Gross weight vs Glidespeed

[midlifeflyer]

I know in the ArrowIII that I'm practicing in lately, if you want to sink like a rock, it will do that for you happily.

The Arrow is know for it's safe glide: glides like a safe!

 

[uwochris]

I don't know if this was mentionned above already, but there is one important thing to consider with L/D max.

When you are flying at the best glide speed, you are actually attempting to achieve the maximum ratio of lift to drag- this was mentionned above.

However, something to keep in mind is that L/D max is associated with a specific AOA, NOT an airspeed. Small trainers and Cessnas don't have angle of attack indicators, so the only resource we have is the ASI. This ratio is obtained from the coefficient of lift and drag curves, and finding the area where the ratio between the two is greatest.

While your best glide speed changes with weight, winds, etc, the specific AOA to use to achieve L/D max remains CONSTANT with altitude, weight, etc. This is important.

Now, let's consider what happens when weight increases; if weight were to increase, more lift would be required to maintain an equilibrium. There are 2 ways to generate lift - increase the AOA or increase IAS. Since L/D max is associated with a fixed AOA, you only have 1 option- increase IAS.

Therefore, when gross weight increases, use a slightly higher IAS for the best glide ratio.

Also, endurance is NOT affected by winds, but range is. With endurance, we fly at the minimum point on the power required curve- this provides minimum sink rate. Range is a function of distance... headwinds and tailwinds can therefore have an impact.

 

[midlifeflyer]

Now, let's consider what happens when weight increases; if weight were to increase, more lift would be required to maintain an equilibrium. There are 2 ways to generate lift - increase the AOA or increase IAS. Since L/D max is associated with a fixed AOA, you only have 1 option- increase IAS.

That is an =excellent= additon.

 

[Airpiraterob]

simple (?) math

mabye this was already said in a different way but i hope its something that can add to the discussion.

oh and definite credit to UWOCHRIS, absolutely correct info.

-best L/D does indeed remain constant. the speed at wich this is attained is not. best L/D speed increases with weight.
-best L/D is found at ONE angle of the wing to the airstream (Angle of attack) any higher or lower angle of attack increases the drag of the wing. usually this AOA is at a few degreese positive (3 perhaps)
-so whatever speed you are flying at, if the wing angle AOA is at 3 degreese positive in this case, youre flying at your best L/D
-racing gliders use water ballast. true. they do this to make the best L/D happen at a higher speed in relation to the first point above. how is following below:

look at this:
you only get a certian "coefficient of lift" at your best L/D (a Cl of say .2)
given the same AOA as a constant (+3 deg.) you need to go faster to generate more lift. again this is assuming that only ONE angle is used and that being the best L/D angle in this case. so...

a plane with a 200 square foot wing at .2 Cl (coefficient of lift) gives you 357#'s lift at 60mph, and 635#'s lift at 80mph. so this glider should have the same performance at 80mph as it does at 60 but weighs 278#'s more!

(wing area x rho/2 x velicity in FPS squared x Cl = pounds of lift)

the changing variable is the pounds of lift that has to equal the weight of the plane, as that goes up the only thing you can change in the equation is the velocity quotient.

AH HA! you were about to point something out to me but i caught it....the drag increase of travelling much faster is the penalty. your range at the best L/D will decrease as the drag sharply increases. with a 30% increase of speed you get a 78% increase of lift.....you also get a 78% incease of drag!! so your range at that higher speed is dimished...but not by much! (its all porportional to the square of the number...twice as fast...4 times the drag) so overall given no wind at all, the best L/D for maximum range glide is found at the lightest possible weight for the airplane. (get an enema in a sauna before you fly it this way for maximum effect)

just some notes:
the drag at 80mph is very minimal anyway and 78% on top of not much drag to start with is still not much anyway

i had alot of fun in the Schweitzer 1-26, the best glide and the minimum sink speeds were only 5mph apart as i remember. something like 59 and 54mph. i think....its been a while.

 

[KickSave]

Rob, excellent post.. you had me right up until this point:

>>> (get an enema in a sauna before you fly it this way for maximum effect)<<<


Can't I just leave the flight bag full of FAR AIM's at home instead of that?


/Dave

 

[Airpiraterob]

you can do all 3 and have absolute maximum weight savings

on an additional note, if you were wanting to loose alot of altitude, the common idea is to do the forward slip on approach. Thats good and everything however in a glider they slow to minimum sink speed...or a few knots less even very close to the stall and then do the forward slip. you drop like the preverbial rock towards a black hole. and in any headwind at all you loose the altitude and dont gain any distance. i tried to replicate this in a c150 once and well....it didnt work too well.

 

[dmspilot00]

Vnugget... you deleted your post, I thought what you said brought up an interesting point. Did you find evidence to show that you were wrong?

Also I wanted to point out something about that weight-change airspeed formula. That formula can be used with almost all speeds from stall speed, landing speed, and glide speed.

When you do use it (especially to calculate stall speeds), if you want an accurate answer you should start off with Calibrated Airspeed (KCAS), apply the formula, and then convert the answer back to indicated. Usually there will be a chart/graph to convert between the two. Near stall speed the difference between KCAS and KIAS in a 172 is significant.

The same formula can be used to calculate speeds under a G load, btw.

 

[VNugget]

I was right, but I found out that Airpiraterob already said it, so my post that said "_______, and no one has said this" was rendered quite.... dumb. (In case anyone missed what we're talking about, it's the point that a heaver (but otherwise identical plane) would, in real life, have more parasite drag to contend with, and thus haver a lower (L/D)max, and therefore not be able to glide as far.)

But, I have something else to attack now.

GeorgeTG said, Beware of the fact that L/D max is not the best glide speed.
Best glide is achieved at the tangent to the power required curve.

There is no contradiction there. Note that the Thurst required (which is identical to the total drag) curve is NOT the same as the Power required curve. Keep in mind that Power = Thrust * Velocity.

Best glide does occur at the lowest point on the Thrust required curve, which is (L/D)max (proof here), which does coincide with the tangent of the Power required curve.

Minimum sink, on the oher hand, occurs at the lowest point of the Power required curve, NOT at (L/D)max. This is because the glide angle is multiplied by the vertical component of velocity, just like the thrust req'd curve is multiplied by velocity to get the power req'd curve.