Polars and G-forces

[Ken Kochanski]

As we all know, adding ballast to a glider shifts the polar, allowing
higher speeds at better L/D ratios.

What happens when we are pulling, say 1.5 gees, as we thermal. Does
our L/D and sink rate reflect the 1 gee weight ... or the 1.5 gee
'force'.

[Greg Arnold]

Ken Kochanski wrote:

> As we all know, adding ballast to a glider shifts the polar, allowing
> higher speeds at better L/D ratios.
>
> What happens when we are pulling, say 1.5 gees, as we thermal. Does
> our L/D and sink rate reflect the 1 gee weight ... or the 1.5 gee
> 'force'.

The 1.5g force shifts the polar. This causes the minimum sink (and
stall and maximum L/D) to be at a higher speed, which is why we fly
faster when thermalling than when flying at minimum sink in level flight.

[Andreas Maurer]

On 2 Jan 2004 16:17:33 -0800, (Ken Kochanski)
wrote:


>What happens when we are pulling, say 1.5 gees, as we thermal. Does
>our L/D and sink rate reflect the 1 gee weight ... or the 1.5 gee
>'force'.

If you pull any G-force, your wing needs to create more lift.

There are two ways to do this:

1. You fly faster than in level flight, but with the same AoA,
therefore with the same Cl and Cd (= L/D). Sink rate will be higher
due to higher speed.

2. You fly at the same speed as in level flight, resulting in the
necessity to fly at a higher AoA to get a higher Cl.

The higher Cl results in a different (usually higher) Cd of the
airfoil, and in a higher induced drag. Higher drag means higher sink
rate...

In other words: The more G's you pull, the higher your sink rate will
be.



Bye
Andreas

[ir. K.P. Termaat]

Andreas Maurer > wrote in message >...
> On 2 Jan 2004 16:17:33 -0800, (Ken Kochanski)
> wrote:
>
>
> >What happens when we are pulling, say 1.5 gees, as we thermal. Does
> >our L/D and sink rate reflect the 1 gee weight ... or the 1.5 gee
> >'force'.
>
> If you pull any G-force, your wing needs to create more lift.
>
> There are two ways to do this:
>
> 1. You fly faster than in level flight, but with the same AoA,
> therefore with the same Cl and Cd (= L/D). Sink rate will be higher
> due to higher speed.
>
> 2. You fly at the same speed as in level flight, resulting in the
> necessity to fly at a higher AoA to get a higher Cl.
>
> The higher Cl results in a different (usually higher) Cd of the
> airfoil, and in a higher induced drag. Higher drag means higher sink
> rate...
>
> In other words: The more G's you pull, the higher your sink rate will
> be.
>
>
>
> Bye
> Andreas

Hi Andreas,

What you say is basically right. A small typing error though. Cd = Cl
x D/L, where D/L is 1/GR, where GR = glide ratio at the specific speed
flown.
If g-load increase the polar of the glider shifts somewhat to the
right and down, as happens when filling the wings with water. For a
g-load of 1.3 the shift of the polar is sqrt(1.3) to the right and
down. A g-load of 1.3 corresponds to a banking angle of about 40° at
110 km/h.

If g-load increases, L has to increase as you correctly say. This can
be done in two ways acc. to the lift formula: L = (1/2 x rho x v^2) x
S x Cl
1. increase flying speed (very effective because it appears squared in
the lift formula)
2. increase Cl, by increasing angle of attack (about linear effect on
Cl)

In both cases the sinkrate increases relative to level flight with
g-load = 1 because of the downward polar shift (drag increases in both
cases).

The drawback of flying with higher speed is that the circle radius R =
v^2 /(9.8 x tg phi), where phi is banking angle, increases
quadratically with speed and one may easily circle quite much away
from the core of the thermal.
The drawback of flying with a higher angle of attack is that one
cannot circle slower then stalling speed at the higher g-load
(sqrt(1.3) higher then in level flight in this example. Moreover
inside the thermal the glider is more difficult to handle (some more
then others).

In the practical situation one will circle with a somewhat higher
speed and a somewhat higher angle of attack in trying to get up as
quickly as possible.

Karel, NL

[Derrick Steed]

it reflects the wing loading, so it is the 1.5G force - as you put it.

Rgds,

Derrick.