REFLECTOR:VG's
Jim Sower
reflector@tvbf.org
Mon, 08 Dec 2003 19:19:27 -0600
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<... Van should issue a warning ...>
I would guess that a hazard exists only to the extent that you are in turbulence that
will put more than 6 Gs on the airplane or doing hard aerobatics while paying no
attention at all to Gs (hard to visualize). The only time I am personally interested
in maneuvering speed is when I am involved in either aerobatics, ACM or serious
turbulence. My Velocity is unsuited for the first two, and I am unsuited for the last
(all of my encounters with thunderstorms over the years have been decidedly negative
:o(.
It's safe to assume that installing VGs will have the direct collateral effect of
reducing maneuvering speed and the science you cite below should quantify the effect,
but it's a good bit more involved than is really necessary. Since lift is a function
of V^2, and maneuvering speed is a stall speed, for any given weight, etc.,
maneuvering speed will be stall speed times the square root of the G-limit (all
airspeeds Indicated). For an airplane that is stressed to 6 Gs, Vm = Vs * (6 ^ .5) =
Vs * 2.45. Likewise, for an airplane stressed for 4 Gs, Vm = Vs * 4^.5 = Vs * 2.
In the interest of picking nits, canard maneuvering speed can be further complicated.
Vs in a Velocity is a function of aircraft weight and CG. Vs will be lower at aft CG
and higher at forward CG since stall happens at canard stall, and at a given weight
you run out of pitch authority at a higher speed at forward CG than would be the case
for aft CG. Quantitatively, those few knots are probably not significant enough to
worry about.
Of course, in turbulence, you don't know if the load will be positive or negative, so
one could argue that T-storm penetration speed should be Vs times the negative load
limit or Vs * 3^.5 = Vs * 1.73. Naturally, this would be independent of whether or
not VGs are present since they have no effect on negative stall.
And it goes on and on ....
It is a handy thing to know about but my own Vm = arbitrarily twice Vs ... Jim S.
steve korney wrote:
> Here is an interesting article on VG's
>
> Vortex Generators - Stall Speeds and Maneuvering Speed
>
> Various opinions have recently been given on the effect of vortex generators
> on stall speeds and maneuvering speeds. I believe that it is appropriate
> that an engineering approach should be shown as well. If the calculations
> below can be verified then Van should issue a warning that a potential
> hazard can exist.
>
> In Section 15 of Van's Manual a definition is given of maneuvering speed.
> 1. The maximum speed at which full abrupt controls can be applied.
> 2. The minimum speed at which limit G-load can be produced.
>
> So in the RV6 the G limit has been established to be 6g's and at 132 mph a
> full abrupt pull on the stick will produce 6g's before it stalls. At lower
> speeds less than 6g's will result and at higher speeds more than 6g's will
> result.
>
> So how is this related to the stall speed?
> The answer is in the lift equation.
> L = Cl x r x V squared / 2 x A
>
> L = lift
> Cl = coefficient of lift
> r = air density
> V = velocity
> A = wing area
>
> In straight and level flight the Lift is equal to the gross weight (1600
> lbs)
>
> In a 6 g turn, Lift must be 1600 x 6 = 9600 lbs
>
> Lift(1) = 1600 stall speed is 54 mph and an abrupt pull will give only 1 g
> max
> Lift(2) = 9600 if we don't know this speed we can calculate it.
>
> Lift(2) = 6 x Lift(1)
>
> by substituting in the lift equation
> Lift(2) = Cl x r x V(2)squared/2 x A
> 6 x Lift(1) = 6 x Cl x r x V(1)squared/2 / A
>
> Cl x r x V(2)squared/2 x A = 6 x Cl x r x V(1)squared/2 x A
>
> This reduces to
> V(2)squared = 6 x V(1)squared
> V(2)squared = 6 x 54 x 54 = 17,496
> V(2) = sqrt (17,496) = 132 mph
>
> So an abrupt pull at 132 mph will give 6g and then a stall, just like Van
> says.
>
> Now we have some hard evidence that the vg's reduce the stall speed on RV's.
> >From the lift equation, everything must remain the same except the
> coefficient of lift. So adding vg's changes the coefficient of lift. It is
> in effect, changing the airfoil. The mechanism has been studied to death.
> The turbulent flow created by the vgs allows the boundary layer to remain
> attached farther back on the airfoil at high angles of attack, and allows
> greater angles of attack before separation occurs.
> Does the maneuvering speed change too? You bet it does.
>
> Let's use Terry Jantzi's numbers.
> We can see evidence since the 2g stall speed has changed.
>
> First, without the vgs installed
>
> 6 x V(1)squared = 6 x 56 x 56 = 18,816
> sqrt (18,816) = 137 mph (119 knots)
>
> So Terry would be able to pull 6 g's and no more at 119 knots
>
> With the vgs installed
>
> 6 x V(1)squared = 6 x 53 x 53 = 16,854
> sqrt (16854) = 130 mph (113 knots)
>
> So with the vg's installed he can pull 6 g's at only 113 knots
>
> Is there a hazard?
> If he has calculated the maneuvering speed to be 137 mph from the no vg
> condition and assumes it is good for the with vg condition, how many g's is
> he able to pull with full abrupt stick movement at 137 mph?
>
> 137 x 137 / 53 x 53 = 6.7 g's
>
> In Terry's case the change in stall speed was minimal.
> What happens when the change is greater and the stall speeds are lower?
>
> Larry says that his RV4 slow flight speed has gone down from 40 mph to 30
> mph. He must be skinny. Van says the stall speed should be 48 mph.
> Van's maneuvering speed would be
> Sqrt (6 x 48 x 48) = 118 mph
>
> Since Larry's RV4 with the vg's will fly at 30, I am going to assume that at
> gross weight it will stall at 40 mph.
>
> If Larry flys at 118 mph with the vg's on and pulls back hard on his stick,
> he will expose his RV4 to
>
> (118 x 118) / ( 40 x 40) = 8.7 g's
>
> Wow! Larry's plane flown solo could turn around in my yard.
>
> His maneuvering speed with the vg's should be
> Sqrt(6 x 40 x 40) = 98 mph
>
> That's a fairly large change.
> It is a potential hazard?
> I think so. The correct maneuvering speed should be determined.
>
> Kinda makes you think...
>
> Best... Steve
>
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<... Van should issue a warning ...>
<br>I would guess that a hazard exists only to the extent that you are
in turbulence that will put more than 6 Gs on the airplane or doing hard
aerobatics while paying no attention at all to Gs (hard to visualize).
The only time I am personally interested in maneuvering speed is when I
am involved in either aerobatics, ACM or serious turbulence. My Velocity
is unsuited for the first two, and I am unsuited for the last (all of my
encounters with thunderstorms over the years have been decidedly negative
:o(.
<p>It's safe to assume that installing VGs will have the direct collateral
effect of reducing maneuvering speed and the science you cite below should
quantify the effect, but it's a good bit more involved than is really necessary.
Since lift is a function of V^2, and maneuvering speed <b>is</b> a stall
speed, for any given weight, etc., maneuvering speed will be stall speed
times the square root of the G-limit (all airspeeds Indicated). For
an airplane that is stressed to 6 Gs, Vm = Vs * (6 ^ .5) = Vs * 2.45.
Likewise, for an airplane stressed for 4 Gs, Vm = Vs * 4^.5 = Vs * 2.
<p>In the interest of picking nits, canard maneuvering speed can be further
complicated. Vs in a Velocity is a function of aircraft weight <b>and</b>
CG. Vs will be lower at aft CG and higher at forward CG since stall
happens at canard stall, and at a given weight you run out of pitch authority
at a higher speed at forward CG than would be the case for aft CG.
Quantitatively, those few knots are probably not significant enough to
worry about.
<p>Of course, in turbulence, you don't know if the load will be positive
or negative, so one could argue that T-storm penetration speed should be
Vs times the <b>negative</b> load limit or Vs * 3^.5 = Vs * 1.73.
Naturally, this would be independent of whether or not VGs are present
since they have no effect on negative stall.
<p>And it goes on and on ....
<p>It is a handy thing to know about but my own Vm = arbitrarily twice
Vs ... Jim S.
<p>steve korney wrote:
<blockquote TYPE=CITE>Here is an interesting article on VG's
<p>Vortex Generators - Stall Speeds and Maneuvering Speed
<p>Various opinions have recently been given on the effect of vortex generators
<br>on stall speeds and maneuvering speeds. I believe that it is appropriate
<br>that an engineering approach should be shown as well. If the calculations
<br>below can be verified then Van should issue a warning that a potential
<br>hazard can exist.
<p>In Section 15 of Van's Manual a definition is given of maneuvering speed.
<br>1. The maximum speed at which full abrupt controls can be applied.
<br>2. The minimum speed at which limit G-load can be produced.
<p>So in the RV6 the G limit has been established to be 6g's and
at 132 mph a
<br>full abrupt pull on the stick will produce 6g's before it stalls. At
lower
<br>speeds less than 6g's will result and at higher speeds more than 6g's
will
<br>result.
<p>So how is this related to the stall speed?
<br>The answer is in the lift equation.
<br>L = Cl x r x V squared /
2 x A
<p>L = lift
<br>Cl = coefficient of lift
<br>r = air density
<br>V = velocity
<br>A = wing area
<p>In straight and level flight the Lift is equal to the gross weight (1600
<br>lbs)
<p>In a 6 g turn, Lift must be 1600 x 6 = 9600 lbs
<p>Lift(1) = 1600 stall speed is 54 mph and an abrupt pull
will give only 1 g
<br>max
<br>Lift(2) = 9600 if we don't know this speed we can calculate
it.
<p>Lift(2) = 6 x Lift(1)
<p>by substituting in the lift equation
<br>Lift(2) = Cl x r x V(2)squared/2 x A
<br>6 x Lift(1) = 6 x Cl x r x V(1)squared/2
/ A
<p>Cl x r x V(2)squared/2 x A = 6 x
Cl x r x V(1)squared/2 x A
<p>This reduces to
<br>V(2)squared = 6 x V(1)squared
<br>V(2)squared = 6 x 54 x 54 = 17,496
<br>V(2) = sqrt (17,496) = 132 mph
<p>So an abrupt pull at 132 mph will give 6g and then a stall, just like
Van
<br>says.
<p>Now we have some hard evidence that the vg's reduce the stall speed
on RV's.
<br>>From the lift equation, everything must remain the same except the
<br>coefficient of lift. So adding vg's changes the coefficient of lift.
It is
<br>in effect, changing the airfoil. The mechanism has been studied to
death.
<br>The turbulent flow created by the vgs allows the boundary layer to
remain
<br>attached farther back on the airfoil at high angles of attack, and
allows
<br>greater angles of attack before separation occurs.
<br>Does the maneuvering speed change too? You bet it does.
<p>Let's use Terry Jantzi's numbers.
<br>We can see evidence since the 2g stall speed has changed.
<p>First, without the vgs installed
<p>6 x V(1)squared = 6 x 56 x 56 = 18,816
<br> sqrt (18,816) = 137 mph
(119 knots)
<p> So Terry would be able to pull 6 g's and no more at 119 knots
<p>With the vgs installed
<p>6 x V(1)squared = 6 x 53 x 53 = 16,854
<br> sqrt (16854) = 130 mph (113
knots)
<p> So with the vg's installed he can pull 6 g's at only 113 knots
<p>Is there a hazard?
<br>If he has calculated the maneuvering speed to be 137 mph from the no
vg
<br>condition and assumes it is good for the with vg condition, how
many g's is
<br>he able to pull with full abrupt stick movement at 137 mph?
<p>137 x 137 / 53 x 53
= 6.7 g's
<p>In Terry's case the change in stall speed was minimal.
<br>What happens when the change is greater and the stall speeds are lower?
<p>Larry says that his RV4 slow flight speed has gone down from 40 mph
to 30
<br>mph. He must be skinny. Van says the stall speed should be 48 mph.
<br>Van's maneuvering speed would be
<br>Sqrt (6 x 48 x 48) = 118 mph
<p>Since Larry's RV4 with the vg's will fly at 30, I am going to assume
that at
<br>gross weight it will stall at 40 mph.
<p>If Larry flys at 118 mph with the vg's on and pulls back hard on his
stick,
<br>he will expose his RV4 to
<p> (118 x 118) / ( 40 x 40) =
8.7 g's
<p>Wow! Larry's plane flown solo could turn around in my yard.
<p>His maneuvering speed with the vg's should be
<br> Sqrt(6 x 40 x 40) = 98 mph
<p>That's a fairly large change.
<br>It is a potential hazard?
<br>I think so. The correct maneuvering speed should be determined.
<p>Kinda makes you think...
<p>Best... Steve
<p>_________________________________________________________________
<br>Tired of slow downloads and busy signals? Get a high-speed Internet
<br>connection! Comparison-shop your local high-speed providers here.
<br><a href="https://broadband.msn.com">https://broadband.msn.com</a>
<p>_______________________________________________
<br>To change your email address, visit <a href="http://www.tvbf.org/mailman/listinfo/reflector">http://www.tvbf.org/mailman/listinfo/reflector</a>
<p>Visit the gallery! www.tvbf.org/gallery
<br>user:pw = tvbf:jamaicangoose
<br>Check new archives: www.tvbf.org/pipermail
<br>Check old archives: <a href="http://www.tvbf.org/archives/velocity/maillist.html">http://www.tvbf.org/archives/velocity/maillist.html</a></blockquote>
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