Light on nose and consequent effects - a CoG problem?

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metzgergva
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Light on nose and consequent effects - a CoG problem?

Post by metzgergva »

Hi,
I have read a couple of times and experienced myself that you can roll on the main gear quite long. That should not be the case.
I also found it confusing that when you trim to the TO mark the plane pitches up and stalls unless you push against it. It should fly with about climb speed depending your load.
I found that when I load her up with 4 people that the forward trim is close to max in cruise and the takeoff is even more brutal on pitching up.
I found that the aircraft looses its pitch stability when you fly level and fully loaded, meaning it reacts wrongly when changing power.
It misses inertia in pitch direction and the elevator is oversensitive.

All that lead me to check the CoG empty and consequently how it moves with fuel and pax added and it explains directly what I experienced in flight.

With just pilots in front the CoG is barely before the CoL and with adding pax in the back it goes behind CoL. That should never happen and that is why planes have a defined CoG range.
And the Cog is very close to the main gear putting litte weight on the front end.

So the basic laws of flight stability are not met reading the internal FSX data which is what FSX finally flies with. Accusim certainly has an important influence with it's outside FSX calculations to overcome limitations of the FSX flight model, but at the end it is FSX that simulates the flight behaviour that are based on the data that accusim delivers to FSX.

Setting CoG and CoL right is vital for flight stability at any phase of flight and should be easy to correct.

I love all the features of accusim and the systems and maintenance, but the C172 as a trainer aircraft is very stable and follows the rules of flight.
Happy flying!
Alexander

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jgraddon
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Re: Light on nose and consequent effects - a CoG problem?

Post by jgraddon »

Good call!

smokeyupahead
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Re: Light on nose and consequent effects - a CoG problem?

Post by smokeyupahead »

Great explanation! Bravo...


David
Cheers,
David DD

voth
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Re: Light on nose and consequent effects - a CoG problem?

Post by voth »

hi

sorry for using your thread for this but as it is about CoG i feel free to request your help :p

today wanted to check if my fully loaded 172 is within the CG limits but i had two problems with it:

1.) the manual (page 46)says: "Reference Datum: Lower portion of front face of firewall."
so far so good - where is it? i assume somewhere between engine and pax compartment?

2.) i would need some distance from the ref.datum to the "load sections" like the seats or the fuselage to calculate the momentum wouldn't i?

so is this data anywhere provided or did i just miss it?

metzgergva
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Re: Light on nose and consequent effects - a CoG problem?

Post by metzgergva »

voth wrote:hi

sorry for using your thread for this but as it is about CoG i feel free to request your help :p

today wanted to check if my fully loaded 172 is within the CG limits but i had two problems with it:

1.) the manual (page 46)says: "Reference Datum: Lower portion of front face of firewall."
so far so good - where is it? i assume somewhere between engine and pax compartment?

2.) i would need some distance from the ref.datum to the "load sections" like the seats or the fuselage to calculate the momentum wouldn't i?

so is this data anywhere provided or did i just miss it?
Typically these data are in the flight manual accompanied by an actual aircraft balance report and a calculation is provided that you need to check prior your flight.
A2A has not used real world data in the aircraft.cfg but you could make the calculation from their geometric data base in the aircraft.cfg

The datum and the empty CoG are set to 0.0.0
[WEIGHT_AND_BALANCE]
...
reference_datum_position = 0, 0, 0
empty_weight_CG_position = 0, 0, 0

You find the leading edge and MAC data under
[airplane_geometry]
...
wing_root_chord =5.06000 //Feet
....
wing_pos_apex_lon =1.5 //Feet, longitudinal distance from reference point, negative going aft

From that you can calculate that the CoG empty is at 29.6% MAC (1.5/5.06) (which is my point in the argumentation)

And all load stations under
[WEIGHT_AND_BALANCE]
...
and as example
station_load.0 = 0.00, 0.50, -0.20, 0.00, Pilot
meaning the pilot's weight is located 0.5 feet forward from the datum position, etc.

Hope that helps!
Happy flying!
Alexander

voth
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Re: Light on nose and consequent effects - a CoG problem?

Post by voth »

yes helps a lot, thank you for explaining

Alec246
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Re: Light on nose and consequent effects - a CoG problem?

Post by Alec246 »

http://www.youtube.com/watch?v=Uwy29IfglWY

Here's a video demonstrating that while a C172 can rollout with nose gear in the air, it requires power, As soon as we hear the pilot chopping the power, the nose goes back to the ground

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Killratio
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Re: Light on nose and consequent effects - a CoG problem?

Post by Killratio »

Alexander,

When you say "fully loaded" are you talking 4 passengers plus full fuel?

Darryl
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>


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metzgergva
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Re: Light on nose and consequent effects - a CoG problem?

Post by metzgergva »

Daryl, sure not as MTOW would be exceeded.

My point is that the empty plane is currently at 29.6 % MAC. Pilots bring it a tad forward and fuel backward. CoL seems to be at 33% MAC, which is a typical value and FSX default. Adding pax and luggage and reducing fuel so that MTOW is not exceeded brings CoG behind CoL! While if CoG empty would be 22% the full plane would be around 30% and by that stable from an aerodynamics point of view.

Unfortunately I don't have a C172R balance sheet, so I do not know what the real world values are but for GA stable trainer aircraft usable CoG range would typically be 20-30% MAC with sweetspot at 25 and the rear limit 3-5% before CoL. With 25% MAC loading it should fly normal cruise speed with neutral trim.

The actual plane may deviate a bit but not much. And every plane is a bit different depending on avionics, tank configurations and extra equipment.

I'm sure that just getting the CoG starting point and its movement more closer to the reality and I guess A2A has all data available, it will solve all issues reported in my original post.
Happy flying!
Alexander

Caldemeyn
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Re: Light on nose and consequent effects - a CoG problem?

Post by Caldemeyn »

Maybe this post by Mr. Killratio could be an answer to some problems concerning CoG (4th post) http://a2asimulations.com/forum/viewtop ... 07&t=35876

Q_Flyer

Re: Light on nose and consequent effects - a CoG problem?

Post by Q_Flyer »

It's fantastic to have one of the purveyors of some of the greatest flight models that FS has seen, inputting into this debate.

I posted exactly what you said regarding just after take-off, Alexander, with trim set to the T/O marker, and the need to then assert a lot of forward pressure on the yoke, to avoid a stall..... but I was told by some on Avsim that this was "normal behaviour" ... :roll:

I really hope A2A will listen to and consider Alexander's advice and suggestion here, regarding CoG..... it makes such logical sense for the problems we are seeing.

Cheers.

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Killratio
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Re: Light on nose and consequent effects - a CoG problem?

Post by Killratio »

Cheers Alexander..just making sure.

The R is prone to rear CoG and in fact, personally, I learned very early on to refuse to fly it with four adults on board, even at or below MTOW. Legal, within limits, supposedly "safe" but to tell the truth, I've never liked the way it feels loaded like that. Personal preference for sure but hey..I'm PIC!!

Stalling with 4 adults is VERY unpleasant and it takes far too much effort to get the nose down. Any slow flight is unpredictable in that configuration and I've had a nasty experience on takeoff as well, with a gust, which left me with nose high, no control effect and drifting towards trees at the side of the runway.

Regardless of .cfg files, FSX limitations etc, the R is certainly not a "big 152" like the older C172's (which for pure flying fun, I much prefer...AND I miss 40 deg of flap!!) but it is more comfortable. My understanding is that compromises in cfg had to be made to produce overall effect and, as usual, in those cases, the more extreme ends of the envelope may well suffer.

I don't do FM work but I can tell you that having flown R models since 1998, I (and other vastly better/more experienced pilots on the beta team) are generally happy with the end result.

Is it perfect..no, I haven't yet seen that in FSX...I do think you are right though, CoG is contributing to some of the percieved differences. Many of the criticisims are, however, coming from pilots with experience (some considerable) of the older models, assuming that the R performs very similarly. This is not the case.

regards

Darryl
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>


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metzgergva
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Joined: 06 Jun 2012, 03:39

Re: Light on nose and consequent effects - a CoG problem?

Post by metzgergva »

Thank you Daryl for your insight. I have in the meantime received a C172R manual and can read the movement of CoG with loading it up to MTOW.
A maximum rear allowed value of 36.4% must bring it very much close if not at CoL and that causing instability and your described stall issue of that specific model. But unless otherwise proven by data I have difficulties to accept that the approved CoG range goes behind CoL for a GA aircraft. And from the manual I can read that CoG empty is about 24% MAC.

We will see what A2A flight dynamics engineers will come up with. Hopefully soon.
Happy flying!
Alexander

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Killratio
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Re: Light on nose and consequent effects - a CoG problem?

Post by Killratio »

Alexander,

Thanks for the figures and agreed, hopefully there will be some info/input soon. As very much a "stick and rudder" guy, it is always interesting to see some of the science behind handling.

best regards

Darryl
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>


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Romeo_Tango
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Re: Light on nose and consequent effects - a CoG problem?

Post by Romeo_Tango »

metzgergva wrote:Hi,
I have read a couple of times and experienced myself that you can roll on the main gear quite long. That should not be the case.
I also found it confusing that when you trim to the TO mark the plane pitches up and stalls unless you push against it. It should fly with about climb speed depending your load.
I found that when I load her up with 4 people that the forward trim is close to max in cruise and the takeoff is even more brutal on pitching up.
I found that the aircraft looses its pitch stability when you fly level and fully loaded, meaning it reacts wrongly when changing power.
It misses inertia in pitch direction and the elevator is oversensitive.

All that lead me to check the CoG empty and consequently how it moves with fuel and pax added and it explains directly what I experienced in flight.

With just pilots in front the CoG is barely before the CoL and with adding pax in the back it goes behind CoL. That should never happen and that is why planes have a defined CoG range.
And the Cog is very close to the main gear putting litte weight on the front end.

So the basic laws of flight stability are not met reading the internal FSX data which is what FSX finally flies with. Accusim certainly has an important influence with it's outside FSX calculations to overcome limitations of the FSX flight model, but at the end it is FSX that simulates the flight behaviour that are based on the data that accusim delivers to FSX.

Setting CoG and CoL right is vital for flight stability at any phase of flight and should be easy to correct.

I love all the features of accusim and the systems and maintenance, but the C172 as a trainer aircraft is very stable and follows the rules of flight.
I suspect that this is the reason that every one is experiencing the over sensitive elevator control.
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