CG sensitivity 3

[Macm_020171]

Hello,

What is making an aircraft design more or less sensitive to CG location. I heard in the past that aircraft might me more CG sensitive than other aircraft, so CG might be quickly or no so quickly out of the envelope.

 

[rb1957]

that'd be the airplane's stability derivatives ... how quickly the airplane's stability changes (with CCG position).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

Its all about the aircraft configuration - wing location and size, empennage or canard location and size, engine location, fuel location, payload location, weight distribution, control surface sizes, etc, etc.

 

[IRstuff]

It's a design result; you can trivially design an A/C that's unstable by putting its CG in a bad place



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[Macm_020171]

I wanted to follow up on this interesting subject. I was working on a program during conceptual design of the airframe layout and systems integration. It came out that the CG was very delicate and at some point we were not able to change much the structural layout and systems architecture, or distribution along the aircraft in order to save weight. So at some point the CG was ruling on top of weight savings and the aircraft was heavier than target. I'm not an expert in aircraft design, but it made some noise to me that program didn't want to change the design of the aircraft to make the CG a bit more flexible allowing to improve the structure layout to save weight.

 

[rb1957]

airplane design is a horrible set of compromises. Get them right and you have a winner, get some wrong and you have a lame duck.

in the case in question, the guy at the head of the table decided that the consequences of changing the CG range (anything from nothing to a complete redesign of the aerodynamics) were more severe than the benefit of controlling the airplane weight. Yes, increasing the weight would also have significant consequences for the structural design (anything from nothing to extensive redesign), but that was his decision.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

The wider the acceptable CG range the less efficient the plane will be, mostly from flying at a higher CL for the horizontal stabilizer than necessary. Since the aspect ratio for the horizontal stabilizer is generally lower than for the wing, the induced drag component is further penalizing.

It is of interest to know what the sensitivity was. It should not be stability; it's more likely efficiency that is lost.

In early design I would expect that one could just move the wing location to place the center of lift as required. A lot of planes get a nose weight added as it is far too easy to build a tail-heavy airplane.

 

[Macm_020171]

rb1957, we were at a conceptual design phase, a bit far from PDR, I would think that maybe tweaking the aircraft design might have gotten us to have a more efficient airframe and overall performance of the aircraft, but you're right that program might have other constraints such as budget and schedule.

 

[rb1957]

A HUGE part of airplane design is deciding when you've "optimised" enough. The quotes are very intentional ... no design is fully optimisied, well not with out a whole bunch of qualifying statements. In order to keep the design space small enough to come to grips with I think you need to say these parameters are primary and won't change (AUW, number of engines, number of pax, cost, schedule). Cost and schedule will obviously change (as time goes by) but the sense is not it increase the price to trade off some weight (which would also gain you range) ... neither of which are "primary parameters" (on my list, not to say your project).

But design decisions taken at conceptual design phase will only be proven far in the future ... but they can be proven wrong quite quickly, and a brave design team will acknowledge this and change direction (even at the cost of schedule) rather than keeping on pushing forward.

One example of a bad design decision is the DC-10 vs the L-1011. The DC-10 beat the L-1011 to market and established itself, and the L-1011 quickly became a market loser.

Another example of a bad design decision is (IMHO) the 737 Max. The basic airframe was pushed too far, a much earlier design than the A320 it had much lower ground clearance. This exposed it to the higher by-pass engines (bigger nacelles). The '90s redesign pushed the nacelles to have flattened bottoms. Boeing had to do something to counter the A320 Neo (the design that killed Bombardier's C-series, but that's another story). You can see the trouble they had in that it took them a year to solidify the 737 Max. Whatever happened behind closed doors, they took a bunch of decisions, and the rest is history (no belittling the hundreds of people killed).

FWIW, I think a type design should be limited to 40 years production, at which time it must be recertified. That would stop the continual "grand-fathering". Over that 40 years the design would be "brushed up", as the '37 was done, but in my world Boeing would have "clean-sheeted" the '37 in the '00s (when they were "effing" around with a variety of other designs).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

The more detail one examines on the MAX, the less blame the MAX should have. The 737 series did get new wings and cockpit and engine when they went from Classic to NG; not sure how much more clean a sheet one gets on an aluminum tube. The main problem was accepting a system that produces false stall warnings. That problem predates the NG. The next problem is that non-Western airlines don't do upset training - recall that that was the primary fix for the AF447 crash. Everyone assumed that a reminder of what the trim cutout switches were for would be the temporary solution after Lion Air. Then Ethiopian did zero additional training - those pilots didn't even respond correctly to the 30-40 year old stall warning. There is a reason that, after blaming Boeing down to the guys emptying trash cans, that Ethiopia has more MAX planes on order -and- they have not released their final accident report, now 2 1/2 years late, apparently because the other reviewers asked about the training and handling of the Emergency AD, the FCOM update, and likely the Preliminary report on Lion Air.

What took Boeing a long time was solving the following problem: What to do about pilots who do exactly the opposite of every step in the procedure book, because the FDR on ET 302 shows the pilots did every step backwards from the manual. That's a problem that even Airbus has failed to handle.

 

[rb1957]

737 NG ... is pushing the envelop due to ground clearance ... how many other planes have flattened nacelles ? who would choose to do this, if there was any other way ?

737 MAX ... I would like to know if the American pilots had similar problems (with a single AoA vane failure causing the system to misbehave)? Did US pilots (porperly trained ?) have awareness of the MAX issues ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[3DDave]

The only training needed was for trim system failure - the cause did not matter.

In all the extensive investigations there were only two aircraft that ever saw MCAS activated, one with an improperly calibrated AoA sensor and one where the AoA sensor was likely removed by a bird strike.

What every report has also been notably silent on is the training for using the trim cutout/disable switches and whether those had ever been used in the NG. My guess is total systemic complacency on the part of training organizations and continuous cutbacks in airline spending on training for unusual circumstances. Instead the training dollars seem mainly spent on how to babysit the autopilot. If the autopilot has a problem it's a coin flip for some airlines if anyone will walk away.

The outcome of the two crash flights should have been exactly like the first known flight - the one the day before the Lion Air crash with the exact same plane and exact same miscalibrated sensor, where the pilots expressed some annoyance at the trim behavior, not a case of being thankful to be alive, but irritated at having to fall back to manual trim and 90 minutes of flight with the false stall warning and stick shaker hammering away. Misunderstanding how to manage a trim problem is understandable for Lion Air. The Ethiopian ET302 FDR speaks for itself.

 

[rb1957]

you're reinforcing my opinion about the MAX. Flawed sure, but unfairly maligned. But I think Boeing unreacted to the first accident.

In any case I think the MAX was a compromised design, a 60s design competing with a 90s one ... the issue being that the 737 was designed for low by-pass engines and the A320 for high by-pass.

Not many planes have been in production for more than 40 years. It doesn't sound that unreasonable a cut-off.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Ng2020]

There appears to be widespread misconceptions about grandfathering.
Are you familiar with the changed product rule?
Whilst the 737 type certificate dates back to the 60's, this does not mean that the Max & NG certification basis is at Amdt 25-0 across the board.

 

[kontiki99]

This is kind of an interesting topic. I see it on two different levels.

At one level, I have a buddy building an experimental Zenith 750 STOL that seats 3. It's a really nice sheet metal CNC cut kit. The CG envelope is already laid out in the documentation and the location of a lot of the weight is pretty much fixed by design owing to the control surface mechanisms.

He does have a lot of latitude owing to his choices for battery placement, engine selection etc. He's going in with a fuel injected super charged Camry engine, so he's installing an aux fuel tank with additional fuel pumps to drive the fuel injectors.

His CG envelope is weight limited at the FWD CG. It looks like a short rectangle chamfered on the forward side. I suggested to him that he build everything that had a fixed location and weight it to find the resulting CG. Then choose the locations of the rest of the equipment so when complete, max weight was closer to the aft CG limit, ideally fully loaded with pax, fuel, cargo and try to set it up so the CG only shifted forward as fuel burned.

On a different level, at work, we support large freighter aircraft. The MD11s, A300s and I think the 777s have CG management computers that transfer fuel around to manage the CG so that the aft horizontal stab doesn't have to exert any downward lift. They do that because downward lift causes induced drag which is not fuel efficient. Aft tanks are located in the horizontal stab on some aircraft, I don't know about all. On a swept wing aircraft burning fuel on the inboard tanks will tend to shift the CG aft too.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[rb1957]

"I suggested to him that he build everything that had a fixed location and weight it to find the resulting CG. Then choose the locations of the rest of the equipment" ... that's a good idea. I'd suggest placing (not fixing) the heavy components that he has some flexibility in position, then weighing and seeing if he has an issue or can better place these heavy pieces so as to improve the final result. Once bolted in place, things are harder to change ! Also weight with full fuel, empty fuel (simulated fuel weight is sufficient), and pax (200 lbs per ?) ... sure these should be close to the middle of the CG envelope but it'd be nice to know you're still within the box.

"CG management computers that transfer fuel around to manage the CG so that the aft horizontal stab doesn't have to exert any downward lift" ... I'm not so sure this is completely correct. If the H. Stab had zero lift the the airplane's pitch stability would be "not much". I think the fuel management is to keep the H. Stab force small ('cause it is inefficient due to drag, and 'cause it increases the wing lift) ... small but not zero. Also there may be some movement of the wing CP due to transonic effects which can be trimmed out, but moving the CG would reduce this trimming (and the H.Stab force).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.