Debunking Glider Spoiler Turns Causing Spin Thinking

All this discussion seems to have been started by my posts on June 3 and June 4. After having gone around a few times, maybe it is more clear where I was coming from. Does anyone have any problems with this table? IF so, can you clearly explain why?

Examples: G-load (lift / weight) at various bank angles and L/D ratios:

L/D Infinite-- 0 deg 1.00 30 deg 1.15 45 deg 1.41 60 deg 2.00
L/D 10:1-- 0 deg .995 30 deg 1.15 45 deg 1.40 60 deg 1.97
L/D 5:1-- 0 deg .981 30 deg 1.13 45 deg 1.36 60 deg 1.87
L/D 2:1-- 0 deg .894 30 deg 1.00 45 deg 1.15 60 deg 1.41
L/D 1:1-- 0 deg .707 30 deg .756 45 deg .817 60 deg .894

Yes, at typical sailplane glide ratios, changes in the drag vector (due to landing gear, spoilers, etc) have little effect on the magnitude of the Lift vector. (As I said on June 3.) Yes, Lift is almost the same magnitude as Weight, and the lift-wise component of the G-loading vector is almost the same magnitude as the total G-loading vector. For all practical purposes, there's nothing to argue about here. But in theory, there is a difference, and if you start looking at really extreme cases, the difference becomes large. The more of the total weight is supported by the drag vector, the smaller the lift vector must be.

S

On Friday, June 12, 2015 at 10:20:53 AM UTC-7, wrote:
On Friday, June 12, 2015 at 9:01:01 AM UTC-7, jfitch wrote:

The L/D/W vector triangle is a construct of convenience, not a Holy Trinity. It carries accuracy but no further insight than any other interpretation. Physically, it is most intuitive to think of the aerodynamic forces on a wing section as the sum of the pressures on its surface. This is mathematically complex though,

Hmmm.. every time I try to introduce more complexity I'm accused of changing the subject and trolling.

I agree that splitting the aerodynamic forces into L and D is in some sense arbitrary. I disagree that is not enlightening. Every time we talk about effect of G-load on airspeed at the stall angle-of-attack, etc, we are making use of that split, i.e. we are talking about the lift-wise component of G-loading. It is a useful thing to do.

I say she IS a Holy Trinity and I intend to worship at her altar again very soon!


You said some days ago: "In steady state, unaccelerated flight, Lift = Mass and the wing loading is constant. It does not matter what your sink rate is. The wing loading is the same with flaps, gear, and spoilers out as it is clean. If Lift Mass then the glider is accelerating, either up, down or by changing direction."

I say no, in an unbanked unpowered glide, in the steady-state condition, Lift is less than Weight, and Lift changes whenever the L/D ratio changes.


You said: "It doesn't matter if the glider is descending - the wing loading is the same. The only thing that will change that is accelerated flight.. That is an increasing or decreasing rate of descent (not constant) or a change in velocity (that includes turns which are a change in velocity by definition). A sustainer glider will have the same wing loading in level (unaccelerated) flight as it does in a glide. "

But I say not so. I say the lift-wise component of the G-loading-- i.e. the Lift/Weight-- is slightly less in a descending glide than in level flight (all relative to the airmass of course.)


You said "It is best not to think of the "speed at which the wing stalls".. That can be anywhere from 0 to beyond redline, depending on a bunch of conditions. You should think of "the angle of attack at which the wing stalls" which is for all intents invariant on a glider."

I say that's all true, and it's also true that the airspeed we see at the stall angle-of-attack is highly dependent on the square root of the lift-wise component of the G-loading, and that's why we ought to care about the lift vector or the lift-wise component of the G-loading or whatever we want to call it.

The L D W concept IS useful and enlightening, because airspeed at stall is highly related to the magnitude of L. Not to the total aerodynamic force.. If we care about airspeed at stall, then we care about the lift-wise component of the G-loading.

S

What you have discovered is merely a quirk of the (arbitrary) definition of the vector L. How it may change with changes in L/D ratio over the normal performance envelope will have no effect on what a pilot does or should do. It has less effect on the airframe structure than many other factors and I doubt any designer considers this as an independent problem.

We should worry less about the airspeed at stall (a wing will stall at ANY airspeed, at ANY wing loading) and worry more about angle of attack. In most regimes of flight, AOA is the one thing you can immediately affect - airspeed, lift, drag, etc. are consequences of it and will settle to steady state after some time goes by. You will have much better insight starting with that, than L and D vectors. If you are stalled or near stall, specifically you must reduce the angle of attack and let L, D, W, airspeed, wing loading, and pretty much everything else fall where they may.

Well, if you don't think very steep descent angles (relative to airmass) are relevant to soaring, how about very steep climb angles (relative to airmass)?

In the future we can look forward to motorgliders with powerful electric (or rocket?) motors that will afford very steep glide climb angles, such as is currently the case in the world of RC soaring.

A large glide angle (poor L/D ratio, steep glide path relative to airmass) reduces the magnitude of the lift vector, and the lift-wise component of the G-loading vector. Likewise, a large climb angle (steep climb path relative to airmass) also reduces the magnitude of the lift vector, and the lift-wise component of the G-loading vector.

(The lift-wise component of the G-loading vector is simply Lift / Weight).

For wings-level flight-- 40:1 L/D ratio--

The three numbers in each line of the table a

a) Thrust to Weight ratio,
b) Lift to Weight ratio (i.e. liftwise component of G-loading vector),
c) stall speed in powered climb, divided by stall speed in exactly horizontal flight.

The first line (0 T/W) is a descending case; all the following lines are climbing cases.


T/W L/W Stall speed / stall speed in exactly horizontal flight
0 -- .99969 -- .99984
..1 -- .9972 -- .9986
..5 -- .878 -- .937
..75 -- .680 -- .825
1 -- 0 -- 0

In the last case, where thrust equals weight, setting the wing to the zero-lift angle-of-attack yields a steady-state climb regardless of airspeed, so the concept of stall speed becomes meaningless.

In the motorgliders of the future, we'll gain a hands-on feel for the way that a steep glide or climb angle reduces the lift-wise G-loading component, and the stall speed!

S

Ok, one last try:
The force created by the wing is usually described as a vector, wich can be decomposed into a vector perpendicular to the flight path (lift), and a vector antiparallel to the flight path (drag). The sum of the two, i.e. the total vector of the wing, is exactly opposed to the weight vector of the aircraft and rules the g-load. In non-accelerated flight it exactly equals the weight, ruling in 1 g - regardless of L/D ratio.

Now if you suppose that it's the lift part which is opposed to the weight, then L/D would influence g-load. But this is just not how physics works on this planet.

Bert TW

"In the last case, where thrust equals weight, setting the wing to the zero-lift angle-of-attack yields a steady-state climb regardless of airspeed, so the concept of stall speed becomes meaningless.*"

Sorry , I was a bit sloppy there. If thrust = weight and l/d 40:1, airspeed must be xxx. If we hold thrust = weight and unoaqd L to zero, so L/D becomes 0, ....

Re "one last try" immediately above-- please take a moment to re-read my previous post. It is the lift-wise component of the G-loading vector-- closely related to what your panel-mounted G-meter reads-- not the total G-loading vector-- which is determined by the L vector, or the L/D ratio, whichever way you want to look at it. In previous posts I spent some time discussing the difference betweeb the total G-loading vector, and the lift-wise component of the G-loading vectorm

I stand by the tables I posted today and on June 4. Do you have a specific correction to offer?

S

Phone-phart, disregard second-to-last, will repost. S

"In the last case, where thrust equals weight, setting the wing to the zero-lift angle-of-attack yields a steady-state climb regardless of airspeed, so the concept of stall speed becomes meaningless.*"

Sorry, that was a bit sloppy. If thrust=weight, no thrust is available to counteract drag, and so airspeed must be zero in the steady-state case. Angle-of-attack is undefined, so the wing cannot be considered to be stalled. Progressively higher T/W ratios (greater than 1:1) allow for progressively higher airspeeds, with the wing held at the zero-lift angle-of-attack, as the aircraft climbs straight up. Obviously we have abandoned the concept of a 40:1 L/D-- we have shoved the stick forward to unload the wing to zero lift as we climb straight up, so L/D is zero.

"Now if you suppose that it's the lift part which is opposed to the weight, then L/D would influence g-load. But this is just not how physics works on this planet.*"

A key point is that g-load is really just an expression of force. That's all it is. It is the vector sum of all the real forces acting on the aircraft, except gravity. Then we divide by weight to get a dimensionless expression.

When we talk about g-load, we really aren't saying anything that we couldn't express just as well by talking about the actual aerodynamic and thrust forces generated by the aircraft.

In a glider, where there is no engine, the g-load is the vector sum of all the aerodynamic forces generated by the glider. In coordinated flight, this would simply be the vector sum of lift and drag.

When we say that g-load affects stall speed, we really should say that the lift-wise component of the g-load vector affects stall speed. Not the total g-loading vector. But all we're really saying is that the magnitude of the lift vector affects stall speed. No extra information or content is added by bringing the concept of "g-loading" into the discussion.

The L/D ratio affects the magnitude of the lift vector, and also affects stall speed. In a powered climb, the T/W ratio affects the magnitude of the lift vector, and also affects stall speed. As per the tables I posted on June 4, and yesterday.

S