best glide speed of a warrior

"john smith" wrote in message
...
73 kts is the stall speed at max gross and aft CG.

Vs(actual)=[sqrt(actual wt/gross wt)]*Vso(gross)


I am not talking about stall speed but it makes sense to me that the
calculation should be the same for the change in glide speed,

so if my best glide speed is 73kts at gross wt (1055 kg), then with just me
and half fuel at about 850 kg
this would give a speed of 66kts, which is a significant difference (
especially from the 75 the flying school suggests)

now that i think about it your equation makes sense. lift required should
be proportional to weight. but lift generated by the wing is proportional to
v^2. So for the same angle of attack it makes sense that the speed should
be a factor of
sqrt ( actual wt/gross wt). in the stall example you are just talking about
a different angle of attack ( 16 degrees) instead of the best glide angle of
attack (4 degrees)

thanks for the help John.

Terry

"Hilton" wrote in message
nk.net...
Terry wrote:
the best glide speed of a warrior is quoted as 73 kts. but best glide
should
be at best lift /drag ratio...

It is.


which I recall is at about 4 degree angle of attack.

Who told you this? CFI? POH? Seems awful low to me; I've never seen an
AOA quote for Vbg for a GA aircraft.




The 4 degrees comes from my theory books ( trevor Thom) and I understand it
is pretty much the same for all light aircraft. which is why the wing is
usually set at this angle when the aircraft is level ( the riggers angle I
believe they call it) But since we dont have an angle of attack meter the
number is pretty much academic. the principle is that the best glide speed
is at a constant angle of attack and if the weight changes the speed to
achieve that angle of attack must change. I just wanted to understand how
much it changes.




But this must be a function of weight.

The airspeed is, not the AOA.

yes I agree, I didnt write this well but the change in airpseed is what I am
after, since we dont know tha aoa


So my questions are what weight is the 73 kts based on?

Best lift over drag as you suggest.


My flying school actually rounds it off to 75kts, regardless of weight.

Stupid. Fly 73, not 75 - if you can't fly 73, you need practise. There
are
numerous rules of thumb, to get it accurately, scale by sqrt(weight) /
sqrt(gross) - ensure you use CAS, not IAS.

Im certainly doing the practice and good pt about using CAS


Hope this helps,

It does, thanks
Terry

"Maule Driver" wrote in message
m...
The best glide speed in the book is calc'd at gross weight. At lighter
weights, the best glide will be lower. The calculation for best glide at
various weights is straight forward but I don't know it. I'm going to
guess
that it will be about 6 knots slower at empty weight plus a standard pilot
(can't wait to find out what the real number is)


See responses above, your 6 kts was a good guess,

Flying that speed accurately is valuable but perhaps not as valuable as
understanding the other major variable in doing an engine out glide to a
spot on the ground. That variable is wind. Since this is a ground
reference manuever so to speak, wind plays a signinficant role. The task
at
hand is generally "best glide speed for distance over the ground". If
you
are flying upwind to a spot, best glide speed will be higher. Downwind to
a
spot, a slower than best glide speed will take you further. Accurately
flying 73 knots to make maximum distance over the ground into a 20 knot
headwind is a mistake. (I'd guesstimate that 80-85 knots would be more
like
it).

excellent point about the wind, i can make sense of that, spend less time
going against the wind.



I don't know the formula for doing the calc and doubt you will have it if
faced with the problem. When faced with the problem, I had a gps driven
glide computer in my glider which would give you precise guidance based on
wind, glide polar, and vertical air motion.

I dont intend to be doing the calcs on the spot but to have some numbers in
the back of my head for different wts. I always have a fair idea of what my
wt is at any time.


A couple of rough rules of thumb
- err on the high side for upwind max distance glides - you really need to
go faster upwind and there is disportianate penalty for being slower than
optimal
- err on the high side for downwind - you can get a little more distance
by
going slower than best glide speed, but not much. Just fly best glide if
in
doubt

I'm really interested to see what the actual speeds are for different
weights and wind conditions.

Im interested in the wind calcs too now. thanks Maule. by the way what is
this Maule thing that you drive?

Terry

"G.R. Patterson III" wrote in message
...


Maule Driver wrote:

Flying that speed accurately is valuable but perhaps not as valuable as
understanding the other major variable in doing an engine out glide to a
spot on the ground.

Several experts (people like Barry Schiff) claim that the best speed to
use is
actually a speed which will keep you in the air longest. That gives you
the most
time to attempt a restart and use the radio. The only time you should use
best
glide is when you actually need to in order to reach a suitable spot to
land.


Wont the best glide speed also keep you in the air the longest? if not what
speed will?

Terry

Yup, brian fart.
The equation is the same, just change the Vso to Vglide and the best
speed at reduced weight will be correct.

Jay Somerset wrote:

It was either a brain fart, or a typo. :-)

d&tm wrote:

Wont the best glide speed also keep you in the air the longest? if not what
speed will?

No, best glide doesn't do that. The best loiter speed is a bit lower than that
for the best angle of climb. It's usually not published, but you can determine
it pretty well by empirical experimentation.

George Patterson
The desire for safety stands against every great and noble enterprise.

On Tue, 14 Dec 2004 06:50:15 +1100, "d&tm"
wrote in
::


Wont the best glide speed also keep you in the air the longest?

I believe 'best glide' speed is ebullient to maximum L/D speed which
will provide the maximum distance obtainable (in still air) for a
given AGL altitude.

if not what speed will?

As Maule Driver stated in Message-ID:
, 'minimum sink speed'
will yield the maximum time aloft (in still air or not) for a given
AGL altitude.

Both speeds are easily found if you have a polar diagram of aircraft
(airfoil) performance plotted on a grid with airspeed on the X axes
and vertical speed on the Y axes. There's an example here
http://www.afg.ethz.ch/Info/polaren.xls; click the 'Polar Chart' tab
at the bottom.

I usually have my student's fly final at 60. I would think we'd have
noticed if stall speed were 73.

"d&tm" wrote in message
...
The 4 degrees comes from my theory books ( trevor Thom) and I understand
it
is pretty much the same for all light aircraft. which is why the wing is
usually set at this angle when the aircraft is level ( the riggers angle I
believe they call it)

The angle is called "angle of incidence", and while not just one factor
controls the choice of angle of incidence, most often the primary factor is
the desire to keep the cabin relatively level during cruise. Cruise speed
usually results in an angle of attack well below the "best glide" angle of
attack (and an airspeed well above Vbg).

I don't know off the top of my head what a typical best-glide angle of
attack is, but it's above a normal cruise angle of attack, and if 4 degrees
is a typical angle of incidence (and thus a typical cruise angle of attack),
the a typical best-glide angle of attack would be greater than 4 degrees.

Pete

BTW: There is a formula to get Vy, Vx, and Vso (and probably best
glide) for each weight. I don't have it in front of me right now. That
formula is on the Flight Engineer FAA written exam.

-Robert, CFI