Overweight or out of CG

[MauleSkinner]

Gotcha! And on that point, I agree with you wholeheartedly as well. The "level turn" was assumed; doesn't apply the same way to a coordinated descending turn from base to final.

It doesn't?

 

[TangoWhiskey]

It doesn't?

I don't believe so, but am willing to educated.

If I roll into a 45 degree bank in a descending constant airspeed turn (not trying to maintain altitude), I'm not going to pull the same load factor as if I try to maintain a constant-altitude with that same bank angle.

 

[MauleSkinner]

I don't believe so, but am willing to educated.

If I roll into a 45 degree bank in a descending constant airspeed turn (not trying to maintain altitude), I'm not going to pull the same load factor as if I try to maintain a constant-altitude with that same bank angle.

"It depends on the circumstances."

Basically, if you're in a constant descent- or climb-rate turn, the load factors on the airplane are the same as a constant-altitude turn.

On the other hand, if you are in level flight, and roll into a 45-degree bank WHILE initiating a descent, you're reducing your load factor the same as you would be by pushing the nose over while wings level...the load factor will decrease UNTIL you're stabilized in the descent (constant rate). At that time, you'll also stabilize at the same load factor as a constant-altitude turn (per your chart) for the bank angle you're using.

 

[gismo]

"It depends on the circumstances."

Basically, if you're in a constant descent- or climb-rate turn, the load factors on the airplane are the same as a constant-altitude turn.

On the other hand, if you are in level flight, and roll into a 45-degree bank WHILE initiating a descent, you're reducing your load factor the same as you would be by pushing the nose over while wings level...the load factor will decrease UNTIL you're stabilized in the descent (constant rate). At that time, you'll also stabilize at the same load factor as a constant-altitude turn (per your chart) for the bank angle you're using.

That's exactly correct. The notion that a constant descent somehow reduces the wing loading is a commonly held (incorrect) belief. To lower the wing loading you must be accelerating (increasing your velocity towards or decreasing your velocity away from the earth).

 

[TMetzinger]

One other point - all the discussion so far has focused on the takeoff, and for that I agree it's better to be within CG and overweight provided you have enough runway for the departure.

But you need to look at your CG and weight upon landing too.

 

[Ghery]

I was told some years back that the plane in the movie "Flight of the Phoenix" (I forget if it was the original or remake) crashed after going over the hill in the last scene because of forward CG and the pilot died.

You made this a theoretical question, but it was practical for me on a recent phase check for my student solo privileges. Between the CFI and me in a Skyhawk we were 550 pounds in the front seat. I calculated the W&B like seven times and even only fueled to the tabs, we were forward CG. I had a 20lb bag of kitty litter with me ready to ride in the tail to balance us out. At full fuel there's really no way to make it work: you're either forward CG or overweight.

So here's my question. Page 6-9 of the POH says the moment arm for the front seat is 37. But it's actually 34-46, for the seat adjustment. I'm tall, so I ride pretty far back. Just how far back? I guess I should measure.

Time to buy a 182.

It seems the 182 has a similar issue. In my calculations with Pilot/Passenger (350 lbs), full fuel, rear seats empty, and no baggage, the CG is 1/2" forward of the limit. I have been told that the 182's, especially the newer models, tend to be nose heavy. I asked the pilot who ferried mine about this and he said that just be prepared for the nose to drop significantly when power is cut. So, I am thinking that perhaps this common expectation is because with two persons in front only, the CG is so far forward. So, if with two persons, I always added 80 lbs (max limit) to the most rearward area, this would shift the CG 2" rearward and bring it within the envelope.

Perhaps this would make the nose heavyness less so. I have yet to fly the airplane and want to reduce my risk of nose damage...

I just ran the W&B spreadsheets for the C-172N and C-182P in our club. 550 pounds in the front seat of either with nothing in the back seat or baggage compartment and you are still aft of the forward CG limit. Not sure where the problem might lie. Oh, and the C-172N has the Penn Yan 180 hp conversion, so with the extra 250 pounds on the max gross you're still about 200 pounds under max, even with full long range (50 gal useable) tanks. Only about 100 pounds under max gross with full tanks (75 gal useable) in the 182.

 

[jesse]

Every chart I've seen on FAA tests and training publications indicates that load factor and stall speed increase with bank angle.

Per the graph below, a 60-degree bank introduces a load factor of 2 and an increase in stall speed of 40% over that experienced in level flight. How am I misinterpreting this?

You're assuming a level turn. I can bank as steep as I want, without the risk of stalling, provided I don't intend on maintaining altitude.

 

[jesse]

The difference with the 182 is that you can *ADD WEIGHT* to the back and fix the CG without putting yourself over gross.

If you're two rather large guys in a 172 and want to stay within CG, stay under gross, and depart with full fuel you're kind of hosed. 182 - just add some weight in the back and you're set.

Of course there are limits to the above. But you can handle the problem better with a 182 then you can a 172.

 

[Capt. Geoffrey Thorpe]

The difference with the 182 is that you can *ADD WEIGHT* to the back and fix the CG without putting yourself over gross.

If you're two rather large guys in a 172 and want to stay within CG, stay under gross, and depart with full fuel you're kind of hosed. 182 - just add some weight in the back and you're set.

Of course there are limits to the above. But you can handle the problem better with a 182 then you can a 172.

Have one guy sit in the back.

 

[skidoo]

Have one guy sit in the back.

Great idea! When me and my instructor go out for the first flight, I think I'll ask him to sit in the back

 

[MauleSkinner]

Great idea! When me and my instructor go out for the first flight, I think I'll ask him to sit in the back

You'll never get anywhere in aviation with that whimpy attitude! Demonstrate your mastery of PIC authority and TELL him to sit in the back!

 

[skidoo]

The difference with the 182 is that you can *ADD WEIGHT* to the back and fix the CG without putting yourself over gross.

If you're two rather large guys in a 172 and want to stay within CG, stay under gross, and depart with full fuel you're kind of hosed. 182 - just add some weight in the back and you're set.

Of course there are limits to the above. But you can handle the problem better with a 182 then you can a 172.

On my particular 182, I have lost 100 lbs usable due to the turbo, and a few more lbs for other stuff. With two in front and full fuel it is out of limits, but with 64 gallons or less, it is within. The CG at full fuel is 38.7" and gradually moves forward to 37.1" when empty. But, the limits change as the 182 gets lighter too. The actual CG doesn't change much over the weight range, but the limits do. The limit at full fuel is 39.3". and at empty fuel is 34.3"

I can add another 120 lbs and be at gross with full fuel and two empty seats. If I add that in the baggage area, that brings it to just within limits. Looks like I will need to be flying mostly between 1/4 and 3/4 fuel loads...

 

[rottydaddy]

I was told some years back that the plane in the movie "Flight of the Phoenix" (I forget if it was the original or remake) crashed after going over the hill in the last scene because of forward CG and the pilot died.

Nope... in the original (there was no actual "Phoenix" in the remake, only models and CG stuff), the plane was making one of several "landing passes" for the cameras, and somehow "stubbed" the mains on the rough terrain, or just touched down too hard.
The force of the impact caused the aft fuse to break off. The plane immediately fell on its nose, and ended up on its back. Forward CG may have been a factor (it does touch down very flat, as if Mantz could not keep the nose up), but it didn't nose-dive off the cliff or anything like that.

The cameras were rolling... if you're curious enough to stand watching a fatal accident, have a look...

http://www.youtube.com/watch?v=n82nN_lqn58

 

[Steve]

Narrative

http://www.check-six.com/Crash_Sites/Mantz-P1.htm

Nope... in the original (there was no actual "Phoenix" in the remake, only models and CG stuff), the plane was making one of several "landing passes" for the cameras, and somehow "stubbed" the mains on the rough terrain, or just touched down too hard.
The force of the impact caused the aft fuse to break off. The plane immediately fell on its nose, and ended up on its back. Forward CG may have been a factor (it does touch down very flat, as if Mantz could not keep the nose up), but it didn't nose-dive off the cliff or anything like that.

The cameras were rolling... if you're curious enough to stand watching a fatal accident, have a look...

http://www.youtube.com/watch?v=n82nN_lqn58

 

[flyingcheesehead]

It seems the 182 has a similar issue. In my calculations with Pilot/Passenger (350 lbs), full fuel, rear seats empty, and no baggage, the CG is 1/2" forward of the limit.

That's weird... It's virtually impossible to load our 182 out of CG if it's at or under MGW. With 650 lbs of people in front, it's at the front limit. With the maximum baggage capacity in the back and the back seaters heavy enough to take the plane up to MGW it's still within the aft limit.

 

[poadeleted20]

You're assuming a level turn. I can bank as steep as I want, without the risk of stalling, provided I don't intend on maintaining altitude.

Only up to a point, like Vne. In order to make an underloaded turn, you must be allowing a vertical acceleration downward, and that means a continually increasing rate of descent. At some point, you must either stop that acceleration and load up the wings again or say good-bye to your wings.