Principles of flight question
Thread Starter
Join Date: Aug 2002
Location: uk
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
Principles of flight question
can anyone help me with this Principles of flight question please?
as forward flight increases, when moving from the hover, in level flight:
a) transistional lift will reduce
b) induced drag will reduce
c) induced flow will increase.
i know that transitional lift drecrease after about 50 kts, but during transistion, the collective is lowered slightly which i beleive decreases the Angle of attack which in turn increase the induce flow. or is it that the molecules flow decreases at increasing speed which should reduce the induced drag?
or am i totally lossing the plot?
any help much appreciated. thank you
as forward flight increases, when moving from the hover, in level flight:
a) transistional lift will reduce
b) induced drag will reduce
c) induced flow will increase.
i know that transitional lift drecrease after about 50 kts, but during transistion, the collective is lowered slightly which i beleive decreases the Angle of attack which in turn increase the induce flow. or is it that the molecules flow decreases at increasing speed which should reduce the induced drag?
or am i totally lossing the plot?
any help much appreciated. thank you
Join Date: Dec 2001
Location: Philadelphia PA
Age: 73
Posts: 1,835
Likes: 0
Received 2 Likes
on
2 Posts
Lots of things happen right at translational lift. But what happens will depend whether you start from IGE or OGE.
IGE- at about the speed when 'translational lift' happens, you'll probably be over-running the ground vortex, so you might see a drop in height. You will probably see (if you're looking) a decrease in torque / increase in RPM with no movement of the collective as the increased induced flow reduces the induced drag on the rotor. THere should be no reduction in collective, unless you do something other folks don't do.
Hope that helps to clear things up.
S.
IGE- at about the speed when 'translational lift' happens, you'll probably be over-running the ground vortex, so you might see a drop in height. You will probably see (if you're looking) a decrease in torque / increase in RPM with no movement of the collective as the increased induced flow reduces the induced drag on the rotor. THere should be no reduction in collective, unless you do something other folks don't do.
Hope that helps to clear things up.
S.
Spur:
(b) is the correct answer. As we accelerate, transitional (or translational) lift increases, induced drag reduces and the induced flow decreases.
For a given collective setting (pitch angle) as you accelerate from a hover:
the induced flow will decrease
the induced angle will decrease
the induced drag will decrease (decrease in rotor drag and therefore increase in RRPM/decrease in engine power required)
the angle of attack and total rotor thrust will increase
and so the aircraft will want to climb.
If we don't want that to happen during the initial stage of takeoff, we reduce the pitch angle with collective, which will reduce the increased angle of attack and total rotor thrust back towards where it was prior to climbing.
Shawn:
As you accelerate the induced flow decreases (not increases) which reduces the induced drag. Anything which brings the Relative airflow closer to the plane of rotation will tilt the Total Reaction on the blade more vertical and reduce drag on the rotor. Typo or am I missing something?
(b) is the correct answer. As we accelerate, transitional (or translational) lift increases, induced drag reduces and the induced flow decreases.
For a given collective setting (pitch angle) as you accelerate from a hover:
the induced flow will decrease
the induced angle will decrease
the induced drag will decrease (decrease in rotor drag and therefore increase in RRPM/decrease in engine power required)
the angle of attack and total rotor thrust will increase
and so the aircraft will want to climb.
If we don't want that to happen during the initial stage of takeoff, we reduce the pitch angle with collective, which will reduce the increased angle of attack and total rotor thrust back towards where it was prior to climbing.
Shawn:
As you accelerate the induced flow decreases (not increases) which reduces the induced drag. Anything which brings the Relative airflow closer to the plane of rotation will tilt the Total Reaction on the blade more vertical and reduce drag on the rotor. Typo or am I missing something?
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
shawn
you said > at about the speed when 'translational lift' happens, you'll probably be over-running the ground vortex, so you might see a drop in height.
this ground vortex causes a rise in pressure under the helicopter, does it?
ive been trying to say this to nick, only instead of ground vortex, i used ground cussion.
this ground vortex causes a rise in pressure under the helicopter, does it?
ive been trying to say this to nick, only instead of ground vortex, i used ground cussion.
Join Date: Apr 2000
Location: West Midlands, UK.
Age: 73
Posts: 294
Likes: 0
Received 0 Likes
on
0 Posts
I'm confused
'.....as the increased induced flow reduces the induced drag on the rotor.'
' .....as we accelerate, .... induced drag reduces and the induced flow decreases'.
'.....as the increased induced flow reduces the induced drag on the rotor.'
' .....as we accelerate, .... induced drag reduces and the induced flow decreases'.
