Optimum CG • discussion on 'Pilots of America' forum

[Garfly]

 

https://www.pilotsofamerica.com/community/threads/optimum-cg-test.141807/

 

 

[facthunter]

Tail heavy is most efficient. Nose heavy safer and more stable. Anywhere in the allowable range is safe if checked properly and should cover all flight conditions of fuel use till all useable fuel exhausted..  Nev

[spacesailor]

In reality,  a lot of sircraft will have the C G changing during the flight , due to fuel use .

My H B is nose heavey on takeoff , and as the flight proceeds, the fuel tank over my legs ( between firewall  &  instrument panel ). Gets lighter moving the C G rearwards , making ' stall's ' on landing more possible .

spacesailor

[facthunter]

Most aircraft will experience a C of G change as fuel is used but you are supposed to ensure it ALWAYS stays in the allowable range even to the stage of using all the fuel it can use. It's not airworthy otherwise. I prefer the graphical presentation where the Planes weight and basic index are shown as a start point and you go down the page from the top with three fuel use points noted.  Nev

[spacesailor]

Or have a half kilo of lead to slde on a pole from as far rearwards as necessary , to as far forward as necessary! .

After all we know the aircraft managed before. 

But it's all a guessedtimste ?.

spacesailor

[facthunter]

Absolutely NOT permitted to be a part of the deal in "Normal" circumstances There's no guess work if it's done right and plenty of crashes when it wasn't or the load shifted. Nev

[spacesailor]

I Have been asked to Loan lead weights to glider members for getting the c g optimally correct .

I believe it Is normal practise. 

A much better idea would be to ' use ' something like " baggage  ", or even a small reserve of fuel to get that C G under your control . The idea of using " elevator " trim to offset a heavey tailend seems a waste of fuel, with that extra 

Induced drag .

spacesailor

[Bruce Tuncks]

The best Libelle pilot ever ( gosh I'm showing my age here) was asked where he liked the CG....  "an inch behind the aft limit" was his reply....  then his eyes grew big...  "Bugger, I forgot to tell those whackers who bought it about the lead in the tail"

We also had a glasflugel 304 at the club. This thermalled very well with a light pilot, but very poorly with a heavy one.

For Spacey's benefit, the tail loads are usually DOWNWARDS so extra lead there reduces the induced drag... but there is more to it than this, the overall performance is better for some gliders at aft C of G. Personally, I was too scared to go back behind the aft limit, but I well remember Harry Schneider fixing a bag of sand with a string pull-tie to the rear fuse of a new glider they had designed and were certifying. I dunno how far behind the aft limit they had to demonstrate spin recovery at. The string was clearly there to empty the sand out if spin recovery was not happening.

[Bruce Tuncks]

[facthunter]

I did say NORMAL' The industry does it the way it does. as a developed method over time. You can load it the way you wish as long as it's LEGAL or you may lose your life or your house.  Nev

[Bruce Tuncks]

I'm sure it was illegal nev, but he sure showed it went well. I wonder if the "certified" CG position is not compromised by "safety" considerations... But, as I said, I was too afraid to find out.

[facthunter]

I wouldn't like to be encouraging it more that I already have and that doesn't recommend going outside the ALLOWABLE range. Boring but safe. Perhaps that's why I've never had an inadvertent stall and have survived so far. Nev

[Bruce Tuncks]

Well this guy survived ok as well Nev. As I said though, for the reasons you have stated, I personally was law-abiding .

[facthunter]

Plenty of pilots think rules are for others but not them. Are you  comfortable sharing a thermal with such people?  They shouldn't be lauded or praised.  Nev

[facthunter]

One inch behind the aft position on a glider is not the same as the same figure on a plane with a short Moment ARM to the tailplane. Gliders have the tailfeathers a long way back to reduce the down force needed otherwise so the design already reduces the effect.  Long wingspans complicate slow flight in a sailplane and they do enter a spin fairly easily.  Nev

[RFguy]

It's interesting to examine NORMAL or UTILITY category aft (and fore) CG  limits if provided.  Nev you are right an inch on a sailplane is (generally)  alot less  (% MAC) than an inch on a short empanage powered fixedwing.   Bruce, is the fastest way down in a Libelle to induce a  nose down spin ?

Edited April 22, 2023 by RFguy

[Bosi72]

In my opinion, and happy to be corrected, "Optimum CG" would be the position of CG which would produce angle of attack where Lift/Drag ratio is at maximum, which is around 4-6deg for majority of aircrafts.

 

[Bruce Tuncks]

Never tried any spins in a Libelle. The airbrakes are very effective and it comes down real fast if you open them. I think you can maintain a 45 degree downwards at rough airspeed  but I've never tried that either.  Gosh what a boring pilot huh.

The only real spin I induced in a glider was in a Ventus in Nevada. I was framing a picture of a snow-flecked mountain-top with the camera at the clear-vision panel and framing the pic with the rudders. So I didn't see the ASI or anything else, but that Ventus sure kicked into a spin. ( I was about 17,000 ft at the time, and all alone,  so there was no danger at all )

Actually, I reckon height is too hard to get and too easy to lose anyway, especially in a glider. So aerobatics of any sort never appealed.

[Bruce Tuncks]

Bosi, that sounds ok to me. I think you can measure angle of attack with a spirit-level, but I never tried that. I was really quite happy with the climb performance of the Libelle anyway, but I did move the c of g back....  there is a mounting-point for lead bits just forward of the rudder for this purpose.

[Bruce Tuncks]

We would wear a chute and sit in the glider on scales inside the hangar to get the figures to calculate just how much lead. It never occurred to us to go back behind the aft limit, but the point we all aimed at was that limit. Except for this quite fat mate, who was quite slow in races and we didn't figure out it was the c of g which was hurting him.

I realize now that we also could have carried more water-ballast, although it would have been safe but illegal to use "unused" cockpit weight allowance quite safely as water-ballast extra.

In the olden days, Libelles had a rough airspeed of 135 knots, and we used to do start-runs at this speed, full of water. Glasflugel had used an old standard in their certifying.

The most recent AD was to reduce this to 89 knots or so. But no wings were ever ripped off using the 135 knots idea. Maybe I'm lucky that we didn't use the extra water-ballast, but my guess is that we were just a little slower and the wings were strong enough anyway.

 

[RFguy]

The less the tail has to do the better for efficiency. Bruce do you recall, for those gliders when set at the "design  aft-limit" , in response to a impulse pitch input (up or down step), in terms of dynamic pitch stability, were they over damped (return very  slowly to original attitude) or underdamped (borderline unstable probably aft CG for that airspeed/loading) IE entered an oscillatory condition like a car spring without a damper )   or somewhere in between ?

[Bruce Tuncks]

My strong guess is that the 1" did not change the damping to underdamped or even critical. I wish I knew more, but I would have to have a disposable ballast setup like Harry Schneider used once. ( a bag of sand with an emptying pull-string) to test this out. 

Sorry, but I only ever heard  of one guy who flew like this. He won the championships, but he was pretty good anyway. Would he have won if he had just used the approved figures and bolted lead inside the fin?   I think so, but who knows?

Incidentally, test flying is very difficult to actually do in practice. I once tried to figure out if turbulators helped a Mosquito glider, and if so, just where to stick them. I had sections of turbulator-tape ahead of pitot tubes connected to a spare vario .

I thought that a differential comparator setup would be the most reliable, so there was an identical setup on each wing . Well, I was only after an improvement, not solving unknowns.

Well, after several weeks, I gave up! it was all too hard. The wings are NOT identical and I found no position of turbulator tape that improved things  (as measured by the pitot tubes ) at all speeds and flap settings.

 

[Area-51]

2 hours ago, RFguy said:

The less the tail has to do the better for efficiency. Bruce do you recall, for those gliders when set at the "design  aft-limit" , in response to a impulse pitch input (up or down step), in terms of dynamic pitch stability, were they over damped (return very  slowly to original attitude) or underdamped (borderline unstable probably aft CG for that airspeed/loading) IE entered an oscillatory condition like a car spring without a damper )   or somewhere in between ?

This is a great diagram regarding optimum CG... correlate the image to dynamic pitch stability at best glide. The optimum CG position will result in a stable constant decent at exactly the 'best glide' velocity, with neutral pitch trim... moving the CG rearward will cause the airframe to gradually pitch up until airspeed bleeds off and the nose pitches down once again; and this can repeat all the way down to an actual hands off landing in a perfect environment... moving the CG forward will pitch the airframe nose down, and into the ground.... positioning the CG too far from the optimum best glide position amplifies these effects until dynamic stability collapses... depending upon the individual airframe optimum CG will change either side of Best Glide; which is the primary purpose of pitch control/trim surfaces... A longitudinal sliding mass is the simplest pitch control device, and things can go very wrong when the mechanism fails.

 

Test proofing an airframe design within Xplane is a great way to study affects without getting dead.. one design recently looked at had a violent tendency to barrel roll immediately if the CG was moved beyond the rearward position by just 5.0mm; the spin was mostly unrecoverable when affecting full pitch up deflection during the upper quadrant of the manoeuvring speed in straight level flight moving the CG 5.0mm forward again allowed for instant recovery of a controlled barrel roll.

 

The perfect CG position on the perfect theoretical airframe will see the airframe return to a stable straight ahead vector immediately at any speed through any vectored direction of fluid (air) when all control pressure is released back to a neutral position.

Edited April 23, 2023 by Area-51

[facthunter]

The best efficiency in a conventional layout plane as where the tail carries some of the Planes mass by providing positive LIFT of it's own. Where it needs a downforce to achieve the required A/C attitude that is more force for the wing to carry. The ELEVATOR determines the AoA of the wing. It's not a weight shift aeroplane.   Nev

[skippydiesel]

What is the effect on stall speed/recovery, in a turn/bank, when the aircraft is essentially balanced using weight rather than  elevator position???