v-glide an vy on cessna172
 

Hi,

Does anyone know why vy is 74 and v-glide is 68 on the cessna 172 sp.I taught they would be the same as they both go for best l/d ratio ?

Mucky

I'll wait for the experts to correct me... but my understanding is that, with power on, the airflow from the propellor over both the wing and the tailplane results in a different speed for the same angle of attack when compared to power off. Thus, Vy and Vg will both be at the same alpha, the difference in speed being due to the different airflow.

FFF
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I think differently. The two speeds aren't necessarily the same.

Vy is the point where there is the greatest excess power. It is the difference between two independent curves ie power available & power required.

Vg occurs at the best L/D ratio ie minimum drag, a different curve from the other two. Min drag is just that. The lift required is being obtained for the least amount of drag.

Power available changes with speed.
Power required is derived from Thrust required x TAS. This the same as Drag x TAS. Multiplying the two together gives a different curve to the original Drag curve.


For the sake of argument, change the engine characteristics slightly and the power available can change, leading to a different point between Power Avail. & Power required curves where they're furthest apart.

This hasn't necessarily changed V min drag so hasn't changed Vg.

To add to Tinstaafl's comments:
the simple theory has a number of assumptions which simply are not correct for simple aeroplanes
eg a simple drag polar with no power effects etc
a constant power output & propeller efficiency, or constant THP (thrust horse-power), which is far from true for a fixed pitch propeller

If anyone is interested there's a detailed performance analysis of the 172 online here.
(there is an error there too, I see!)

djpil, your quote

(there is an error there too, I see!)

can you expand a bit please? I've read Dr Lowry's article, but can't see what you're referring too

Hawk

The only thing I noticed was that one of their fundamental data numbers - subsequently used to derive further information - was based on the measured glide performance through a thickness of the atmosphere. It's a form of average across the changing density altitude. This then gets used to determine performance for specific density altitudes.

Perhaps this a negligible error given short distance over which it was measured? Given that the propeller performance is also from a line of best fit so isn't exact either?

Sorry, my problem disappeared when I spent more time looking at it - I had assumed a different notation.