Aft CG and DFE's

[StressMerchant]

Thanks, Peregrine. Well whether it's called the "neutral point" or "stick neutral point" it't not a good place to be.

So can anyone (Stressors) tell me what I must have done to live through that? Changed the airspeed? flap setting? or power? Because something made it less unflyable.

jim

This is a bit like the flutter debates. How do you recover if you encounter flutter? Well, to start with, just don't go there. If you do end up there, there may be one or two things you can try, but none are guaranteed.

I've never flown an aircraft with a CG back far enough to cause real stability problems. Peregrine and some others may have flown the variable stability demonstrator during flight test course, but I'm not sure how close they took the demonstrator to true neutral stability.

I did once see an aircraft airborne with a CG too far aft. It was a new prototype on an early program flight, as part of the envelope expansion. The problem was immediately obvious from the ground, manifesting as a very uneven flight path. The pilot was obviously struggling to keep the aircraft on the desired flight path, the 1 hour sortie was converted into a single circuit. Amongst the lessons learned is not to contract out the stability analysis to the lowest bidder. From the in-cockpit footage the pilot was obviously having to apply constant corrections to the pitch control, but to his credit both he and the flight test engineer handled it really calmly and professionally.

What could you do? Well, reducing power as much as possible may help. In general, for single tractor propeller engines, power reduces longitudinal stability. I emphasise the "in general", as it does depend on the aircraft configuration. The effect of power is to change the slipstream over the tailplane, and depending on the engine thrust line it could also change the pitching moment.

Coming back to the original question from Chalkie, I am still intrigued by the DFE discussion. Seems everyone here is in agreement with Chalkie, yet the majority at the DFE meeting seemed to disagree. Are we missing something?

[jimdavis]

Thanks, Peregrine. Well whether it's called the "neutral point" or "stick neutral point" it't not a good place to be.

So can anyone (Stressors) tell me what I must have done to live through that? Changed the airspeed? flap setting? or power? Because something made it less unflyable.

jim

This is a bit like the flutter debates. How do you recover if you encounter flutter? Well, to start with, just don't go there. If you do end up there, there may be one or two things you can try, but none are guaranteed.

I've never flown an aircraft with a CG back far enough to cause real stability problems. Peregrine and some others may have flown the variable stability demonstrator during flight test course, but I'm not sure how close they took the demonstrator to true neutral stability.

I did once see an aircraft airborne with a CG too far aft. It was a new prototype on an early program flight, as part of the envelope expansion. The problem was immediately obvious from the ground, manifesting as a very uneven flight path. The pilot was obviously struggling to keep the aircraft on the desired flight path, the 1 hour sortie was converted into a single circuit. Amongst the lessons learned is not to contract out the stability analysis to the lowest bidder. From the in-cockpit footage the pilot was obviously having to apply constant corrections to the pitch control, but to his credit both he and the flight test engineer handled it really calmly and professionally.

What could you do? Well, reducing power as much as possible may help. In general, for single tractor propeller engines, power reduces longitudinal stability. I emphasise the "in general", as it does depend on the aircraft configuration. The effect of power is to change the slipstream over the tailplane, and depending on the engine thrust line it could also change the pitching moment.

Coming back to the original question from Chalkie, I am still intrigued by the DFE discussion. Seems everyone here is in agreement with Chalkie, yet the majority at the DFE meeting seemed to disagree. Are we missing something?

Thanks so much Stressors - as always you bring sanity to aerodynamic, design and structural problems. We are lucky to have you to call on.

[AEH]

I have had the misfortune to fly an aircraft well aft of CG limits - unwillingly and NEVER EVER AGAIN.

I cannot comment on stick forces in this particular instance as it was in a Tiger Moth which has truly awful control harmony under the very best of circumstances but the pitch forces hardly change between Vs and Vne - but my eyes water in one and the scarf droops in the other.

I was not sure whether I was PIC or PAX but I boarded the front seat in my mentors Tiger Moth. I had a flying jacket over an flightsuit and as we were only going away for one evening to the World Gliding Championships in Mafikeng in Dec 2000, I could wear the same clothes the following day - one advantage of an open cockpit. My toothbrush, toothpaste, razor, deo, cellphone and spare undies were in my flightsuit. Bob had packed everything else into the baggage bay. We were running late and being younger I put the cars away and closed up the hangar.

Soon after takeoff the problems became apparent. Some explanation is required first. The Tiger Moth has a large baggage bay aft of the rear cockpit in the turtledeck. it has a long moment arm from the CG. The fuel tank is above and ahead of the front cockpit which is on the CG the tank being ever slightly ahead. A Tiger Moth uses two springs as a pitch trim device that place a load in either direction other than neutral. This makes any force other than where you put the trim rather heavy. I had the spring bias fully forward to no avail. All tigers fly very differently to each other, not like a Cherokee or C172, and this one usually needed a lot of forward trim.

The slightest turbulence (which with a low wing loading is lots and lots) the nose would pitch either up or down. I reacted by putting in lots of opposite stick only to find it often had no effect whatsoever. When I mentioned it to Bob his comment was "Oh yes it does that". He saw no reason to return. The fuel tank which we were depleting at a rate of 40L/hr was slightly ahead of the normal CG which meant it was worse as time went on. It was absolutely frightening having the nose come up and with full forward stick the only thing that changed was the moisture content of my underwear. It entered a phugoid of sorts that was only just controllable. Sometimes it went nose down and even full up elevator hardly had any effect until the speed built up slightly. The hard part was trying to act BEFORE it entered a pitch up/down cycle, not knowing which way it would depart. So it called for lots of input immediately on identification of any pitch change. If it started to get near a stall my option was to roll it onto a wingtip. And a Tiger's roll rate is probably measured by the same rate as the second hand of a stuck clock, if not a calendar. I was a not happy camper.

I did however have 5km of beautiful runway, with no wind on which to effect a decent landing in front of my peers. I touched down with a touch of power and the subtlety of a butterfly landing on a teflon petal. I was going to do a very gentle wheeler.

Or so I thought- I was in for a real surprise! As the main gear touched we shot up like a politicians hand out of a brown paper bag. Again and again. I could not have done it any smoother. I would have challenged Neil Armstrong or Winkle Brown to do it better.

On my 5th attempt I said to Bob I was going around. We had used a good 4km of available runway by now. His response was "Not bloody likely - let me have a go". It was his plane and he had tons of hours in it. If he wanted to bend it - it was his prerogative. He flew slowly about a ft above the runway, pulled the throttle back, then buried the nose into the ground which takes a lot of pitch change in a Tiger. You could feel the legs taking strain and the unnatural tendency to bounce skyward again. I was not sure to be impressed or frightened. But we wandered along like a pangolin until there was no speed left to keep the tail up. We taxied in silence the 5km back to where the action was.

I was expecting Bob to unpack his prized Anvil collection. I was close. He had a 6lb hammer, 4x 1in square 1 ft long angle bar tie-downs and a very heavy large canvas cockpit cover. This would have prevented a C-130 from getting airborne in a gale! His tiny overnight bag was also squashed in with 6 qts of oil (the only time really needed in a Gypsy engined aircraft).

I took out the front stick and placed the hammer, tie downs and the canvas cover in the front cockpit for the ride back making sure nothing touched the rudder bar. He could not be persuaded to leave these prize possessions on the trash heap and replace then with lighter better components. It flew considerably better. At least I had the rear cockpit on the way home. Never again.

[Peregrine]

I find it of concern that of the stories posted describing the difficulties related to apparently flying close to the neutral point, nowhere was mention made of preflight preparation in determining the CG of the aircraft before launching off. Meticulous planning in this regard, especially when flying a new type for the first time is a clear requirement. Perhaps this will be hoisted aboard by readers …

[jimdavis]

I find it of concern that of the stories posted describing the difficulties related to apparently flying close to the neutral point, nowhere was mention made of preflight preparation in determining the CG of the aircraft before launching off. Meticulous planning in this regard, especially when flying a new type for the first time is a clear requirement. Perhaps this will be hoisted aboard by readers …

A valid concern. There have only been two posts where pilots admitted to flying with a dangerously aft CG.

The first was mine. I had no way of knowing where the CG was. The owner/designer/builder had been flying the aircraft safely and happily for two years before I flew it. After removing the seat back I was sitting possibly 2 inches further aft than the owner. There were no charts or graphs to calculate the new CG. However I did learn one important lesson - never fly someone else's home built.

The other was AEH flying a Tiger moth. Although I owned one for for nearly half a century I never consulted the Mass and Balance docs. As Alan pointed out every Tiger is rigged differently and the graphs are meaningless. But I do seem to remember a 20 pound limit for stuff in the baggage compartment.

I also remember having difficulty recovering from a flat spin with my fat friend Bernie Marriner in the back, and nothing in the luggage compartment. Recovery was easily handled by rocking the stick and throttle fore and aft together until the nose went down and normal recovery worked fine.

As you know, I am a stickler for safety, however there are times when common sense dictates that you pull your nose out of the books and adopt a practical approach.

jim

[Peregrine]

OK, Jim. Your last statement is rather contradictory, but nuff said. By the way, I found details of the CG limits of the Tiger, but it’s a bit late now I guess.

[Chalkie]

Jim said it: never fly someone else's homebuilt. I think he meant 'own designed homebuilt' or as in my case 'plans built homebuilt.'

A friend had built a Pietenpol Aircamper and needed a conversion. The aircraft was at Syferfontein and had been test flown by other pilots, the owner now needed a conversion. I said, OK; but we will position to Grand Central as I did not want to spend days driving to Syferfontein and flying from there.

The day arrived, the briefings were done. I was to occupy the back hole and the owner the front hole so that I could get to know the aircraft and then instruct the owner. I had no reason to think the aircraft was unsafe to fly, 'but surely sir, others had already flown it and all were happy,' and so we taxied out. Ground run completed, the only problem was a bad intercom system with mis-matched headsets and so we lined up...

I slowly added power and ran down the runway. The tail took a long time to come up with full stick forward and eventually we were airborne; still with stick within 1/2inch of full forward and not enough runway in front of us. I had a few words to say on the intercom and the answer was 'pull the power back and it flies fine.' Well, I disagreed as I felt if I pulled the power back we would descend. We staggered around the patch and I managed to do a 3 pointer still with full power and full stick forward.

Lots of words were exchanged and I insisted the C of G calculations be looked at again by someone who understood the sums. Long story, the engine mount for the O-200 engine was extended by 10 inches to correct the CG. TEN INCHES, 254mm for the millenials.

[jimdavis]

Jim said it: never fly someone else's homebuilt. I think he meant 'own designed homebuilt' or as in my case 'plans built homebuilt.'

A friend had built a Pietenpol Aircamper and needed a conversion. The aircraft was at Syferfontein and had been test flown by other pilots, the owner now needed a conversion. I said, OK; but we will position to Grand Central as I did not want to spend days driving to Syferfontein and flying from there.

The day arrived, the briefings were done. I was to occupy the back hole and the owner the front hole so that I could get to know the aircraft and then instruct the owner. I had no reason to think the aircraft was unsafe to fly, 'but surely sir, others had already flown it and all were happy,' and so we taxied out. Ground run completed, the only problem was a bad intercom system with mis-matched headsets and so we lined up...

I slowly added power and ran down the runway. The tail took a long time to come up with full stick forward and eventually we were airborne; still with stick within 1/2inch of full forward and not enough runway in front of us. I had a few words to say on the intercom and the answer was 'pull the power back and it flies fine.' Well, I disagreed as I felt if I pulled the power back we would descend. We staggered around the patch and I managed to do a 3 pointer still with full power and full stick forward.

Lots of words were exchanged and I insisted the C of G calculations be looked at again by someone who understood the sums. Long story, the engine mount for the O-200 engine was extended by 10 inches to correct the CG. TEN INCHES, 254mm for the millenials.

Wow Chalks, that's a very scary story to read late at night! 10 inches is unbelievable! It says a hell of a lot for the basic design, and your ability to waggle the stick intelligently, that the aeroplane didn't eat you.

Out of interest, did you establish how it had been flown before? Was it only solo? Or were the previous rear seat occupants less substantial human beings than yourself?

jim

[southside]

Clearly that 70% have never inadvertently taken off with an excessive aft C of G and had the living daylights scared out of themselves and lived to tell the tale.

So when oom Piet tells you he is just loading a couple of boxes in the back, and your enquiry as to what is in the boxes is met with a dishonest answer, and then you don’t feel the weight of the boxes yourself as PIC, you will quickly discover the answer to this thread on rotation.

With an excessive forward C of G, you will die using all your strength to bring the nose up as you hit the ground nose down.

With an excessive aft C of G, you will die with little strength required on the yoke pushing the nose down, full deflection of the elevator having no effect, ultimately probably stalling and spinning, still dying as you hit the ground nose down.

When you question someone as a PIC about what they are doing or have done to the aeroplane that you are going to fly, whether it’s maintenance related, loading of cargo, refueling ect it’s not because you don’t trust them, it’s because you don’t trust anyone. You need to check it yourself.

Some may have been dishonest, and some may have made a mistake.

I learnt this the scary way.

I was lucky to survive it.

[Volo]

I am enjoying these CG stories . It brought to mind the story of the legendary Pat Beattys B5 glider.
He had set out to design and build this glider with a swept wing - the objective being to have nothing in front of the leading edge including the pilot , however during the build he could not get the CG envelope to work as the tail of the aircraft ultimately became to heavy .
He could not sweep the wings any further because flaps moved the lift centre to far aft during deployment so he ended up creating 2 x steel bombs on 2 metre long probes ahead of the wing leading edge to restore the CG .
He arrived at Baragwanath airfield (circa 1980) one Sunday morning to stage his first flight in it .
I was the duty Tug pilot that day and it fell to me to do the tow.
Pushing on these bombs I expressed my reservation about there airworthiness but Pat reassured me in his laconic casual way that there would be no problem as they would only come off if he exceeded 6G .
I warned him that if it looked like I was not going to make over the mine dump at the end of 03 I would be dumping him to which he said he was OK with that but not to worry as it would be ok.
3/4 down the runway I was airborne but he firmly still on the ground .
Just as I was contemplating dumping he came off the ground in a sudden pitch up.
Straight away it set the bombs off in a sort of rhythmic oscillation and I was being rhythmically jerked fore and aft in my seat.
This strange motion went on for the next 10 minutes in a slow climb to 1500 ft gradually decreasing until it stopped .
He released and landed without incident never to fly it like that again .
It was heavily modified after that and went on to fly for several years after that without the bombs .

[Chalkie]

Wow Chalks, that's a very scary story to read late at night! 10 inches is unbelievable! It says a hell of a lot for the basic design, and your ability to waggle the stick intelligently, that the aeroplane didn't eat you.

Out of interest, did you establish how it had been flown before? Was it only solo? Or were the previous rear seat occupants less substantial human beings than yourself?

jim

One of our regular posters here on avcom was the second guy to fly the aircraft, I will look for his pseudonym and PM him. The original test pilot was a Chippie owner and I guess he flew solo for the first few test flights. He complained of bad elevator response and so a rubber extrusion was made to fill the gap between the two 4130 pipes of the elevator hinge area. A piece of fabric as used on Tiger Moth would have been a better idea.

The second 'test pilot,' flew the aircraft, the rubber seal caused more problems as it came loose and the aircraft ended up in a field with no substantial damage. It would be better if he told the story...

I was told "the engine mount has too much torque offset and the 65HP was much more than the original 27HP," so I was told if I pulled the power back it flew better. Well, it didn't. It went down like a brick with power reduced from full power and two-up.

Since that day I don't trust anyone. On two occasions, two different aircraft, the preflight phase has taken me 3 days and then things still went wrong on both those aircraft... One aircraft I was asked to inspect as it was nearing completion by another AP, resulted in me writing 2 pages of snags. One for the engine, one for the airframe. The AP-builder asked the owner not to bring me back again as I was "too fussy."

I did tell the owner to inform the AP that in aviation, as in life, there are always two ways to do a job. One of them is the right way.

[AEH]

I specifically posted my story here as a learning point for me and so hopefully others will not make the same mistakes. I was very fortunate to have lived through some of these experiences and I am meticulous about CG ever since that flight.

[JoeMat]

This is REALLY scary.
If this was a discussion between two students with 5 hours each in the logbook I'd say 'Very healthy discourse'.

But that this can be the subject of debate amongst supposedly real fliers frankly scares the cr@p right out of me.

Anybody who's ever loaded imprudenly will will relive that nightmare even 20 years after having his wings clipped. Like me.

With this type of expertise around I'm happy I haven't set foot in the air in decades.

Like the immortal Van said 'They're all dangeroos'' up there.

[JacoS]

My 2c :

Airspeed has an effect on how heavy the control column "feels"

CG location will just bias it in a certain direction. Tail heavy will be light to pull up, but hard to push down and vice versa.

[Chalkie]

My 2c :

Airspeed has an effect on how heavy the control column "feels"

CG location will just bias it in a certain direction. Tail heavy will be light to pull up, but hard to push down and vice versa.

NOPE. Please don't try to fly an aircraft with aft CG to prove me wrong.