FineTuning Jets CG
 

Jet Flyer Brothers . All jet models come with a specified CG from the maker. Just like how we fine tune prop jobs CG by the amount of stick we use (down) when we invert to fly neutral and adjust CG piont, is it safe to asume the same for jets also when we flip invert , say it take a lot of down stick to fly level will it be safe to start moving rearwards CG. . or just forget it and stick to recomended CG. Thanks guys

RE: FineTuning Jets CG
 

Sure...

Question: Do jets operate under the same laws of flight as prop driven airplanes do?
Answer: Yup. Just with a kerosene smell!


Harley Condra
BVM REP
JetCat REP

RE: FineTuning Jets CG
 

Harley, I have never quite understood this method of CG checking wherein "fly inverted" if no down is required you are properly CG'd. I have always worked it as a percentage of MAC. Fine tuned based on landing speed.

Can you point me in the direction of a good explanation of the inverted flight method, how and why it works? Or someone with some expertise in this area explain?

Thanks,

Sean

RE: FineTuning Jets CG
 

I have a pretty good trimming chart but my IT skills suck, so I can't post it. If someone lets me know how to post a PDF here - I'll put it up!

RE: FineTuning Jets CG
 

Email to [email protected] and I will post it.

RE: FineTuning Jets CG
 

Dunno about a high-falutin’ explanation ;-) but the overly simplistic version that works for me is:

If your CG is slightly off, then the aircraft at neutral elevator / stab will naturally shove its nose up or down a little as the incorrect CG induces a pitch moment. To compensate we sometimes tend to use elevator trim to keep the nose up on a nose heavy aircraft, or down on a tail-heavy aircaft. However, if you are carrying up elevator to maintain level flight with the nose heavy aircraft, then when you fly inverted you will have the nose-heavy tendency of the aircaft AND the up (now = down) elevator BOTH trying to shove the nose towards the ground.

By doing the inverted check, you are basically just finding out whether you have any elevator trim compensating for a CG-induced pitch condition, because while it cancels the CG-induced pitch when rightside up, it adds to the pitch when inverted.

Like I said – that’s just my “flying for dummies” intrerpretation ; aerodynamic experts feel free to shoot this down and edcuate us.

Gordon

RE: FineTuning Jets CG
 

http://www.my87.com/rcu/trimmingchart.pdf

RE: FineTuning Jets CG
 

yoga fly

you cant trim it like prop.

first tanks in jets are big so fuel is not aways on cg

secont legs are heavy also. in case of an f-15 three legs waith goues forward when gear is up

RE: FineTuning Jets CG
 

Gordon,
I couln't have said it better. (probably not as well....)

Harley

RE: FineTuning Jets CG
 

Hi Sean. Correct or not, I "get" the inverted test method. And %mac is something I've used since I was 15, then tweaked based on longitudinal stability. But I must confess I am completely lost with your reference to tweaking based upon landing speed.

Help me out here? Thanks, Don.

RE: FineTuning Jets CG
 

I don´t use at all the inverted flight method to fine tune the c.g. settings. Thrust angles and settings can fool this method and you can end up doing a nice flat spin to the ground. (ask how I know it..;) )

I usually check the pitch control authority at landing. If I can´t hold the plane´s nose up at touchdown while flying at the lowest possible landing speed the CG is too forward. On the other side If the airplane have full pitch control but it "wobbles" (specialy at windy days) too much at this same moment, the CG is too aft.

Of course, I always use the MAC method for finding the cg as a initial estimate, considering not only the wings, but strakes, fuselage,canards, or everything in the plane that could act as a lifting surface.



Enrique

RE: FineTuning Jets CG
 

Awesome chart! Thanks for sharing it.

RE: FineTuning Jets CG
 

Don, read Enrique's post. That's how I fine tune as well. You want the least amount of elevator to lift the nose for the lowest possible stall speed at landing.

On an F-15 once I had the CG so aft I needed down elevator for landing. That was too far, but the principle applies.

RE: FineTuning Jets CG
 

Here is another way that totally excludes the thrust vector and is very easy / safe to do:

1) Climb pretty high
2) Go to idle, leave the flaps up but lower the LG as it may affect the CG and you probably want to optimize for when the LG is lowered.
3) Turn into the wind, right in front of you.
4) Start a 45 degrees dive, release the stick

If the plane accentuates the dive angle, it is tail heavy. If the plane recovers very quickly by itself and balloons up, it is nose heavy. If it continues down at 45 degrees or slowly recovers, it is right on.

I have yet to do this on a jet but this is how I usually do it for gliders and pattern planes.

Arnaud

RE: FineTuning Jets CG
 

I'm not exactly a jet guy (yet) although I own some stuff that I intend to fly one of these decades. However I am an aero engineer and I do or have done stability calculations for everything from gliders to missiles, even canard fighters.

The upright/inverted method works for aerobatic planes, with basically straight wings. This is a quick measure of how far in front of neutral the CG is for a plane. For a good aerobat with a plenty big tail, you want it almost neutral. That way it goes where you point it and you don't have to fight to keep the plane at an attitude. Having little elevator trim to keep the nose up means you don't need much down elevator when inverted. It also indicates that the CG is very close to neutral balance.

It would not work for any or every jet. For conventional configurations, it might work. L-39 or MB339 with straight wings and tail may behave like aerobats. Doing this with F-16, F-4, canards is asking for trouble. With these, they may behave well at med-high speed, but then would pitch-up themselves when you raise the nose. That is the definition of unstable. They do not have the linear stability response that 'square' airplanes do.

The dive test is also not really applicable. This is developed for gliders. They are also trying to have minimum stability, just a little more than the aerobat. With a dive test for a jet in landing mode , you would want a quick but groovy recovery to a trimmed approach path from a 45 deg dive. Same if you are throttle off on approach and bump a little up elevator then back to normal. You want the nose to quickly and solidly return to the normal approach attitude.

Mainly, Enrique has it. Watch for pitch-up at low/approach speeds.

RE: FineTuning Jets CG
 

I played with CG changes on an F-18C and on an F-18F. I incrementally removed a strip of lead from the nose and flew it. It passed the so called "inverted" test. (Once again, my ignorant self only really knows this one [sm=tongue_smile.gif] ) But the F-18f turned into an absolute bear to land, while maintaining civil flying properties.

Funny how the super hornet displayed these characteristics before the non super hornet. After discussion with you smart folk of RCU, we blamed it on the decidedly large and prominent leading edge extionions found on the superhornet as compared to the non super hornet.

Needless to say I added lead back to both Hornets! :)

That was pretty different than any other airplane (jet or non) CG changes I've tried,a self admitted small number though! [&:]

Raf

RE: FineTuning Jets CG
 

Sean and Johng...I'm sure I'm over-thinking this but here goes:

If a "min up elev near stall speed" is the goal then is it not possible you'd have down elev when trimmed for higher speeds ? And if so do you think the increased profile and induced drag of the elev is compensated for by a reduction in induced drag for the wing? Do you gain or lose top end performance?

My own goal thus far (being newer at all of this) is simply to avoid big trim changes while inverted as well as the "pitchies" (a highly technical term :D)

Thanks for your responses, I'm learnin' a bit here and there, Don.

RE: FineTuning Jets CG
 

My experiance is whatever down you need for inverted flight in a swept wing airplane is just that, what you need. I have seen no real information or proof that how well my airplane flies upside down without stick input has anything to do with my CG.

I did notice on my lightning that barely any input was needed, but as mentioned above, it is basically a straight wing airplane.

RE: FineTuning Jets CG
 

So what method, how exactly do you guys balance a jet model.?
K

RE: FineTuning Jets CG
 

BY the looks of it in short STICK to the makers recomended CG , im sure they have done countless hours of R&D to get it to a point they recomend. and yes there are too many variables in jets as heavy fuel load , heavy UC , delta wing vs conventional wing type etc.
One feel for a nose heavy CG wud be I guess how much the nose drops or how much UP one has to feed to hold in a 45deg banked turn, and\or if she runs out of elev travel upon final flare,
anyways great to see there are so many with near correct answers. , maybe some expert jet test pilots can chime in ali mac , dave s, etc

RE: FineTuning Jets CG
 

I disagree. Sure, use their recommendations for a starting point, but that doesn't mean you should stick to that CG. A perfect example is the Bandit, where I and many others have found that the aircraft is much better behaved (e.g. much easier to flare) with a CG rearward of the manufacturer's recommended range.

Also, the assumption that the manufacturer has done a lot of work to get the correct CG can be completely invalid depending on the manufacturer involved. I know of a number of cases where manufacturers went ahead and released a kit/ARF with an incorrect CG because the prototype was lost before extensive CG testing could be done, and they didn't want to wait until a 2nd or subsequent prototype could take up the slack.

RE: FineTuning Jets CG
 

any dihedral or anhedral in the wing/stab/canards all play a roll on trim change in inverted flight so unless you have a perfectly flat (no dihedral/anhedral) wing/stab you're gonna get a trim change even with perfect CG

RE: FineTuning Jets CG
 

why is this the case? seems counter-intuitive...

RE: FineTuning Jets CG
 

This seems backward to me if it is nose heavy it would seem that the dive would get steeper and if tail heavy it would balloon. If the above statement is correct, how come?