flight support aircraft - Cape to Cairo Challenge RIP

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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by p38 » Tue Aug 06, 2019 7:26 pm

The most dangerous aspect in my opinion of many low wing aircraft with full glass canopies are the fact that you wont easily get out unaided if it tips over onto it's back on landing.

Is this simply not the case in this instance?
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by ddevos » Tue Aug 06, 2019 7:32 pm

p38 wrote:
Tue Aug 06, 2019 7:26 pm
The most dangerous aspect in my opinion of many low wing aircraft with full glass canopies are the fact that you wont easily get out unaided if it tips over onto it's back on landing.

Is this simply not the case in this instance?
No, since the Sling 4 has cockpit doors, albeit small ones.
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Bundu » Wed Aug 07, 2019 6:55 am

Question..aircraft with a to rear cg or at rear cg limit or passed rear cg limit can enter a flat spin and cannot recover, especially if engine quits.. also, how long after takeoff was this accident? Why would they attempt turn around if field is not in sight anymore.. hence question if how far from takeoff did this happen?
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by PJL » Wed Aug 07, 2019 7:07 am

Bundu wrote:
Wed Aug 07, 2019 6:55 am
Why would they attempt turn around if field is not in sight anymore..
The turn around was pure speculation on my part and there is no evidence that this actually happened.

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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Mrb13676 » Wed Aug 07, 2019 7:33 am

Chalkie wrote:
Tue Aug 06, 2019 5:36 pm
Any aft CG condition would make an aircraft pitch sensitive, so "getting the nose over" would be easy.
I may be completely incorrect here, but surely an aft CG requires greater forward pressure on the elevators and if sufficiently aft of the envelope may require downward elevator deflection in excess of that available especially at low speed i.e in a stall situation?
I’m happy to be corrected...
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Chalkie » Wed Aug 07, 2019 8:40 am

Mrb13676 wrote:
Wed Aug 07, 2019 7:33 am
Chalkie wrote:
Tue Aug 06, 2019 5:36 pm
Any aft CG condition would make an aircraft pitch sensitive, so "getting the nose over" would be easy.
I may be completely incorrect here, but surely an aft CG requires greater forward pressure on the elevators and if sufficiently aft of the envelope may require downward elevator deflection in excess of that available especially at low speed i.e in a stall situation?
I’m happy to be corrected...
A few years ago I went to a CAA Gr1 instructor conference, the aft CG debate became heated as the CAA answer was the same as yours, but unfortunately, from experience, it is wrong.

You see it is not only a matter of the couple from mass (weight) to elevator arm becoming shorter it is also the unbalancing of the four forces. Mass is greater therefore lift must be greater which makes drag greater and therefore thrust must be greater. The thrust / drag couple will cause a nose up pitch moment but at the same time the lift / mass couple is becoming weaker, but still balancing the nose up moment with its own nose down moment. If CG were to move further aft the aircraft would eventually become unstable as the CG moves aft and the higher angle of attack moves the lift (centre of pressure) forward closer to the mass which is moving aft.

Now remove the thrust (engine failure) you would expect the drag to pull the nose up, but remember the lift / mass couple are both working together to pull the nose down and the parasite drag from the propeller would also contribute to a nose down moment.

So depending on how far aft the CG is this might still result in nose down pitch or in extreme case of aft CG, the aircraft would become totally unstable and only someone with training in how to fly an aircraft with aft CG out of the envelope, would be able to do so.

LiftDrag.png


Perhaps an aerodynamicist can explain it better. From experience flying aircraft overgross and aft CG, the aircraft becomes unstable and pitch forces become light.
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by TC » Wed Aug 07, 2019 9:33 am

I would suggest that as the CG moves further aft, the control PRESSURE becomes less, the the control TRAVEL for the same change in pitch would be greater?
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by jimdavis » Wed Aug 07, 2019 9:41 am

Chalkie wrote:
Wed Aug 07, 2019 8:40 am
A few years ago I went to a CAA Gr1 instructor conference, the aft CG debate became heated as the CAA answer was the same as yours, but unfortunately, from experience, it is wrong.

You see it is not only a matter of the couple from mass (weight) to elevator arm becoming shorter it is also the unbalancing of the four forces. Mass is greater therefore lift must be greater which makes drag greater and therefore thrust must be greater. The thrust / drag couple will cause a nose up pitch moment but at the same time the lift / mass couple is becoming weaker, but still balancing the nose up moment with its own nose down moment. If CG were to move further aft the aircraft would eventually become unstable as the CG moves aft and the higher angle of attack moves the lift (centre of pressure) forward closer to the mass which is moving aft.

Now remove the thrust (engine failure) you would expect the drag to pull the nose up, but remember the lift / mass couple are both working together to pull the nose down and the parasite drag from the propeller would also contribute to a nose down moment.

So depending on how far aft the CG is this might still result in nose down pitch or in extreme case of aft CG, the aircraft would become totally unstable and only someone with training in how to fly an aircraft with aft CG out of the envelope, would be able to do so.


LiftDrag.png

Perhaps an aerodynamicist can explain it better. From experience flying aircraft overgross and aft CG, the aircraft becomes unstable and pitch forces become light.
[/quote]

BEAUTIFULLY explained Chalks, and dead right, as usual! :D

I have only once flown an aircraft with a seriously aft CofG and it is truly terrifying. Stick forces were extremely light, but it required massive stick travel to take effect. There was a large amount of elevator movement that had no effect - then it would suddenly pitch violently up or down right near the end of the stick travel. Victor Smith called it "... stick neutral ..." I don't remember the exact term but it is a recognised aerodynamic phenomenon. I was exceedingly lucky to live through it.

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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by southside » Wed Aug 07, 2019 11:36 am

Thank you Chalkie and Jim, that is a pearler of info. I have learnt something new!
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Chalkie » Wed Aug 07, 2019 4:39 pm

TC wrote:
Wed Aug 07, 2019 9:33 am
I would suggest that as the CG moves further aft, the control PRESSURE becomes less, the the control TRAVEL for the same change in pitch would be greater?
Perhaps, but not in my limited experience. Remember in an aft CG state you really do not want to induce a PIO. (Pilot Induced Oscillation) There are some SAAF trained Gr 1 Test Pilots on this forum. Perhaps one has done the aft CG training. Fighter aircraft are loaded aft so as to be unstable, so that they can manoeuvre. GA aircraft are designed to be stable platforms within CG limits.

In the case I mentioned above the SACAA answer to a question in an exam was with CG movement aft the arm between elevator and CG was lessened and therefore make the controls less effective. Half of the attendees said, no way, I was one of them! This was debated but the SACAA would not budge.

OK, knowing that. But surely if I have a FWD CG then the arm is lengthened and my elevator authority should be better. (Back in 1965 my maths teacher taught the class; "If ever you say 'but surely sir.' you will 'but surely' be wrong." This is a truism in life.)

Take a C206 (Jim can take a Cherokee 6) remove all the seats and do a circuit before you drop 5 or 6 parachutists. I challenge you to land on the main wheels, it is extremely difficult to do because the CG is so far forward. "But surely sir" I should have more elevator authority, well the fact is you don't.

You don't because the CG (weight) and CoP (lift) are so far apart and the couple they make is strengthened, this forces the nose down and the elevator and horizontal stabiliser can only produce so much down force. A landing on the main wheels with full flap is damn near impossible.
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by pwnel » Wed Aug 07, 2019 4:50 pm

Chalkie wrote:
Wed Aug 07, 2019 4:39 pm
Take a C206 (Jim can take a Cherokee 6) remove all the seats and do a circuit before you drop 5 or 6 parachutists. I challenge you to land on the main wheels, it is extremely difficult to do because the CG is so far forward. "But surely sir" I should have more elevator authority, well the fact is you don't.
For this reason I believe is why CC suggests to tie a 25l can of water down as ballast in the 172's luggage space when doing bush flying.
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Rave » Wed Aug 07, 2019 5:53 pm

PJL wrote:
Wed Aug 07, 2019 7:07 am
Bundu wrote:
Wed Aug 07, 2019 6:55 am
Why would they attempt turn around if field is not in sight anymore..
The turn around was pure speculation on my part and there is no evidence that this actually happened.

PJL
It is mentioned somewhere that the aircraft was 30 Nm out and they lost them off radar? Maybe flat spin is also not out of the question?
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Orthin Opter » Wed Aug 07, 2019 6:12 pm

Rave wrote:
Wed Aug 07, 2019 5:53 pm
PJL wrote:
Wed Aug 07, 2019 7:07 am
Bundu wrote:
Wed Aug 07, 2019 6:55 am
Why would they attempt turn around if field is not in sight anymore..
The turn around was pure speculation on my part and there is no evidence that this actually happened.

PJL
It is mentioned somewhere that the aircraft was 30 Nm out and they lost them off radar? Maybe flat spin is also not out of the question?
Progression from engine troubles to c of g problems to flat spins? Perhaps it was just a loss of power and a stall at the normal glide speed.
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Alan Robertson » Wed Aug 07, 2019 6:14 pm

Deepest condolences to family and friends on what is an unimaginable loss.

Clearly these were respected and competent pilots flying a proven aircraft, and hopefully the learning from this tragedy will add, if only slightly, to the collective safety of the community.

On a separate note and with all due respect to the unfortunate circumstances, I find the learning in this thread to be absolutely fascinating. There are many highly experienced aviators who share their knowledge so generously and constructively on Avcom, and I for one am grateful for it. Thank you.

RIP Des and Werner
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Re: flight support aircraft - Cape to Cairo Challenge RIP

Unread post by Mrb13676 » Wed Aug 07, 2019 8:35 pm

pwnel wrote:
Wed Aug 07, 2019 4:50 pm
Chalkie wrote:
Wed Aug 07, 2019 4:39 pm
Take a C206 (Jim can take a Cherokee 6) remove all the seats and do a circuit before you drop 5 or 6 parachutists. I challenge you to land on the main wheels, it is extremely difficult to do because the CG is so far forward. "But surely sir" I should have more elevator authority, well the fact is you don't.
For this reason I believe is why CC suggests to tie a 25l can of water down as ballast in the 172's luggage space when doing bush flying.
I fly around with 30kg of water in the baggage compartment unless we put passengers in the back. Does wonders for the landing in my (admittedly inexperienced) hands
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