If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Polars and G-forces
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'. |
#2
|
|||
|
|||
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. |
#3
|
|||
|
|||
|
#4
|
|||
|
|||
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 |
#5
|
|||
|
|||
it reflects the wing loading, so it is the 1.5G force - as you put it.
Rgds, Derrick. |
Thread Tools | |
Display Modes | |
|
|