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Acro and forwartd CG

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mark.bitterlich(at)navy.m
Guest

Posted: Tue Oct 20, 2009 10:13 am Post subject: Acro and forwartd CG

I am not an expert, but in my opinion a lot of the time it comes down to

control.   



As a person who has personally explored the areas well outside of the

normal CG range of many aircraft (due in most part to my PAST personal

weight range, and also sometimes due to my personal stupidity), I have

found that you can run out of pitch authority at slow speeds with too

forward of a CG.  When landing, this can lead to some really unfortunate

situations to put it mildly.  Doing aerobatics, you can actually stall

the control surface before the wing.  Basically you become a test pilot.

Sometimes you live and learn.  Sometimes you don't.  I got lucky.  I am

much more cautious these days.  Honestly, I'd much rather exceed gross

weight limits than play with CG limits by any significant amount.  Sure,

you can exceed them a little bit and get away with it... But I honestly

would try to stay within published limits when exploring the total

flight envelope of an aircraft.  I.E.  Aerobatics



I concur that AFT CG's reactions seem to go seriously bad faster than

forward CG excursions. I'm curious what others will offer when it comes

to this.  A good question.    



Mark Bitterlich

  



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MarkWDavis

Joined: 03 Jan 2007
Posts: 104
Location: Syracuse, KS

Posted: Tue Oct 20, 2009 12:28 pm Post subject: Acro and forwartd CG

The ultimate nose down attitude is likely to be increased during recovery 

from a departure from controlled flight or spin resulting in the need for 

much more altitude for the round out and recovery.  Just as your pitch rate 

of change increases drastically as you approach aft CG, similarly the pitch 

rate decreases as you push toward forward CG limits.  Cross countries 

pushing the envelope of CG or gross weight are one thing, pushing the same 

envelopes performing acro is more Chuck Yeager territory.  Don't do it.



Mark Davis

N44YK

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gyoung

Joined: 09 Jan 2006
Posts: 211
Location: Republic of Texas

Posted: Tue Oct 20, 2009 3:21 pm Post subject: Acro and forwartd CG

The forward cg limit is based on pitch control authority. The normal place you see the effect is during flare where you need more speed or prop blast to keep from running out of elevator. That can bend your airplane if pressed too far. Acro is a whole nother thing. 



Aft cg limits are based on longitudinal stability. At the aft limit pitch is very sensitive but there is still an acceptable stability derivative, I.e. Tendency to return to level when disturbed. The margin to the neutral point may be only a couple % mac. Exceeding the aft limit can make it difficult to impossible to control. Going past the neutral point will definitely kill you. 



Ex-aero engineer

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Bill1200

Joined: 19 Oct 2008
Posts: 111
Location: medford oregon

Posted: Tue Oct 20, 2009 3:42 pm Post subject: Re: Acro and forwartd CG

Thanks

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wlannon(at)persona.ca
Guest

Posted: Tue Oct 20, 2009 7:34 pm Post subject: Acro and forwartd CG

Bill:



You have already received some good advise on this subject, particularly the 

comments from Greg Young

I presume from your email address you are flying a 52 rather than a CJ.  I 

do not have any specific knowledge of the 52's flight characteristics but I 

think the following comments will apply.



Generally a forward CG will (varying with how far forward): --

1.  Decrease cruise speed due to increasing trim drag.

2.  Increase stall speed.

3.  Improve stall recovery but may increase recovery time to level flight.

4.  Decrease elevator authority.



Items 1,2 & 3 you can probably live with.

If you are operating a CJ with all the original avionics etc. removed (195 

lbs) and no (or not enough) ballast installed #4 can ruin your day or worse.

The worst case scenario is - solo, no baggage, out of gas (and therefore no 

power).  There will not be enough elevator authority to make a safe landing.



If, in the Most Forward condition (varies with aircraft type - for the CJ it 

is minimum fuel, no baggage, full oil), your CG is ahead of the forward 

limit (or you suspect it is) you should flight test as follows:

1. Load in most forward condition.

2. Approach at normal speed and pretend you have a tail wheel.  If you can 

touch down nose high, at IDLE power, at stall speed

your forward CG is OK  Should be able to handle the worst case

with a little more speed.



Walt



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Bill1200

Joined: 19 Oct 2008
Posts: 111
Location: medford oregon

Posted: Wed Oct 21, 2009 7:38 am Post subject: Re: Acro and forwartd CG

Thanks again, good info from all. 

I do fly a 52 with Russian avionics removed and ballast weight in place. The values I found for CG (I think from the yakuk site) are; -12.91 to -6.77 utility and -9.36 to -6.77 for acro. When I fly solo, full fuel, 12 qts oil, I'm at -13.47! Worse as fuel is used. I've landed solo,near empty fuel and haven't noticed lack of elevator authority, but I'll keep it in mind.

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