes?
In addition, I believe the term produces certain confusion among RC pilots.
“Stalls depend only on angle of attack, not airspeed.
Is the term “stall speed” proper for model airplanes?
Stalls of airplane wings depend only on critical angle of attack (AOA), regardless the air speed.
However, lacking AOA indicators, full scale pilots have used the speed indicators as an indirect way to avoid reaching the critical angle of attack, beyond which the wing stalls.
Since model airplanes have no speed indicator, is the term “stall speed” proper for model airplanes?
In addition, I believe the term produces certain confusion among RC pilots.
“Stalls depend only on angle of attack, not airspeed.
The problem for an RC flyer is that the speed that he sees is not the same as what shows on an airspeed indicator. What we call speed can take many different forms, but we call them all "speed".
True Speed That's the actual speed of the airplane through the air. Headwinds and tailwinds don't affect true speed.
Ground Speed That's the speed that the airplane is making over the ground. Point to point, if you will. Headwinds and tailwinds do affect ground speed.
Indicated Airspeed This is what shows on an airspeed indicator, and what most directly affects the ability of a wing to make lift.
As an example, an airliner at high altitude may have an indicated airspeed of, say 300 and yet his true airspeed is more like 500. And if he has a 100 mph tailwind then his ground speed will be 600.
The problem for RC flyers is that we are seeing "Ground Speed", which may have little relation to the speed that most affects lift. Wind makes a big difference in what we see as "speed". Think about this: If our model has a stall speed of 20 mph (indicated airspeed) and we are flying with a 15 mph tailwind, then our airplane will stall with a ground speed of 35 mph. Conversely, if we have a 15 mph headwind, our model will stall at 5 mph groundspeed.
I prefer to think of the elevator stick as my stall indicator. If the model requires full aft stick to stall, then I know how much stall margin I have by feeling how much aft stick I'm using. This isn't exact science either because of trim changes due to CG, power, flaps, etc. But for a simple model, knowing how much aft stick is required to cause a stall is the most direct indicator that we have. And it works in level flight, in steeply banked turns, and even when doing a loop.
Dick
#6
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RE: Is the term “stall speed” proper for model airplanes?
I have no interest in "stall speeds", I can't know what the plane's speed is anyway, so I fly it based on what I observe it doing.
With experience, knowing when to expect the stall comes.
And one learns which airplanes should never be flown that slowly!
With experience, knowing when to expect the stall comes.
And one learns which airplanes should never be flown that slowly!
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RE: Is the term “stall speed” proper for model airplanes?
Paul, That's kind of what I meant. It's the planes behaviour that tells you whats going on. But If Im used to flying a lightly loaded indoor J3 cub that stalls at 10mph and you hand me a heavy jet model then telling me that it's stall speed is 30mph will have some meaning in terms of what to expect in the way of handling...
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RE: Is the term “stall speed” proper for model airplanes?
Well, you know when the stall speed is too low....the nose go's down....you also know when the AoA is too high...the nose lowers...
So...er...I vote "Yes"...I vote "NO"....."yes"..."No"...Yea..No...
Well it is important...because if you go lower ...and you are close to the ground...er...."Bang"...so yes....
but I agree you do not know how close you are to stall speed....except that generally you are on the backside of the power curve...and the sink rate go's up...
and even if you don't stall ...and you sink into the ground.....it is bad...so ....."No"
I hope this clears everything up....!
Steve
So...er...I vote "Yes"...I vote "NO"....."yes"..."No"...Yea..No...
Well it is important...because if you go lower ...and you are close to the ground...er...."Bang"...so yes....
but I agree you do not know how close you are to stall speed....except that generally you are on the backside of the power curve...and the sink rate go's up...
and even if you don't stall ...and you sink into the ground.....it is bad...so ....."No"
I hope this clears everything up....!
Steve
#9
RE: Is the term “stall speed” proper for model airplanes?
Models differ from their full scale counterparts in proportions. Some models have greater power than their full scale counterpart and enjoy a greater prop to wing length ratio with the result that power on the model will add more vector lift in proportion to full scale. So... even if there was a way to measure airspeed on a model, the numbers wouldn't correspond.
A couple of months back, the maiden of a large Aeronca Champ offered a gross illustration. It was about a 19lb airframe powered by a OS 1.6 twin, which got airborne and wallowed around the pattern at full power with nose slightly up. Attempts to drop the nose to gain speed didn't work as it would not transition from sustained flight by vector lift to airspeed lift only. It was fortunate that a good pilot who carefully managed the elevator was at the stick who was able to milk out the best lift between airspeed generated and vector generated.
A couple of months back, the maiden of a large Aeronca Champ offered a gross illustration. It was about a 19lb airframe powered by a OS 1.6 twin, which got airborne and wallowed around the pattern at full power with nose slightly up. Attempts to drop the nose to gain speed didn't work as it would not transition from sustained flight by vector lift to airspeed lift only. It was fortunate that a good pilot who carefully managed the elevator was at the stick who was able to milk out the best lift between airspeed generated and vector generated.
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RE: Is the term “stall speed” proper for model airplanes?
Sall speed is stall speed, whether you have a meter telling you what it is or not.
the term is valid.
Observing stall is the only ,easure we have as rc pilots so its more seat of the pants flying than full size I guess.
where the topic becomes blury is when 3D flying makes a mokery of conentional terminology since it's considered to be post stall flight.
However with 3D flight the propeller is providingboth the lift and the thrust so are the wings stalled? In a conventional sense yes but the prop wash is large enough to keep airlfow attached....
so is it really stalled....?
the term is valid.
Observing stall is the only ,easure we have as rc pilots so its more seat of the pants flying than full size I guess.
where the topic becomes blury is when 3D flying makes a mokery of conentional terminology since it's considered to be post stall flight.
However with 3D flight the propeller is providingboth the lift and the thrust so are the wings stalled? In a conventional sense yes but the prop wash is large enough to keep airlfow attached....
so is it really stalled....?
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RE: Is the term “stall speed” proper for model airplanes?
I'm not sure what the issue is here other than it is obviously related not to the stall speed but to the pilot's ability to sense or measure it.
In flight the wing does not know if there is a pilot on board or not. It also is not able to read an AoA or airspeed indicator. It just knows that when the air doesn't stick to the top surface anymore that it is stalled. At that point lift may or may not drop off significantly but the drag certainly goes up to a much higher value. If there is no power from a big jet engine or other propulsion source operating on steroids then the model quickly looses speed due to the drag and lift drops off and the nose drops in response.
In flight the wing does not know if there is a pilot on board or not. It also is not able to read an AoA or airspeed indicator. It just knows that when the air doesn't stick to the top surface anymore that it is stalled. At that point lift may or may not drop off significantly but the drag certainly goes up to a much higher value. If there is no power from a big jet engine or other propulsion source operating on steroids then the model quickly looses speed due to the drag and lift drops off and the nose drops in response.
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RE: Is the term “stall speed” proper for model airplanes?
Exactly - that stuff is happening regardless of whether the speed is known.
It may not be so useful to the pilot to know what speed that is, but I think for the designer it is a lot more important.
It may not be so useful to the pilot to know what speed that is, but I think for the designer it is a lot more important.
#14
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RE: Is the term “stall speed” proper for model airplanes?
As the OP pointed out, there's no such thing as stall speed in general, just stall speeds in various conditions. You can stall a plane flying as fast as it will go in level flight at full throttle if you want to, and if you have enough elevator. Still, it's a good idea to point out to beginners the danger of slowing down too much in particular situations, as in the dreaded "downwind turn," where the high groundspeed you get when flying downwind can fool the pilot into thinking that he has plenty of airspeed (and therefore, technically, a low angle of attack) and so can make a tight turn without letting the nose drop. I suppose you can and perhaps should explain this without saying "stall speed." While it's true that you can't estimate air speed well, it's just as true that, because you can't estimate air speed, you can't estimate your angle of attack either, since the angle of attack depends in part on the airspeed.
#15
RE: Is the term “stall speed” proper for model airplanes?
Good post Top-Gunn.... my simple addition would be the only thing more dreadful than a downwind turn is the base leg turn onto final with a strong cross wind blowing on the tail.. That has eaten my lunch twice. We are usually careful of the downwind turn but often fail to remember the crosswind turn, perhaps because sometimes it favors us.
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RE: Is the term “stall speed” proper for model airplanes?
ORIGINAL: Teachu2
I've never seen a model designer publish a stall speed for a model - but every model airplane has one!
I've never seen a model designer publish a stall speed for a model - but every model airplane has one!
V = √(2*M*g/ρ*S*CL_max)
where
M = mass in kg
g = 9.81 (at 1G)
ρ = 1.225 at sea level
S = wing area
CL_max from airfoil data
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RE: Is the term “stall speed” proper for model airplanes?
What needs to be remembered after being really driled into new pilots is that the term Speed in aviation is actually "Airspeed".
"Stall speed" should correctly be termed "stall airspeed".
Once we know we are always refering to air speed over the wing then it becomes easier to remember that downwind we have a lot less lift (hence planes tend to be flown on the elevator downwind).
"Stall speed" should correctly be termed "stall airspeed".
Once we know we are always refering to air speed over the wing then it becomes easier to remember that downwind we have a lot less lift (hence planes tend to be flown on the elevator downwind).
#20
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RE: Is the term “stall speed” proper for model airplanes?
ORIGINAL: TimBle
What needs to be remembered after being really driled into new pilots is that the term Speed in aviation is actually ''Airspeed''.
''Stall speed'' should correctly be termed ''stall airspeed''.
Once we know we are always refering to air speed over the wing then it becomes easier to remember that downwind we have a lot less lift (hence planes tend to be flown on the elevator downwind).
What needs to be remembered after being really driled into new pilots is that the term Speed in aviation is actually ''Airspeed''.
''Stall speed'' should correctly be termed ''stall airspeed''.
Once we know we are always refering to air speed over the wing then it becomes easier to remember that downwind we have a lot less lift (hence planes tend to be flown on the elevator downwind).
Really good idea
bad idea
We don't really have "a lot less lift" downwind at all. We don't even have a lot less lift in downwind turns. We can, and often do force our models on downwind turns into having a lot less airspeed, but it really is a basic truism (that is true) that airplanes aren't affected by constant winds.
We force the lot less airspeed by pulling too great an Angle of Attack, trying to get a normal looking airspeed. We either get too much AOA or a gust hits the plane and the existing AOA goes over the top. We're trying for what we think is a normal airspeed, but crause a dangerous AOA which often is "a lot more" AOA.
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RE: Is the term “stall speed” proper for model airplanes?
yes I can see that that can happen.
What I nooce on all my planes is that on the down wind leg it tends to drop the nose ever slightly so I fed on just a touch of up elevator, not to much but just enough to maintain level flight.
Yes I can compensate for the lowe airspeed by opening the throttle and increasing airspeed but it not always desirable.
Of course I can let her simply ride the ride but when its blowin 35km/hr the plane covers ground pretty quickly. For aerobatics, you need to slow it down or you don't have the room to fit in all the manuevres required for that leg.
I have not noticed my planes ever needing any special compensation when turning. Aileron, touch of elevator and manage the throttle to tighen or ease the turn and she gets arond through the wind pretty solid. Dependin on bank angle I may feed in a bit of rudder but on my Extra no rudder is needed since the CG is pretty much where it needs to be.
Flying, is has to be remembered, is very much a feel activity. We are either able to "feel" what the plane is doing and act accordingly or we can follow a well drilled set of rules. I find it easier to "feel" the plane out. This won't work for everybody.
What I nooce on all my planes is that on the down wind leg it tends to drop the nose ever slightly so I fed on just a touch of up elevator, not to much but just enough to maintain level flight.
Yes I can compensate for the lowe airspeed by opening the throttle and increasing airspeed but it not always desirable.
Of course I can let her simply ride the ride but when its blowin 35km/hr the plane covers ground pretty quickly. For aerobatics, you need to slow it down or you don't have the room to fit in all the manuevres required for that leg.
I have not noticed my planes ever needing any special compensation when turning. Aileron, touch of elevator and manage the throttle to tighen or ease the turn and she gets arond through the wind pretty solid. Dependin on bank angle I may feed in a bit of rudder but on my Extra no rudder is needed since the CG is pretty much where it needs to be.
Flying, is has to be remembered, is very much a feel activity. We are either able to "feel" what the plane is doing and act accordingly or we can follow a well drilled set of rules. I find it easier to "feel" the plane out. This won't work for everybody.
#22
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RE: Is the term “stall speed” proper for model airplanes?
A little research into aerodynamics and flight physics will go a long way. Just making assumptions based on what you think is happening is counter productive and leads to bad habits. There is a century of research and trial and error that has gone on before that will explain everything that is happening during flight. The fact that they are models makes no different in aerodynamics.
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RE: Is the term “stall speed” proper for model airplanes?
ORIGINAL: TimBle
.......Yes I can compensate for the lowe airspeed by opening the throttle and increasing airspeed but it not always desirable.
Of course I can let her simply ride the ride but when its blowin 35km/hr the plane covers ground pretty quickly. For aerobatics, you need to slow it down or you don't have the room to fit in all the manuevres required for that leg......
.......Yes I can compensate for the lowe airspeed by opening the throttle and increasing airspeed but it not always desirable.
Of course I can let her simply ride the ride but when its blowin 35km/hr the plane covers ground pretty quickly. For aerobatics, you need to slow it down or you don't have the room to fit in all the manuevres required for that leg......
Exactly the point that daRock was making. It is YOUR choice to "fly it on the elevator" by that statement.
Free flight flyers and RC glider pilots know that there's no such issue as downwind turns unless we make it so. With free flight the models fly in a trimmed steady circle. Same with gliders when holding a thermal turn. The models in both cases (if the trim is held constant with the RC) will happily turn what the model sees as a constant airspeed circle while the pilot on the ground sees woefully wonky looking turns with really LONG downwind legs. I've had my free flight models in strong enough winds that even the nose into the wind portion is still going backwards. At no time does the model have to "fly on the elevator" as it describes it's flight path downwind.
I wonder if my many hours of observing my free flight models and RC soaring models has accustomed me to the often radical ground speed differences in flying in the wind. I can't remember the last time I had any issues with loseing lift or the controls getting mushy in a downwind turn. But then thanks to the styles of models I fly I know better than to try to slow the model down just because I "THINK" it's going too fast.
Mind you I've also misjudged my last turn to position the model for landing on many an occasion and had to walk a few hundred feet on really blustery days. But I'd rather walk a bit than risk a stall or gusty weather turbulence flipping the model up and over.
Keeping the airspeed up a bit so I can maneuver the model on really windy days with lots of energetic turbulence is paramount for safe flying. We all know how our models get mushy on the controls and slow to react when slowed down and near the stall. Avoiding that situation on the stormy days is the key to survival during takeoff and landing in the first 50 feet where you've got lots of energetic rolling turbulence trying to "bend, spindle and mutilate" the model
Anyhow let's review;
[ul][*] Yes models stall. Just like the real ones they have a stall speed that is linked to the angle of attack of the wing section. This angle is reached at a lower or higher airspeed based on the wing loading of the model.[*] They stall just like real airplanes but often at a lower angle of attack due to Reynolds number effects. The smaller and slower they fly the lower the angle.[*] They can stall at any airspeed if the angle of attack gets high enough. Hence the rather amazing video of that Red Bull racer that snapped out of a turn and dipped his wingtip into the water. And the excitement of seeing an RC pylon racing model snap roll out of a turn and dig in with the ensuing "explosion" and rain of balsa, foam and fiberglass over a 10 foot by 100 foot swath I once saw.[*] We ground based pilots are extremely poor judges of speed. So we need to rely on how the model reacts to our control inputs. If it's getting mushy to respond then you're too slow and it doesn't matter WHAT the ground speed looks like. Get the nose down or throttle up a notch or you're setting up to be bitten by a simple stall performed under misleading conditions if you're traveling downwind on a windy day.
[/ul]
#24
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RE: Is the term “stall speed” proper for model airplanes?
Thanks for the posts, guys!
Thank you Yak 52 for the formula.
The downwind turns that we are discussing now are perfect examples of RCU posts where the term stall speed has been used.
The stall speed increases in turns due to centrifugal force.
The extra g's of a turn or loop, impose higher load on the wing.
It is like the weight (m x g in the formula posted above) increase; hence, the minimum speed before stall increases.
For the particular case of the downwing leg, we tend to cut the throttle down to reduce the ground speed.
Then, we enter the turn too close to the incresed and necessary stall speed.
Thank you Yak 52 for the formula.
The downwind turns that we are discussing now are perfect examples of RCU posts where the term stall speed has been used.
The stall speed increases in turns due to centrifugal force.
The extra g's of a turn or loop, impose higher load on the wing.
It is like the weight (m x g in the formula posted above) increase; hence, the minimum speed before stall increases.
For the particular case of the downwing leg, we tend to cut the throttle down to reduce the ground speed.
Then, we enter the turn too close to the incresed and necessary stall speed.
#25
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RE: Is the term “stall speed” proper for model airplanes?
the excitement of seeing an RC pylon racing model snap roll out of a turn and dig in with the ensuing "explosion" and rain of balsa, foam and fiberglass over a 10 foot by 100 foot swath I once saw.
Regardless, when you remove the lift from one side of a pylon racer when it is in a heavy loaded condition during the turn, the resulting crash happens amazingly quick. Since the usual wing panel affected is the left panel, the snap into the ground occurs in the blink of an eye. It is more exciting when it happens just over your right shoulder and the debris field is 50 feet away.