Question about glide ratios (for the aerodynamically skilled)

As best I can tell, it seems for a given configuration air speed for
best glide angle (I will define best as most shallow angle) changes
with weight, but the angle itself doesn't change much. Hi performance
gliders carry water to increase penetration into headwinds, dump it
to increase time aloft, but still go down about foot for every 20 some
they move thru the air, don't they?

I need, for a fictional piece I'm writing, an efficient long range low
power consumption airplane and don't want it to fail because of
technical errors.

Thanks for your comments

Tina wrote in news:1189955725.117947.201170@
19g2000hsx.googlegroups.com:

As best I can tell, it seems for a given configuration air speed for
best glide angle (I will define best as most shallow angle) changes
with weight, but the angle itself doesn't change much. Hi performance
gliders carry water to increase penetration into headwinds, dump it
to increase time aloft, but still go down about foot for every 20 some
they move thru the air, don't they?

I need, for a fictional piece I'm writing, an efficient long range low
power consumption airplane and don't want it to fail because of
technical errors.

Thanks for your comments

Tha's correct. he L/D doesn't change at all, really (there's some
inconsequential difference in L/D for weight) but for all practical
purposes it remains the same regardless of weight. Only the speed at which
the best glide angle is achieved changes.
Note that this is only true in still air conditions. With a headwind, the
heavier airplane will go further and with a tailwind, the lighter one will.

Bertie

"Tina" wrote in message
ups.com...
As best I can tell, it seems for a given configuration air speed for
best glide angle (I will define best as most shallow angle) changes
with weight, but the angle itself doesn't change much. Hi performance
gliders carry water to increase penetration into headwinds, dump it
to increase time aloft, but still go down about foot for every 20 some
they move thru the air, don't they?

For the best answers, take this question to rec.aviation.soaring.

First of all, a glide ratio (also called L/D for Lift/Drag) of only 20 is
considered very bad for a glider these days. Most training gliders will give
you in the low 30's and high performance gliders might beat 50.

For any given weight, every airframe has one speed that gives best L/D. As
weight increases, that speed increases. Because of winds, you may not always
choose your best glide speed to net the most distance over the ground. In
general, you slow down a bit for a tailwind and speed up a tad for a headwind to
gain the most distance over the ground.

Vaughn

On Sun, 16 Sep 2007 15:15:25 -0000, Tina wrote
in . com:

As best I can tell, it seems for a given configuration air speed for
best glide angle (I will define best as most shallow angle) changes
with weight, but the angle itself doesn't change much.

That's essentially correct. Parasitic drag increases with an increase
in air speed, so the effect of increased weight to increase the speed
at which maximum distance is achieved in still air does decrease the
Lift over Drag figure some.

Hi performance gliders carry water to increase penetration into headwinds,
dump it to increase time aloft, but still go down about foot for every 20
some they move thru the air, don't they?

That is approximately true. But any sailplane worthy of the name has
an L/D of thirty to one or greater with the highest performance
sailplanes today in the fifty or sixty to one range.

I need, for a fictional piece I'm writing, an efficient long range low
power consumption airplane and don't want it to fail because of
technical errors.

It is unclear exactly what sort of aircraft you need. Are you
referring to an actual aircraft that exists in today's world, or a
fictional aircraft? How many seats does your aircraft need to have?
Are you looking for a certified aircraft or will something excremental
do?

Here's the ultimate:
http://www.nasa.gov/centers/dryden/n...-068-DFRC.html
This aircraft is designed to remain aloft for months without requiring
any refueling, nor any fuel at all.

Thanks for your comments

You're welcome.

In article . com,
Tina wrote:

As best I can tell, it seems for a given configuration air speed for
best glide angle (I will define best as most shallow angle) changes
with weight, but the angle itself doesn't change much. Hi performance
gliders carry water to increase penetration into headwinds, dump it
to increase time aloft, but still go down about foot for every 20 some
they move thru the air, don't they?

I need, for a fictional piece I'm writing, an efficient long range low
power consumption airplane and don't want it to fail because of
technical errors.

Thanks for your comments

An aircraft drag coefficient is defined by:

Cd = Cdp + Cd0 + Cl**2/(pi*A*e) whe

Cd = total drag coefficient;
Cdp = sum of the parasite drag coefficients;
Cd0 = airfoil basic drag coefficient;
Cl = lift coefficient (a function of angle-of-attack [alpha])
pi = 3.14159732......
A = wing aspect ratio (sqrt[span**2/S)
e = wing planform efficiency factor
S = wing area

The third term represents the induced drag contribution, due to lift
production.

Best L/D will occur when Cl/Cd = min.


In unaccelerated flight, L = weight = Cl*q*S,

where q = dynamic pressure = 0.5*rho*V**2, whe
rho = air density;
V = velocity through the air

Within limits, best glide occurs at a set alpha; speed varies with
weight.

Thanks everyone.

The airplane we need for the story is a pilotless drone, a fictional
airplane, capable of carrying a few hundred pound payload long
distances -- think in terms of drones used by the DoD now.

The principal author will model it after some of the existing high
performance gliders, that will both save him from working at making
his airplane unique and will make it more believable -- don't you hate
stories that contain technical errors?

I loved one of the (God I hope it was) typos, where Larry asked if the
airplane should be "a certified aircraft or will something excremental
do".

Thanks again, you've offered the insights and verification I hope for.

"Tina" wrote in message
ups.com...

I loved one of the (God I hope it was) typos, where Larry asked if the
airplane should be "a certified aircraft or will something excremental
do".

If you spell "inconvenience" in a certain way, the first selection in
Office's spell check can turn out *VERY* ironic results:

(Actual email):

"I'm sorry for the incontinence, but I will be out of the office tomorrow
for a doctor's appointment."


--
Doug Semler, MCPD
a.a. #705, BAAWA. EAC Guardian of the Horn of the IPU (pbuhh).
The answer is 42; DNRC o-
Gur Hfrarg unf orpbzr fb shyy bs penc gurfr qnlf, abbar rira
erpbtavmrf fvzcyr guvatf yvxr ebg13 nalzber. Fnq, vfa'g vg?

On Sun, 16 Sep 2007 15:15:25 -0000, Tina
wrote:

As best I can tell, it seems for a given configuration air speed for
best glide angle (I will define best as most shallow angle) changes
with weight, but the angle itself doesn't change much. Hi performance
gliders carry water to increase penetration into headwinds, dump it
to increase time aloft, but still go down about foot for every 20 some
they move thru the air, don't they?

I certainly hope they do better than that. An F-33 Bonanza is between
17 and 20:1 clean (depending on how heavily it's loaded.) My old Deb
has almost twice the ratio of a 172. Best glide is @ 120 MPH (give or
take)and that gives a rate of descent between 500 and 600 fpm.

Roger (K8RI)


I need, for a fictional piece I'm writing, an efficient long range low
power consumption airplane and don't want it to fail because of
technical errors.

Thanks for your comments

An aircraft drag coefficient is defined by:

Cd = Cdp + Cd0 + Cl**2/(pi*A*e) whe

Cd = total drag coefficient;
Cdp = sum of the parasite drag coefficients;
Cd0 = airfoil basic drag coefficient;
Cl = lift coefficient (a function of angle-of-attack [alpha])
pi = 3.14159732......

No, pi is closer to 3.1415926536.....
Otherwise quite a good explanation...

Cheers :^P

wrote:
An aircraft drag coefficient is defined by:

Cd = Cdp + Cd0 + Cl**2/(pi*A*e) whe

Cd = total drag coefficient;
Cdp = sum of the parasite drag coefficients;
Cd0 = airfoil basic drag coefficient;
Cl = lift coefficient (a function of angle-of-attack [alpha])
pi = 3.14159732......

No, pi is closer to 3.1415926536.....
Otherwise quite a good explanation...

Cheers :^P


I use the formula for Cake (Ca)