Cessna aft CG limitation - How come? 1

[skyete]

The C182Q has a maximum gross takeoff and landing weight of 2950 lb. When Cessna introduced the C182R (an identical airplane to the Q) in 1981, the MGTOW was increased by 150 lb to 3100 lb with no change to the max landing weight. At the same time, the aft CG limit was reduced by 2.5 inches (from 38.3% of MAC to 34 % of MAC). Why the CG change? At first glance, one suspects the static margin reduction was not acceptable or, perhaps, not enough nose-down elevator authority. Any ideas? Is there a simple way to show this numerically or by analysis? THANKS!
_________________
Mvh, Tom

 

[aviat]

The information you are seeking should be in the type certificate data sheet at the site below.The difference between the MGTOW of the two is made up of fuel only, there is no difference in baggage. You have to burn that much fuel off or else you likely will have to do a heavy landing inspection.
The CG of the extra fuel is at the rear limits, so that is probably why they moved the limits fwd. The elevators have more travel in the R. The TCDC uses inchhes aft of the datum you may have to determine your MAC's.

http://www.airweb.faa.gov/Regulator...33bad86256ecb00553120/$FILE/ATTMU61Z/3a13.pdf

 

[skyete]

Aviat, thanks very much for your post. It's much appreciated, and the TCDS does confirm the change in limits.

However, the reason for the change still seems to have more to do with stability and control (perhaps static margin?). The fuel arm for the 182Q is well within the envelope (at 46.5) so even loading the Q (illegally) with an extra 150 lbs of fuel could put you above max gross, but hardly past the aft limit (you'd have to try real hard with extra aft baggage!).

The R model has no max zero fuel weight restriction, so (for example) you could load an extra skydiver or two, or jettisonable cargo, and still land legally.

What I'm really trying to do is run some numbers to show the prudence of the restriction without chasing through lots of stability and control theory. Any further thoughts would certainly be appreciated. THANKS!

 

[plasgears]

A case study for you:

On a long XC trip in a C172, we were within gross wt and envelope limits at takeoff. It was four adults with brief cases in baggage. On final approach, while over the numbers, I ran out of down elevator. It took power to regain control. Could this be related to the aft CG limit change cited above?

 

[skyete]

Interesting case study, plasgears. The 172 and 182 are different animals, of course, but share many of the same traits.

There are lots of different 172's, but looking at the 1980 172N and the 1981 172P is instructive: The fuel arm is actually a little aft of the rear CG limit. This means that the CG actually moves forward as fuel is burned, so if you're in the envelope at takeoff, you're still in it on landing (unless you were so far forward loaded initially that fuel burn puts you out of forward, but such a loading is highly unlikely).

As they did with the change from 182Q to 182R, Cessna also increased the MGTOW from the 172N to 172P in 1981. The 182 increase was about 5% and came along with the CG limitation change mentioned above. The 172 increase was 100 pounds (about 4%), but there was NO CHANGE to the CG limits. (And unlike the 182, the weight increase was a Design Weight change, applying to both landing AND takeoff). What they DID do, however, was restrict the flap travel to 30 degrees from the previous 40. This was no doubt to maintain nose-down pitch authority, especially given a go-around at the new weight.

In your case, I assume flaps were deflected on final, and like all Cessna high-wing singles, associated with a pitch up to be overcome with down elevator. Nevertheless, I don't think what happened in your case was CG related, but I could be wrong.

As far as "running out of down elevator" in general, an interesting FAA Airworthiness Directive (AD 77-14-09) ominously entitled "Aircraft Controlability" was issued against the 182P and Q airplanes in 1977. Apparently, during flight testing for an autopilot STC, FAA found that there was insufficient nose-down elevator control during flaps-down, power-on flight. The AD required the aft CG limit to be placarded and moved forward 2.5 inches until the horizontal tail spar, if found uneven, could be bashed back into shape with a block and mallet! Turns out that defective tooling was turning out almost imperceptible changes to the tail spar, causing flow separation across the elevator.

Although the 172 and 182 share the same wing (flap limits in the heavier 172's notwithstanding), they do not share the same horizontal tail and the AD mentioned did not apply to 172's. But in your case, maybe something similar was happening? In any event, that's probably more than you wanted to know. Thanks for your post. Hope someone can answer that first CG question. CHEERS, Tom

 

[karlbamforth]

Plasgears you said, "run out of down elevator on finals",.
Sounds damn scary to me I would expect to be holding up elevator, and be running out of up elevator in the flare.
Maybe I misunderstood.

Karl

 

[plasgears]

Karl,

As I came over the numbers, I was clanking against the down elevator stop, and the airspeed was lower than I wanted, ~50 mph. When power was added I regained some semblence of down elevator control. The added power worked on the down elevator to drop the nose and add airspeed.

Could the plane have been out of rig? Possible. A week later, this same ship was involved in a fatal with a low time pilot in IMC. The FBO was padlocked by FSDO after critical investigation.