REFLECTOR:VG's

[steve korney]

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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