Weight and Balance Gurus enter in

[DFH65]

So been doing a bunch of calculations for possible purchase and the empty CG seems to be really far forward. See other thread here: https://www.pilotsofamerica.com/com...-on-late-50s-172s-whats-your-empty-cg.120950/

Anyway all this got me thinking can anyone explain the reason for the front angle on the CG envelope. From what I read on line it has something to do with certification and stick forces. My question is one of curiosity since the condition can be remedied simply with ballast but is this considered a forward CG condition with the normal forward CG flight characteristics? Is it in CG but out of envelope? What flight characteristics would you expect?

 

[Bell206]

the empty CG seems to be really far forward

Don't quite follow your post. You show a 170 chart but reference a 172 previous post. Plus don't offer any wts/arms/moments to look at. As an FYI, some aircraft don't have an Empty Weight C.G. Range, i.e., must be within the chart. Check the TCDS but I think most Cessna do not. If you load your aircraft with a "normal" crew, pax, and fuel and your outside the chart perhaps your empty weight and balance figures are incorrect????

 

[Dan Thomas]

So been doing a bunch of calculations for possible purchase and the empty CG seems to be really far forward. See other thread here: https://www.pilotsofamerica.com/com...-on-late-50s-172s-whats-your-empty-cg.120950/

Anyway all this got me thinking can anyone explain the reason for the front angle on the CG envelope. From what I read on line it has something to do with certification and stick forces. My question is one of curiosity since the condition can be remedied simply with ballast but is this considered a forward CG condition with the normal forward CG flight characteristics? Is it in CG but out of envelope? What flight characteristics would you expect?

You have a problem with your W&B documentation. The CG should never be out of the forward limit even with one light person and low fuel.
And if this your empty CG, you really do have a documentation problem. No 170B should be 1950 lbs empty.

Errors in W&B are too common, and an actual re-weigh is the only solution unless one has all the superseded documents and can find where someone added instead of subtracted, or mixed up forward and aft of datum. Easy to do with a Cessna.

I once did a prebuy on a Taylorcraft that had serious W&B troubles. It had a useful load of 120 lb according to the document.

 

[DFH65]

Sorry should have explained I used an online 170B calculator but put in all the numbers for this 1957 172. A 1957 Cessna 172 is essentially a 170B with a nose wheel so the calculator is similar in many respects. The numbers I put for the above chart intentionally put the bullseye in that area for demonstration purposes they are not empty weight with full fuel.

My question wasn't really about this specific airplane as much as I am curious as to why most weight and balance envelopes have the upper left corner cut off and what that means practically. Is the CG out or range or just some other limitation that makes the envelope not square(if that makes sense).

Here is an example of this planes W&B numbers with two FAA 170 pounders and full fuel with no baggage. Just seems odd that it would be out of the envelope. This tells me it may be a W&B error. If I can't find a newer W&B we will reweigh it for sure.

 

[flyingron]

Control authority. You only see it on the front side of the graph because you're fighting the weight to keep the nose up.

 

[Bell206]

A 1957 Cessna 172 is essentially a 170B with a nose wheel so the calculator is similar in many respects.

Not quite. Each aircraft model is unique when it comes wt/bal even between variants within a specfic model line. So a 170 is not "similar" to a 172. It just creates confusion.

why most weight and balance envelopes have the upper left corner cut off and what that means practically. Is the CG out or range or just some other limitation that makes the envelope not square(if that makes sense).

Not most envelops. The design of the wt/bal chart is more based on where the datum point is located. But in general charts with no negative arms will have the front cut off due as stated above. In order to fly, an aircraft must have more control movement available to lift the nose up. The CG envelop is strictly to insure control authority.

As a side note, helicopters have a separate empty weight/balance chart/limit that resides in the mx manual. If the empty weight/bal is outside this empty weight limit then the operational/loading limits can not be guaranteed. For example, a Bell 206 requires a minimum pilot weight of 170lbs to maintain these limits. ALL light weight 206 drivers carried a weight bag in order to fly.

 

[IK04]

It's called TDF: Tail Downforce.

With the CG that far forward at that total weight, there is not enough elevator authority to provide enough TDF to meet the certification requirements for dynamic stability.

 

[DFH65]

I realize each individual aircraft is unique but as far as the arms go the early 172s are identical to the 170B so a 170B calculator works well and only requires a few tweaks. The CG envelope appears to be exactly the same.

 

[Clip4]

So been doing a bunch of calculations for possible purchase and the empty CG seems to be really far forward. See other thread here: https://www.pilotsofamerica.com/com...-on-late-50s-172s-whats-your-empty-cg.120950/

Anyway all this got me thinking can anyone explain the reason for the front angle on the CG envelope. From what I read on line it has something to do with certification and stick forces. My question is one of curiosity since the condition can be remedied simply with ballast but is this considered a forward CG condition with the normal forward CG flight characteristics? Is it in CG but out of envelope? What flight characteristics would you expect?

I do not believe you can load a 170 with a BEW of ~1300# with 433# of fuel and people and get a CG any where near 36.4.



.

 

[Bell206]

The CG envelope appears to be exactly the same.

To each their own. But that assumption is neither correct or legal.

 

[Lindberg]

You have a problem with your W&B documentation. The CG should never be out of the forward limit even with one light person and low fuel.
And if this your empty CG, you really do have a documentation problem. No 170B should be 1950 lbs empty.

Errors in W&B are too common, and an actual re-weigh is the only solution unless one has all the superseded documents and can find where someone added instead of subtracted, or mixed up forward and aft of datum. Easy to do with a Cessna.

I once did a prebuy on a Taylorcraft that had serious W&B troubles. It had a useful load of 120 lb according to the document.

It's very easy for my '48 170 to be out of the forward limit with two full-size people up front and nothing in the back. And the fuel tanks are so close to the CG they make little difference between full and empty.

 

[dmspilot]

Anyway all this got me thinking can anyone explain the reason for the front angle on the CG envelope. From what I read on line it has something to do with certification and stick forces. My question is one of curiosity since the condition can be remedied simply with ballast but is this considered a forward CG condition with the normal forward CG flight characteristics? Is it in CG but out of envelope? What flight characteristics would you expect?

The envelope represents the allowable weight and CG combinations, it can't be "in CG but out of envelope". The airplane is too heavy for the CG, or the CG is too for forward for its weight — two ways of phrasing the same thing.

 

[DFH65]

Control authority. You only see it on the front side of the graph because you're fighting the weight to keep the nose up.

This is what I was looking for. Thanks!

It's very easy for my '48 170 to be out of the forward limit with two full-size people up front and nothing in the back. And the fuel tanks are so close to the CG they make little difference between full and empty.

This is also helpful for my previously posted question.

To each their own. But that assumption is neither correct or legal.

There is nothing incorrect or illegal about using an online calculator. I am using this one here: http://trumpetb.net/alph/wb170B.html Yes it is labeled as a 170B but I am plugging in the numbers for a 1957 172. It is just doing the math and plotting the graph. If you look at the C.G. envelope graph on that page it corresponds to the C.G. envelope in the TCDS for the 1957 172 exactly so in theory the plot should be correct as well.

172 TCDS info
C.G. Range Normal (+40.8) to (+46.4) at 2200 lbs. (+36.4) to (+46.4) at 1733 lbs. Utility category (+38.4) to (+40.3) at 1950 lbs. (+36.4) to (+40.3) at 1733 lbs. or less

 

[Old97]

I dont remember the details but an AP i know said when he rebuilt his 1959 175 he called cessna and they confirmed their moment range in the book was incorrect.

 

[DFH65]

People make mistakes...maybe instead of 36.9 it is 39.6 ?

I am kind of hoping this was the case. Back in 57 the sheet was done with a typewriter (do you kids remember those?) so this is certainly possible. I am hoping to look through the paperwork again today and see if I can find a more up to date weight and balance. If there isn't one we will have one done anyway.

 

[DFH65]

The envelope represents the allowable weight and CG combinations, it can't be "in CG but out of envelope". The airplane is too heavy for the CG, or the CG is too for forward for its weight — two ways of phrasing the same thing.

Makes total sense. I know the plane has to be flown in envelope. I am just trying to understand the physics and the actual flight characteristics around this. When I become a famous airplane designer this will all come in handy.

 

[Bell206]

I am just trying to understand the physics and the actual flight characteristics around this.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-1.pdf

 

[DFH65]

https://www.boldmethod.com/learn-to-fly/performance/how-does-cg-affect-aircraft-performance/

This was helpful in visualizing the TDF as well.

 

[CharlieD3]

Just for grins and giggles, I looked at the POH for my old rental 172. Extrapolating the chart for no fuel, no people, just oil; the W&B comes out just at the forward edge of the utility category.

Please note, I said extrapolating. I added 4 graph squares, and extended the cg envelope linearly, following the existing envelope forward limits, beyond the lower left of the graph.

That was for a bare bones 172 without an actual (known to be different because of accessories)W&B. Published stock numbers.

Ghost flyer could fly the plane within the envelope with weightless fairy dust for fuel, and still be within limits.

I wouldn't trust some on-line calculator to figure my weight and balance. Especially one that changed the aircraft type, without being sophisticated enough to change the labeling of the aircraft type in the chart. Too many unseen variables for me.

 

[Caramon13]

I remember I had to carry around 50 lb sacks of sand to use as ballast in my Cherokee 180. It was just naturally nose-heavy and with anyone up front and full fuel you almost had to load those in to get into the envelope or to get to where it wasn't RIGHT on the front of the slope.

 

[Dana]

Not most envelops. The design of the wt/bal chart is more based on where the datum point is located. But in general charts with no negative arms will have the front cut off due as stated above. In order to fly, an aircraft must have more control movement available to lift the nose up. The CG envelop is strictly to insure control authority.

The position of the datum has nothing to do with the shape of the envelope; it only affects what the actual cg numbers are. Whether you call a position, say, 30" act of the firewall or 60" act of the prop flange, it's still the same position.

The limits on the left side (forward cg) are determined by having enough elevator authority to flare, and so is weight dependent. The aft limit is based on static stability, which is (generally) not weight dependent.

 

[Ghery]

You have a problem with your W&B documentation. The CG should never be out of the forward limit even with one light person and low fuel.
And if this your empty CG, you really do have a documentation problem. No 170B should be 1950 lbs empty.

Errors in W&B are too common, and an actual re-weigh is the only solution unless one has all the superseded documents and can find where someone added instead of subtracted, or mixed up forward and aft of datum. Easy to do with a Cessna.

I once did a prebuy on a Taylorcraft that had serious W&B troubles. It had a useful load of 120 lb according to the document.

I remember I had to carry around 50 lb sacks of sand to use as ballast in my Cherokee 180. It was just naturally nose-heavy and with anyone up front and full fuel you almost had to load those in to get into the envelope or to get to where it wasn't RIGHT on the front of the slope.

Back when the club had a 1969 Arrow with a 3 blade prop I had to use a 50 pound weight in the baggage compartment to bring the CG back behind the forward limit if I was flying it solo. IIRC putting someone in the right seat eliminated the need for the weight in the baggage area. We sold it a few years ago, so I haven't had to worry about it since. Our C-172s and C-182 don't have that problem.

 

[Bell206]

The position of the datum has nothing to do with the shape of the envelope;

You are correct. I misspoke. My intent was the shape of the chart is dependent on the theoretical CG point, not datum, and the useful load on either side of that CG point. For helicopters it is all weight dependent regardless whether its longitudinal or lateral as any extreme shift can limit control authority. Thanks.

 

[FastEddieB]

I remember I had to carry around 50 lb sacks of sand to use as ballast in my Cherokee 180.

Another ballast idea is to carry collapsible 5 gal water containers. Fill as needed when ballast is needed, take up virtually no room when empty and collapsed.

 

[flyingron]

Most of the big-engine Navions sit on the forward CG limit. There's a lead weight added to the tail to compensate (it's amazing what 9lbs will do when it is that far on the arm). People have done things like moving the battery and avionics back as well.

 

[Bell206]

5 gal water containers.

Just hope it doesn't leak in flight.

 

[FastEddieB]

Just hope it doesn't leak in flight.

That goes for Tequila bottles as well.

DAMHIK!

 

[PeterNSteinmetz]

Another ballast idea is to carry collapsible 5 gal water containers. Fill as needed when ballast is needed, take up virtually no room when empty and collapsed.

Also very useful in Arizona. When full they are emergency water.

 

[DFH65]

Follow up question. In the TCDS for the early 172s the seats are listed at 2 at +36" and 2 at +70"
I see the later Cessna TCDS show a range from like 34" to 46".
It is amazing what a couple of inches will do when it is the bulk of the weight in the plane and it is behind the EWCG.

My question is this...Does anyone know if the 36 inches measured with the seat forward, all the way back or somewhere in between?

Obviously I can measure it next time I am at the airport and find out where the seat CG really is when I am in it.

 

[Bell206]

Does anyone know if the 36 inches measured with the seat forward, all the way back or somewhere in between?

Looking at the station reference chart STA 44.0" is at the bulkhead location in the center of the door. So if I had to guess the 36" puts the seat forward.

 

[Dan Thomas]

The 36" is where most folks will have the seats. They show the 34-46" travel because the seats can indeed run back a long way but it's unlikely that anyone will fly from that position unless they're seven feet tall.

When in the seat, your belt buckle will be very close to your body's CG. Unless you have a 60" waist...