A quiz question re C of G

[Pace]

You take four friends in a four seater. Load their bits and pieces and leave your friends while you shoot off to the Loo having done a C of G calculation.
Unbeknown to you while you were away one of the PAX had run back to his car and loaded a very heavy item into an unnaccessable area of the aircraft .
You take off with the aircraft out of aft C of G. How would you handle the return landing?
Now he has loaded the item in a nose area and you have the aircraft out of forward C of G how would you handle the landing with that?

Pace

[BackPacker]

Long runway, loads of speed (I'd say Vref+10 in a light single/twin). Bleed it off just above the runway, touching down when you almost run out of elevator travel. Or when the aircraft stalls, obviously.

Plus you try to move whatever weight you can move around to counterbalance the inaccessible weight.

With the weight too far forward, in a familiar airframe, I might be going up high to check slow speed handling and determine a minimum handling speed. With the weight too far aft, I'm not going to risk a deep/unrecoverable stall.

Good quiz question though. Interested to hear other responses.

[thing]

Surely you wouldn't take four friends in a four seater.................

[Pace]

Backpacker

This was supposed to be for the novices not experts :)
Many NOVICES dont realise that in both situations extra speed gives extra authority over the elevator and rudder not just for the landing but also in the climbout in such a situation.
In the climb if you normally climb at 70 kts increase that to 80 kts and the same with the landing.
Obviously get weight up front even getting the rear passengers to lean full forward etc.
If its a leggy blond drag her into the front with the two of you at least you will die happy :E

Surely you wouldn't take four friends in a four seater

Thing my names Pace not Surely. Maybe thats the C of G problem you didnt see the one who jumped in covered in a blanket and the others didnt tell you :E

Suppose you could always chuck him out to save the day ;)

Pace

[Pilot DAR]

Good catch thing, I did carry two [adult] friends in a 150 once though!

I agree with Backpacker's thoughts. Having spent many hours flying aircraft which were loaded by other people, I have formed the habit of lifting off singles early, and close to stall speed. Thus, if there is a C of G, or control issue, I have a chance at finding it early, and maybe aborting. If you got airborne without noticing it, and only realized because of an odd pitch trim setting, I'd return to land normally, other than avoiding a stall at any point, and having speed/power available to help with a flare if I needed it.

I once test flew a multi engined aircraft, which was deliberately loaded to gross weight, aft C of G for the test flight. Approaching the stall, the pitch control went rather mushy, and to fly more slowly, it became necessary to apply forward control force. I did not allow the plane to stall, though did apprear to maintaining somewhat controlled flight at a few knots CAS below the stated stall speed for the configuration.

exam C of G thread drift...

while writing an exam once, knowing that it was a multiple choice question, and I had identified the two answers which were definately not correct, the remaining two answers were something like: "You take off in C of G limits, and the C of G does not change throughout the flight" - Nope, not that answer, you burn fuel (certainly in the subject aircraft type for the exam), and the C of G is gonna change, so it must be the other answer. That answer was: "You take off in C of G limits, and because of fuel burn in flight, you land out of limtis" - nope, it can't be that answer, as of course, I did my takeoff and landing C of G calculations, and did not takeoff on a flight where landing out of limits was possible...

So, I answered the former - wrong, it was the latter answer! I complained that it was not fair to have the correct answer on an exam be a viloation of the air regulations. The writer of the exam agreed, and I believe that the question was removed.

[BackPacker]

This was supposed to be for the novices not experts

At 250 hours TT I still consider myself a novice. Especially in the company of you, DAR, IO540, SNS3Guppy and a few others that frequent this forum...:ok:

[Pace]

The other problem is pilots? How many actually do C of G calculations or think they know their aircraft well enough to know what it will takes without?

Pace

[IO540]

Nowadays, the chances are that the leggy blonde will lie about her weight by an amount bigger than the weight of anything somebody could covertly stick in the boot :)

This may sound daft but people are very heavy. A boot packed solidly with gear will probably weigh 1/3 of a modern-day adult male.

A straight overload, while being illegal, is easily dealt with by using more runway. The real problem is with loading too far forward or too far back. It can make the plane uncontrollable, with the situation possibly getting worse as fuel is burnt off.

[peter272]

Some s*d did just that to me in our Cub

we planned a trip away and while I was in the briefing room he switched tents putting in a much bigger one at the back.

On climbout I couldn't trim fully forward so it was a lot of pushing to keep behaving herself. I went straight back to the ground, but it was easier to land gently, super skygod that I am.

I then went and gave him the biggest b*ll*cking of his life. He was a pilot I thought could be trusted and he left our group soon afterwards - b*st*rd!

[Pace]

This may sound daft but people are very heavy. A boot packed solidly with gear will probably weigh 1/3 of a modern-day adult male.

The average weight of people has increased quite dramatically with the UK now being one of the most obese in Europe.

Think a while back 200 ibs was an average for each PAX and baggage?

Pace

[IO540]

he switched tents putting in a much bigger one at the back.

How much did the two respective tents weigh, and what aircraft was it?

[what next]

Think a while back 200 ibs was an average for each PAX and baggage?

When I started flying commercially in 1992, the standard figure was 185lb for an adult male. Now we are at 215lb. But this figure is not static because it depends on the opertation and the total number of passengers.

[BackPacker]

it depends on the opertation and the total number of passengers.

???

Do passengers get heavier, person-for-person, if you cram more of them in a confined space?

I can well understand that the kind of operation would determine average passenger weight (evacuating refugees from some famine-stricken African country vs. a holiday flight to Florida) but I don't see why the average weight would change with the number of passengers.

Or would it be to hide the effects of the standard deviation changing with the number of passengers?

[what next]

I can well understand that the kind of operation would determine average passenger weight...

Since passenger weight includes (standard) luggage, there is a large difference between charter/holidaymakers and scheduled air travel. And the dependeny of the individual passenger weight from the total number of passengers results from long-term statistic analysis. With few passengers only, to be statistically on the safe side, one must assume the highest individual mass.

For an example table (the first one that Mr. google found for me), see here, a variation of this table must be included in the operations manual of each AOC holder:

SUBPART J – MASS AND BALANCE (http://www.civilaviation.gov.eg/HTML/WEB-ARABIC/LOWS_REGULATIONS/ECAR_May.,2009/Part91/App91H.htm)

[Pilot DAR]

Yeah, passenger average weight can also vary with purpose of the flight. I have flown hunters into the bush, who all equppied, were a lot heavier than average. Even parachutists, in my experience, were heavier. Though they did not bring along baggage when boarding.

A review of this sad accident will remind us about loading and passenger weight....

http://www.tsb.gc.ca/eng/rapports-reports/aviation/2004/a04h0001/a04h0001.pdf

[Genghis the Engineer]

CG out of aft: likely result - particularly nasty stall characteristics, tendency to spin off a stall, over control in pitch.

Solution: Fly very gently, particular care to maintain correct approach and landing speeds and no less, use tiny gentle control movements, most especially in pitch. Whilst still at height, move whatever is feasible forwards in the aircraft.


CG out of fwds: likely result - docile stall characteristics, but at a higher stalling speed. Also a lack of pitch authority.

Solution: add about 10kn onto your approach speed, pick a long runway. If possible, move people or kit from front to rear of the aircraft.

G

[Cows getting bigger]

I would consider the effect of burning off fuel. On something like a 172 the reduction in fuel will bring the CofG forward. It may not solve the problem but it will help.

[Pace]

Solution: Fly very gently, particular care to maintain correct approach and landing speeds and no less, use tiny gentle control movements, most especially in pitch. Whilst still at height, move whatever is feasible forwards in the aircraft.

Genghis

Can you elaborate on correct approach and landing speeds. I would have thought aft and forward carrying more speed would give more authority over the tail.
I appreciate the delicate control movements but why standard approach and landing speeds for an aft C of G compared to a forward C of G?
Standard speeds would more likely lead to overcontrol and loss of control than higher speeds?

Pace

[jxk]

When I did my 'multi' in the US I was asked whether it was better to have a forward or aft CofG [providing still within limits] with regard to cruise. I answered aft, was I correct?

[Genghis the Engineer]

Pace - the major issue with being too-aft CG is not the tendency to pitch up (which you can trim, or hold), but the increased elevator authority. Higher speeds are likely to give you greater elevator authority again, which may make things even worse.

jxk - aft CG will give less tail downforce, thus you need slightly less wing lift, therefore you get slightly less induced drag. So, you were correct.

Yeah, passenger average weight can also vary with purpose of the flight. I have flown hunters into the bush, who all equppied, were a lot heavier than average. Even parachutists, in my experience, were heavier. Though they did not bring along baggage when boarding.

A review of this sad accident will remind us about loading and passenger weight....

http://www.tsb.gc.ca/eng/rapports-reports/aviation/2004/a04h0001/a04h0001.pdf

I'm not sure I agree with you there with regard to the causes - whilst the aeroplane was overweight, the amount of ice on the wing was probably much more important.

On the other hand, yes, the planning went wrong and the aeroplane was significantly overweight.

G

[Piper.Classique]

When I did my 'multi' in the US I was asked whether it was better to have a forward or aft CofG [providing still within limits] with regard to cruise. I answered aft, was I correct?
Yes, less drag from the tail, which will have to produce less download to maintain attitude, and prolly less drag produced by the trim tab. Some gliders have a water trim system to pump ballast aft for this very reason. Works however many engines you do or don't have

[englishal]

Loading outside of C of G can be dangerous!.....

I once flew this knackered old C172 to San Luis Obispo airport for lunch. Unbeknown to me the pilot door had a habit of not opening from the outside, and for some reason the pax door wouldn't unlock from outside either. So I locked the pax door, got out and shut the pilot door and had lunch......

I come back to the aeroplane and find I can't get in...Sh*t I thought, what am I going to do, after trying everything...Luckily the baggage door was open...you can see where this is going....So slim here decides "I know, I'll climb in the baggage door"...which I did.

Well I got half way in before the aeroplane did a wheelie, right in front of the Cafe with yours truely stuck half in and half out of the baggage door :O:O

Luckily I managed to enlist the help of a passer by to swing off the prop while I completed the operation. :}

I learned about C of G from that !

[Mariner9]

Seems to me that the preceeding posts in this thread overcomplicate matters and miss the bleedin' obvious.

Adding flaps results in a nose-down pitching moment.

Therefore, CofG too far aft - land with full flaps
CofG too far forward - land flapless.

Simples.

[jxk]

mariner9

Not all aircraft work like this. Do you know what causes the pitch change when the flaps are deployed?

[Genghis the Engineer]

Seems to me that the preceeding posts in this thread overcomplicate matters and miss the bleedin' obvious.

Adding flaps results in a nose-down pitching moment.

Therefore, CofG too far aft - land with full flaps
CofG too far forward - land flapless.

Simples.

Wrongles.

CG too far aft - pitch stability degraded
More flap - pitch stability degraded (certainly on, for example, single engine Cessnas)

Aft CG + flap = potentially uncontrollable aeroplane.

So actually, with an aircraft out of aft CG I'd be tempted to land flapless.


I say again, pitching moments can mostly be held or trimmed out, it's the overpowered elevator outside aft CG that is generally the problem.

G

[Pilot DAR]

An aft C of G within limits will give you a slightly faster cruise speed, as there is a little less induced drag from the H stab having to hold the nose up. I have never actually made it work for me, but the theory is a nice reason to put stuff out of the way in the back.

Decades ago I was fresh on a C 310R, which has the immense nose baggage compartment. In actually doing my preflight weight and balance calculations, I found that it's really there for carrying very light, bulky items (inflated beach toys come to mind) 'cause in most loadings, if you care to have anyone in the front with you, there's no excess weight capacity up front in the nose for baggage!

[thing]

Just a little thread drift here but I fly SIA to Singapore on the A380. I'm assuming that the a/c is chocker with fuel and they are always full with passengers. Lets say there are 400 passengers on board, if each one exceeds the 'average' passenger weight by 30 lbs just by chance then that's a lot of extra fat they have to get off the ground. Do they have a safety margin for that sort of thing or do they just use more runway and heave a sigh of relief as they pull it off the end?

[Pace]

Thing

C of G is more critical than grosse weight as long as you have a long runway.

I was involved in a Cessna 340 Ferry from the USA.

I met the aircraft In iceland to take it on. The thing was a wreck and was fitted with a huge ferry tank filling the whole Pax compartment of the 340 and allowing a few inches between the tank top and celing to slide along to get to the pilot seats.

I made some calculations and with a full ferry tank the aircraft was 1100 ibs over grosse weight which makes a mockery of those who scream with terror if their aircraft is a 50 ibs over grosse.
The big problem I had with that particular aircraft was that it had engine problems.

Loose one over that sea and there is no way the aircraft would stay up.
I refused to move it unless the ferry tank was almost empty.


Pace

[thing]

Thank you.

[Pilot DAR]

huge ferry tank filling the whole Pax compartment of the 340

Hmmm, We took a C 303 Trans Atlantic on internal fuel only, I would have thought a 340 could have made it.

I was copilot on a DHC-6 operating 3000 pounds overgross with ferry fuel in the cabin. The pretakeoff briefing included "in the case of engine failure, land straight ahead". I don't know whose neighbourhood in Rotterdam we would have graced, if one quit upon our initial departure, but it would not have been pretty. While still carrying quite a bit of that overgross fuel, we got the (non deiced) Twin Otter rather iced up over France, and that was not good. I never did calculate our weight before icing it up, but the weight, and drag of the ice, resulted in a rather maximum continuous power descent through many thousands of feet, to the south of France in August, where it happily melted off. I've often wondered if it came off in chunks big enough to make it to the ground and do any harm - so I won't say what year that was! We continued on to our destination of Rhodes, Greece, so the plane was completely ice free for landing...

[blueandwhite]

Now he has loaded the item in a nose area and you have the aircraft out of forward C of G


:ugh:Not quick to learn are you:ugh::E :p

[Pace]

BlueandWhite

I am definately not quick to learn today.

Schedule

Last Friday caught plane London To USA
Sat groundcourse and flight test Citation Recurrent 11 hr day
Sun Caught plane back UK arrived Gatwick 0730 mon morning(no sleep)

Still cannot work out whats wrong with the above that makes "ME" not quick to learn?

So give me a break :{

Pace

[V1... Ooops]

I was copilot on a DHC-6 operating 3000 pounds overgross with ferry fuel in the cabin... we got the (non deiced) Twin Otter rather iced up over France, and that was not good...

I can appreciate your predicament. I just finished ferrying a new production, non-deiced Twin Otter from Victoria to the Seychelles, via Arctic Canada, with the departure towards the Arctic made on Boxing Day.

Prior to departure, I really had no idea how long it was going to take to get to the destination - I figured on 10 days of actual flying, and estimated an additional 5 days of delay due to presence of icing conditions enroute. I had no intention of even getting out of bed in the morning if there was any mention of icing in the forecast.

As things turned out, the longest weather delay I encountered was spent at home - I had to wait 3 days in Victoria for icing conditions over the Strait of Georgia (the first 15 miles of the 10,200 mile route) to clear. After that, I only encountered one additional weather delay, and that was 1 day only in Broughton Island. I wrote a post about the trip on another forum, and included a few pictures - click here (http://www.avcanada.ca/forums2/viewtopic.php?f=25&t=69781&start=29) to read that post.

Anyway, getting back to the topic of C of G, at least the manufacturer of the Twin Otter provides operators with weight and balance guidance for ferry flights that are carried out (with regulatory permission, and under a flight permit, of course) at weights above the MTOW. Although I carried 5,600 pounds of fuel on board, I didn't have to exceed 14,500 pounds at any time - this due to the very light weight of the new production aircraft.

It is trickier than it looks to get the C of G in the right place when making these overweight flights. Over 90% of the payload is the fuel, and it is very hard to convince the fuel in the tanks to move slightly forward or slightly aft as needed - it's kind of like herding cats. :sad:

It is also interesting to note that for this particular aircraft (a DHC-6), the forward limit for the C of G moves aft and becomes more restrictive as aircraft weight increases, but the aft limit for the C of G remains the same. I wonder if this concept also applies to other types of aircraft?

Michael

C of G limitations for ferry flights (prior regulatory permission is required)
http://i979.photobucket.com/albums/ae275/Paneuropean/FerryWeight.jpg

[Mariner9]

Not all aircraft work like this.

All types I have flown do, but as a caveat to my earlier post applicable for any aircraft that pitches up when flaps are applied, apply flaps if CofG too far forward, land flapless if CofG too far aft.

Still simple in my view.

[Piper.Classique]

Still simple in my view.
Ok, but in that case follow rigidly the manufacturer's instructions, check, check, and double check (not that that's a bad thing anyway). Your view is nonetheless wrong.

[BackPacker]

A agree with Ghengis et al here. It's not so simple that flaps generate a pitch up or down moment and thus can alleviate an out-of-envelope situation.

I'm not an aerodynamics specialist, but AFAIK flaps have three functions/effects:

1. They increase the surface area of the wing, thus providing more lift at any given speed, and allowing flight at slightly lower speeds (Vs0 vs. Vs1).
2. They increase the effective angle of incidence of the wing, thus requiring a lower fuselage AoA on landing (compared to wing AoA), giving you a better view over the nose and reducing the chances of a tail strike.
3. They increase drag, thus allowing for steeper descent angles and better speed control.

Because of (1) and (2), if you apply flaps most (but I'm sure not all) aircraft will pitch up to some degree, which you need to counter initially with forward stick. If, due to a too-forward CofG situation you are already against the back stops with the stick, you can now move the stick forward just a bit, temporarily maintaining control instead of suffering an uncontrolled drop of the nose.

But at the same time, when you apply flaps, the wing surface area increase happens at the back of the wing, not at the front (unless you have slats). This means the center of pressure changes rearwards by a tiny amount, making the pitching down moment of the weight up front more significant. And less easy to counter.

On the other hand, because of (2), if you apply flaps then the fuselage AoA decreases: The fuselage sort-of pitches down as measured against the wing AoA and airflow. This means that the elevator now has a few degrees more freedom (measured against the airflow) to move up and generate a downwards force, before it hits the stops.

And finally (3). More drag means that you can fly with a higher thrust setting, given the same flight path and speed. That higher thrust usually translates into a higher airflow over the tail, giving you more authority: less pitch required for the same effect.

So whether applying flaps in an out-of-CofG-envelope situation is beneficial or disastrous will depend on the exact design of the airframe. Personally I would want to apply my flaps carefully and early, giving me time and altitude to retract them if the situation becomes worse. And don't would not want to make any configuration/speed/attitude changes except the minimum required to make the runway, in the last 500' or so. As said, I'll come in at a slightly higher speed and bleed off any excess above the runway, at a safe height.

(Standing by to be corrected on some of the aerodynamic issues...)

[jxk]

Mariner9

The pitch change due to flap deployment has nothing to do CP movement. Can you think of another reason for this flaps affect pitch change?

[Mariner9]

Ok, but in that case follow rigidly the manufacturer's instructions, check, check, and double check (not that that's a bad thing anyway). Your view is nonetheless wrong.

Following the POH is always good, if predictable, advice. However the manufacturers of the a/c types I fly do not provide a section on how to fly if the Cof G is outside the permitted envelope (as per the hypothetical situation posed by the quiz Q.)

In the aircraft I fly, pitch changes markedly when flaps are applied and re-trimming is required. It is noticable (and simple physics) that considerably less nose-up trim is required when my aft buggage bay is loaded to limits, or that more nose-up trim is required when I'm flying without any baggage whatsoever. Could someone explain why this could not be used for the case in point rather than keep stating that the viewpoint is wrong?

[Piper.Classique]

Could someone explain why this could not be used for the case in point rather than keep stating that the viewpoint is wrong?

I rather think backpacker just did

Yawn
However the manufacturers of the a/c types I fly do not provide a section on how to fly if the Cof G is outside the permitted envelope
No, none of them do. That's because you aren't supposed to do it. However, rest assured that stability will be degraded.
Yawn :ugh::bored:

[Mariner9]

With respect Piper, nobody has suggested anything other than that stability would be adversely affected in the hypothetical situations posed.

I advocated a certain course of action, others disagreed. Surely debate is the point of this forum? It did not in my view warrant yawns and headbanging etc but you are of course also entitled to your opinion!

[FlyingKiwi_73]

I haven't read the whole thread but has somebody pointed out the obvious technical flaw?

Doesn't the engine go in the nose? surely (sorry PACE) an ME would not be a novice :-)

I guess i'd be more worried about that burning smell as the lycomming warmed up the luggage :-)

I am a Novice SEP :D

[Tarq57]

One aspect nobody seems to have mentioned is that the couple between the elevator and the CG is a lot shorter with an aft CG situation than with a forward CG. This means that, as well as having less longitudinal stability in an aft CG configuration, the ability of the control surface to overcome it is reduced; it is working against a shorter lever.

In an extreme situation, the tailplane will be developing positive lift in flight, rather than the normal negative lift. Such a beast would be extremely difficult to control. It would be dynamically unstable longitudinally, and I would suggest a loss of control/flat spin very likely. It's unlikely to happen, though, for the reason following:

Most lighties, if loaded way far too far aft, especially before the pilot has climbed in, will squat on their tails. Dead giveaway, that. Avoid trying to fly anything in that condition. (And if it's a taildragger, better do the sums!)

I've flown plenty of aircraft that pitch up when flap is added. Something like a C206 at vfe for the first 10 degrees has a very powerful pitch up with flap. Most of the high wing Cessnas do. More downwash over the tail. I'd definitely avoid touching the flaps if I detected an aft out of CG condition in flight.

[photofly]

One aspect nobody seems to have mentioned is that the couple between the elevator and the CG is a lot shorter with an aft CG situation than with a forward CG. This means that, as well as having less longitudinal stability in an aft CG configuration, the ability of the control surface to overcome it is reduced; it is working against a shorter lever.I don't think that's a factor. The CofG is always quite close to the centre of pressure of the main wing; a CG movement of a couple of inches changes the arm of the main wing's lift vastly more than it changes the arm of the tailplane. Moving the CofG a couple of inches closer to the tailplane that's already 80 inches away has a very small effect on it's arm.
In an extreme situation, the tailplane will be developing positive lift in flight, rather than the normal negative lift. Such a beast would be extremely difficult to control. It's acceptable for the tailpane to generate positive lift; the condition for stability is that the AoA of the tailplane must be more negative than that of the wing. If the main wing has an AoA of (say) +3 degrees, positive pitch stability remains until the tailplane angle also reaches +3 degrees; at +2 degrees the tailplane can generate positive lift and the aircraft remains controllable in pitch.

Have a look at Denker's website, he explains why:
Angle of Attack Stability, Trim, and Spiral Dives [Ch. 6 of See How It Flies] (http://www.av8n.com/how/htm/aoastab.html#sec-aoa-stability)

[Tarq57]

I didn't examine all the information in the linked site very thoroughly, I must admit.
I have never heard of a (conventional) aircraft that normally operates with the tailplane producing an upforce (lift) in stable flight.

Not saying it doesn't exist, nor that I am assuredly correct, but it appears to violate the normal stability equation.

Ready to be corrected, but with diagrams that more accurately represent reality.

[photofly]

I have never heard of a (conventional) aircraft that normally operates with the tailplane producing an upforce (lift) in stable flight.I've just taken some measurements from a print-out from a C182 poh, scale diagram - blown up to full A4-size.

The quarter-chord (close to centre of pressure) is 43 inches aft of the firewall. The allowable CofG range is 33 to 48 inches aft of the same datum. At anything aft of 43 the tail is providing up-force (lift), not down-force. (It would be interesting if you'd try the same measurement on a different type, if you have time.)

Also consider (as Denker points out) airplanes with canards. The wing at the back is the main wing, and had better be flying at a positive angle of attack.

[Tarq57]

Canards are a totally different type of equation.

What I want to know is, what makes you so certain that any CG range aft of 43" from the datum in this case (or any case for a conventional a/c) will cause the horizontal tail to produce an uplift?

My understanding is that it will always be producing a down-force. Just not as much down-force at an aft C/G as at a fwd C/G.

Look at the profile cross section of any airliner (and some light aircraft) and you will find that the tailplane curvature is opposite that for the main wing. A fair pointer as to the direction the lift is designed to act in.

[photofly]

What I want to know is, what makes you so certain that any CG range aft of 43" from the datum in this case (or any case for a conventional a/c) will cause the horizontal tail to produce an uplift?If the CofP is at 43 inches (pushing up) and the CofG is pushing down anywhere aft of 43 inches then the two of them make a nose-up couple. The extra nose down moment to maintain pitch has to come from the tailplane, pushing up (i.e. in lift).

I may have measured wrong, but it would have to be *very* wrong on the sizeable diagram I have in order to be out by more than 5 inches.

Do you have any PoHs for any other types to check?

If the shape of the tail-plane airfoil has greater camber on the lower surface that suggests that with the CofG in the middle of the allowed range the tail generates a downforce (which agrees with my measurement, with the CofG in the centre of its range the tail is still pushing down) but it doesn't address the issue of stability (or lack of it) where the tail is in lift.

I don't actually know what "conventional" wisdom is on this - but Denker's reasoning is quite compelling and backed up by the only diagrammatic evidence I have to hand.

[Tarq57]

<The extra nose down moment to maintain pitch has to come from the tailplane, pushing up (i.e. in lift).>
Or it could possibly be offset by the big heavy thing up front....

[Tarq57]

The thing being that the sizeable diagram you have probably painstakingly measured does not account for the differences in materials used in assembling the aircraft.
The chunk at the front is very, very much more dense than the other bits used in the airframe manufacture.

The dry weight of an 0-540 is around 438lb. Add oil, and a propeller, that's a fair amount of mass up front.

[photofly]

I think you might be misunderstanding what I'm measuring.

The pilot's operating handbook has a engineering section of the aircraft. The wing is shown clearly edge-on, so it's easy to measure and mark the quarter-chord position; The diagram also shows the firewall, and the spinner-to-tail length (28 feet 9 inches) from which the scale of the diagram can be determined.

By measurement, the quarter-chord position falls 43 inches (in scale) behind the firewall.

The weight-and-balance section states that the allowable CofG range is between 33 and 48.5 inches aft of the firewall (datum 0.0).

[Tarq57]

I get that. But how are you making the inference from that data that the tailplane must be generating lift if the C/G is aft of 43"

[photofly]

But how are you making the inference from that data that the tailplane must be generating lift if the C/G is aft of 43"That's not a big leap. It's because the role of the tailplane is to balance the couple generated between the weight (acting at the CofG) and the lift of the wing (at the CofP).

Oftentimes, the CofG is forward of the CofP, so the tail pushes down to keep the nose up (I know we agree on that).

If the CofG is at exactly the same station as the CofP then the tail flies with zero lift (at zero angle of attack, by one definition of AoA).

If the CofG moves aft of that point and goes behind the CofP the tail needs to push up, in lift.

My point is that nothing terrible happens to the pitch stability in that case. Pitch stability disappears only when the CofG moves even further aft, to the point where, in order to generate enough lift to keep the nose up, the angle of attack of the tailplane becomes greater than the angle of attack of the main wing.

[Tarq57]

Well, that's news to me. (unconvinced.)

Is this something you know, or something calculated from comparing the "relevant" chord point of the wing to the C/G location?

[photofly]

I understand you're unconvinced.

I "know" it through reading Denker, and following his analysis carefully. Measuring diagrams of a 182 is strong evidence that the pitch stability doesn't disappear when the tailplane is required to provide lift instead of downforce. It doesn't suggest Denker's analysis is either correct or incorrect but it does support his conclusion.

I would *very* much like to read an equally detailed and well reasoned but different analysis of pitch stability in airplanes that comes to a different conclusion, but I've not found one. My basic flying texts gloss over the subject in two lines.

[Tarq57]

So what sort of aerofoil does the 182 have, and where along the chord is the centre of lift?

[photofly]

Wikipedia says the C182 has an NACA 2412 airfoil.

This NACA technical report (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090937_1993090937.pdf - table 1, page 47) states that the CofP for NACA2412 is ahead of the quarter-chord by 0.5%.

[24Carrot]

Pitch stability need not disappear if the tailplane is producing positive lift.

The pitch stable condition is that the tailplane AoA should be less than the Wing AoA. As the wing AoA is always positive, the tail AoA can be anything negative, and also in a small range of positive numbers up to the Wing AoA.

It's not too hard to explain in words.

If the CG is forward of both wing and tail, then both are stably weather-cocking, and it doesn't matter what the tail AoA is, (although it will be negative).

If the CG is aft of the wing, then the tail is still stable, but the wing has become unstable. Since the moments are equal in trimmed flight, we need a one degree aircraft rotation to have a bigger percentage impact on the stabilising tail moment, compared to the de-stabilising wing moment. So one degree has to be a bigger percentage of the tail AoA than it is of the Wing AoA. So the tail AoA has to be a smaller positive number in this case.

[SNS3Guppy]

Surely you wouldn't take four friends in a four seater.................

I've done it lots of times, but I threw them all out before I landed.

Fortunately they weren't very good friends, and they all had parachutes. The 182 had only one seat; mine.

I've done it lots of times the other way, too. Four jumpers and a pilot. Just saying.

I have never heard of a (conventional) aircraft that normally operates with the tailplane producing an upforce (lift) in stable flight.

Piaggio Avanti. No, it's not a canard.

Most lighties, if loaded way far too far aft, especially before the pilot has climbed in, will squat on their tails. Dead giveaway, that. Avoid trying to fly anything in that condition.

Standard fare in the Cessna 207 was to load the airplane working aft. When the tail started to sag, start loading forward again, and have someone stand at the back of the airplane, hands behind their back, plams up, feeling for pressure from the horizontal stab. When it got too heavy, move weight forward again, stuff everything left in the nose baggage compartment, and go. If the tail was too heavy, have someone hold it while the engine gets started and gets some breeze over the tail, and go.

A bush company I interviewed with years ago would do a quick checkride in a 207 as part of the interview. One takeoff with a 200' pattern to the right, with full nose down trim, then a touch and go and a left pattern with full nose-up trim. The chief pilot watched and observed. How an individual handled the airplane and the problem said a lot of about the individual.

In ag airplanes, we have a widely variable CG. When the airplane is loaded, the hopper contains well over half the total weight of the airplane. When those airplanes are used for firefighting, all the weight goes at once. This means not just a significant change in gross weight, but in CG. Often with a full hopper, the stick is displaced far aft, and nearly full aft trim may be needed. Ag airplanes don't trim out very well; it's trimmed roughly, but the trim may be most of the way aft to hold level flight at a cruise airspeed, more in the climb. During a drop, I usually ran the trim full forward. Speed is very critical. The Bureau of Land Management put out a memo a few years ago after several fatalities in M18's on drops, advising a 15 knot window for the drop. Too fast and the airplane pitched uncontrollably and too slow and it would stall during the drop. Not a lot of margin in there.

Playing outside the CG is very serious business. It's really a bad feeling when you're out of control authority, and it's not enough.

I haven't read the whole thread but has somebody pointed out the obvious technical flaw?

Doesn't the engine go in the nose? surely (sorry PACE) an ME would not be a novice :-)

It does, but airplanes such as the Cherokee 6 and the Cessna 207 have nose baggage compartments right behind the engine.

In the aircraft I fly, pitch changes markedly when flaps are applied and re-trimming is required. It is noticable (and simple physics) that considerably less nose-up trim is required when my aft buggage bay is loaded to limits, or that more nose-up trim is required when I'm flying without any baggage whatsoever. Could someone explain why this could not be used for the case in point rather than keep stating that the viewpoint is wrong?

Yes. I don't think you really understand why the pitch change occurs. You should also be aware that some airplanes pitch up with flaps, others pitch down, and some pitch up with certain flap deflections, and down with others. This is particularly true of airplanes with flowler type flaps.

Under discussion is a situation in which the center of gravity is loaded aft. Pitch stability may already be a problem, and one may have insufficient control authority depending on the nature of the loading problem.

Download on the horizontal stabilizer is a function of lift from the wing. Change the coefficient of lift, or the way in which the wing produces lift, you also change the download. You change the value, and you change the angle of the download on the horizontal stabilizer.

By lowering flaps in an airplane that tends to pitch down with flaps you're getting the pitch-down moment for two reasons; a change in the center of lift and center of pressure on the wing, and the change in the downwash created by the wing. Youv'e changed the angle of the downwash, and it's value on the horizontal stabilizer has also changed. Negative lift is a vector, just like positive lift; change the angle, change the vector, change the lift. You lower the flaps, you need to move the controls and retrim to compensate.

When lowering the flaps, you may inadvertantly exascerbate the situation. Don't think that because with the CG envelope within limits you see desirable things happen with aft CG and flaps, that you'll see something desirable occur when operating outside the CG limits. You may not. You may find that upon lowering flaps, the airplane pitches and that you don't have the authority to counteract the pitch. You may find yourself doing a nose-dive into the ground, because the airplane that you thought you knew with the CG in limits is not the same airplane with the CG out of limits. The parameters of the CG affect control authority, pitch authority, and the actions of the airplane when configured or clean.

You may not be familiar with tailplane icing, but over the last 20 years or so it's received a lot of attention in professional training. A tailplane stall, or a stall of the horizontal stabilizer, works just the opposite of the way your wing stalls. If the tail plane stalls, the airplane pitches nose down. In conventional conditions, you're used to a practice stall experiencing a nose drop as you get too slow and stall, or increase the angle of attack too much in a bank.

In a tailplane stall, which can occur as a result of a CG out of limits, of ice changing the airfoil of the wing or tail, or from damage to the airplane, happens without a warning buffet sometimes, and can be unexpected. Flying faster can make the problem worse, as can lowering flaps. In such a normal stalled condition, one tends to push the nose forward slightly to decrease angle of attack and "break" the stall; this can make a tailplane stall worse. Sometimes pulling back is the only option, but you may also be out of elevator and have no recourse. Flying faster to get out of the stall can make it worse, and lowering flaps can be fatal.

If you're having an aircraft control problem, generally changing the aircraft configuration is the wrong choice. There are exceptions, of course, especially if the control problem is a result of having made the configuration change that got you there in the first place. In such an instance, changing configuration back to where you started is usually advisable (but not always).

The blanket counsel to lower the flaps maybe a very dangerous counsel. Don't go changing the aircraft configuration.

If control is a problem, don't change airspeed. If the airplane is flying and controllable now, simply get back around and land. Don't try to experiment to see what happens if you fly faster or slower. Don't try to lower flaps as a blanket counsel.

It's important to remember as you reason it out that when the airplane is within the CG envelope, you have actions that are expected and anticipated from the airplane, and you have adequate control authority to handle the those actions and reactions. You have the trim force to be able to fly the airplane. This is not necessarily the case for a situation outside the CG. With a far-aft CG, you may be trimmed forward and holding forward pressure on the controls. Lowering flaps may increase the download or decrease it, or it may vary. If you lower flaps and the download on the horizontal stab increases, if you're already out of control authority, you're in trouble.

[BackPacker]

I have never heard of a (conventional) aircraft that normally operates with the tailplane producing an upforce (lift) in stable flight.

I've been told that the A380 (and possibly more Airbus and Boeing types) have a tailplane that produces positive lift. The pitch stability is then not inherent in the design, but created using the fly-by-wire software.

Don't know the specifics though.

The 182 had only one seat; mine.

Ah, well, technically it's not called a four-seater then, is it?:E

[SNS3Guppy]

Depends. It's got one installed seat, but adequate seatbelts bolted into the floor for all on board. When skydiving, jumpers still need seatbelts, so the floor of the airplane constitutes a seat (according to the FAA).

The airplane is still certified for four seats, but operated under a supplemental type certificate for five people.

[India Four Two]

aft CG will give less tail downforce, thus you need slightly less wing lift, therefore you get slightly less induced drag.

Competition glider pilots have been known to add ballast in the tail, to move the CG to the aft limit in order to minimize drag.

Early in my gliding career, I learned about the forward limit the hard way. I was giving a passenger ride in a Schweizer 2-33 to a Dutch man, who was rather large (is there any other kind?) and in those days, we didn't refer to loading charts before flight.

Everything went well until the flare, which basically didn't happen, in spite of application of full back stick! Luckily, the 2-33 has a very rugged wooden skid forward of the wheel, which absorbed the impact and gave us a very short ground run. As they say, "I learned about flying from that."

[V1... Ooops]

I've been told that the A380 (and possibly more Airbus and Boeing types) have a tailplane that produces positive lift...

Ah, I kinda-sorta think that those aircraft are quite unique exceptions to the general rule being discussed here, because fuel is pumped forward and backward during flight in order to minimize the 'negative lift' created by the tailplane.

In other words, these aircraft don't take off and land with the C of G in a position that results in minimal negative lift or possibly even positive lift being created by the tailplane, instead, the C of G gets moved around (by fuel transfer) in order to create a minimum drag configuration during flight.

It's an interesting concept and a clever bit of engineering, but perhaps a bit of a red herring in this discussion.

Michael

[BackPacker]

Looking for a confirmation of the A380 tailplane positive lift story I've been trawling the internet a bit, and most queries direct straight back to various threads here on PPRuNe, including this one...:oh:

Couldn't find any confirmation anywhere. Just stories about various Airbus types that automatically transfer fuel aft in the cruise, and back fwd automatically when the aircraft seems to be getting itself ready to land. But always to minimize negative lift (and thus, induced drag). Never any hint that they actually go as far as generating positive lift.

This thread is perhaps the best read:

http://www.pprune.org/tech-log/130098-aircraft-c-g-wing-pitching-moment.html