Max Weight of Non Lift Producing Components

Elevator always counts as non-lifting.

On Thu, 07 Dec 2017 21:40:48 -0800, Tango Whisky wrote:

Elevator always counts as non-lifting.

Obviously: In every man-carrying aircraft I know of (computer-stabilised
beasts like the F-22 excepted) the CG is in front of the wing's centre of
lift, so the tailplane *must* generate a downforce during stable, trimmed
flight.


--
Martin | martin at
Gregorie | gregorie
| dot org

Le vendredi 8 décembre 2017 14:08:56 UTC+1, Kiwi User a écritÂ*:
On Thu, 07 Dec 2017 21:40:48 -0800, Tango Whisky wrote:

Elevator always counts as non-lifting.

Obviously: In every man-carrying aircraft I know of (computer-stabilised
beasts like the F-22 excepted) the CG is in front of the wing's centre of
lift, so the tailplane *must* generate a downforce during stable, trimmed
flight.

No. (and by the ways, it's the neutral point which is important, not the center of lift of the wing).
What is important for stable flight is that the gradient of lift produced by the elevator over angle of attack is positive so that a pitching moment of the lift will be counter-balanced.
No if there is a gradient of lift over angle of attack for the elevator, the absolute value of the lift produced by the elevator will be zero for a defined angle of attack, negative below this angle, and positive above. the angle of attack for zero lift is a design parameter and is adjusted by chosing the fixed angle of the elevator in relation to the wing chord. Typically this angle is about -4°.

In any modern sailplane, you want to produce exactly zero lift at the elevator at max L/D speed, simply because this is were the elevator drag is minimum. As a consequence, the elevator will produce positive lift below max L/D speed, and negative lift above this speed.

Hey Tango Whiskey, sounds like you got your sierra together..............would you please address the "ideal" CG position in terms of percentage of allowable range?
Thanks,
JJ

Le vendredi 8 décembre 2017 16:03:18 UTC+1, a écritÂ*:
Hey Tango Whiskey, sounds like you got your sierra together..............would you please address the "ideal" CG position in terms of percentage of allowable range?
Thanks,
JJ

Ideal for what? In terms of performance, this would exactly depend on how the elevator is rigged, which differs from type to type.

On my Ventus cM, I fly with max allowable aft CG. But not beacause I want it - I just don't have any choice if I don't want to exceed MTOW ;-)

I doubt there is a "magic spot" across all glider types.
I would say, in general, pick the speed you will spend the most time at in a certain contest, then fly your normal stuff and see if the flying speed (no trim) matches. This means chute, snacks, water bottles, clothing for the ambient conditions, etc.
Adjust CG to allow "0" elevator (meaning chord of tail and elevator are even, thus no lift in either direction) so you can normally fly no trim.
This is least drag.
In engineering, as in life, there are compromises, fix one thing, likely to FUBAR another.

For most of us mere mortals, the biggest difference is the nut behind the stick.........

Let's not forget canard types where the elevator is in front of the CG
and always produces positive lift.Â* Rutan does it quite often with his
designs, but his glider was a bust.

On 12/8/2017 6:08 AM, Kiwi User wrote:
On Thu, 07 Dec 2017 21:40:48 -0800, Tango Whisky wrote:

Elevator always counts as non-lifting.
Obviously: In every man-carrying aircraft I know of (computer-stabilised
beasts like the F-22 excepted) the CG is in front of the wing's centre of
lift, so the tailplane *must* generate a downforce during stable, trimmed
flight.



--
Dan, 5J

On Friday, December 8, 2017 at 7:03:18 AM UTC-8, wrote:
Hey Tango Whiskey, sounds like you got your sierra together..............would you please address the "ideal" CG position in terms of percentage of allowable range?
Thanks,
JJ

JJ, Steve Smith has a good sermon on this, ask him next time you see him. He understands and explains it a lot better than I could.

I think it is cool that the tailplane lifts up at the low speed range, and down at the high speed range. Odd, but cool. So when you have the stick pulled back the tailplane is actually lifting up, and when the stick is pressed forward the tailplane is pushing down. As TW observes, it's all about the lift gradients. And pitching moment.

--Bob K.

At 01:59 08 December 2017, Dan Daly wrote:
On Thursday, December 7, 2017 at 7:27:02 PM UTC-5, Jim wrote:
I admit I am baffled by "max weight of non-lifting parts". I
understand
=
the issue of spar bending moment limits, and I think I understand that
weig=
hing a glider without its wings will give the current non-lifting (i.e.,
no=
n-wing) weight, but I have a feeling there are subtle things here that I
do=
not understand.

I've rigged an ASK-21 and its wings are HEAVY! I haven't weighed
them
so=
I don't know just how heavy they are though. Certainly felt like more
tha=
n 100 lbs each. Ask me how I know. I'll guess (I know, don't guess)
each
=
wing weighs 150 lbs. Likely more. If I subtract 300 lbs from 780 and
then=
subtract that result from 1320 I'm still short of 902 - which I suppose
is=
a good thing.
=20

Glider cockpit load limits are set by the lowest value determined by:
Max AUW
Seat strength limits
Max wt of non-lifting parts
Forward CG limit

For a ASK21 for which I have the actual weighing results:
Max AUW 1320 lbs
Empty weight, fully equipped, 884 lbs
Therefore disposable load is 436 lbs
Seat strength limit is 242 lbs, so with a max weight pilot the other pilot

cannot exceed 194 lbs

Each wing weighs 219 lbs, max weight of non-lifting parts is 904 lbs.
Fuselage, tailplane, etc wt is 884 - 438 = 446 lbs. Hence max load
considering only non-lifting parts limit is 904 - 446 = 458 lbs. The AUW

limit takes precedence.

On this aircraft forward CG limit did not determine max cockpit load.
Min cockpit load is set by aft CG limit.

Steve Smith told me that all sailplanes would climb and cruise better when the CG was at least 80% of the allowable range. I found out the Genesis-2 wouldn't climb in a weak thermal unless the CG was 90%. Fifteen years ago, I rigged up an in-flight CG Shiffter, mainly to investigate the handling of a flying wing with an extreme aft CG. It consisted of a 10# weight inside a 5 foot plastic pipe with fore and aft cables.............blue handle and red handle! Found the handling got a bit more pitchie at 90%, but I was able to climb with most ships. BTW, I found pitch control by shifting CG, didn't work, but aft CG was essential to get the G-2 to go up in a 1 to 2 knot thermal.
Random thoughts from JJ