v-glide an vy on cessna172
Why do it if it's not fun?
Joined: Jul 2001
Posts: 4,782
Likes: 12
From: Bournemouth
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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FFF
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Joined: Dec 1998
Posts: 4,282
Likes: 6
From: Escapee from Ultima Thule
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.
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.
Last edited by Tinstaafl; 25th August 2004 at 19:17.
Joined: May 2002
Posts: 1,210
Likes: 56
From: Melbourne, Australia
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!)
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!)
Joined: Dec 1998
Posts: 4,282
Likes: 6
From: Escapee from Ultima Thule
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?
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?
