Poor handling characteristics of a specific aircraft

[stressmerchant]

I hope you don't mind me using this forum to pick a few brains on a handling problem.

We're busy getting a turbine Dakota into flying condition after long term storage. The aircraft had its first flight a few weeks ago, and the pilot reported major handling problems. Unfortunately I wasn't able to have a detailed discussion with him, but he basically reported that the aircraft tried to roll over, and that it seemed sluggish. This particular pilot has a lot of time on type, and stated it was the worst handling he had ever experienced. Some of the "old hands" have said that this particular aircraft suffered from handling problems years ago, but we have found no official record of problems in the logbooks.

So far we have checked the following:

Control surfaces, positions and cable tensions - correct
Cable runs, pulleys etc - correct
Airspeed indicators - correct
Engine torque instruments - correct

The interesting one was the aircraft symmetry. When we measured, we found that the vertical fin was leaning over slightly. The angle of lean was fairly small, but we have now removed this and replaced the fin in the vertical. We also replaced a wingtip which was slightly out of alignment, although it was strictly speaking within tolerances.

The rest of the structure appears to be within specification. I also checked for any signs of structural deformation under load, but there were no signs of any buckled panels, popped rivets or similar damage.

The offset fin may have caused some handling problems, but my gut feel is that the offset was not enough to cause the major problems encountered. I am concerned that there is something else.

If you were faced with this problem, where would you look? What would you suggest as extra checks?

Blue Skies

StressMerchant

[Genghis the Engineer]

Some years ago, the RAF had a not dissimilar problem with a handful of Shorts Tucanos which would, if stalled with full flaps, roll inverted in about a second. On the whole, if you were nibbling the stall on finals, that was arguably an undesirable characteristic.

So, we spent a fair bit of flight test time and money trying to solve the problem - and failed dismally.

Then a squadron Engineering Officer and Unit Test Pilot somewhere got together decided to swap front and rear fuselages around a bit and fly them. Mysteriously, the characteristic disappeared - so we stopped flight testing and got on with something else. My best guess is that the basic problem was cumulative manufacturing tolerance (given that you could get about 10" variation in the length of a Tucano, and about 2" variation in headroom between aircraft, this wasn't hard to believe).

I've seen a few other vairations upon a theme - although never quite that bad again. This leads me to suspect, if all the controls are in tolerance is that your actual airframe is bent. Not necessarily badly, and probably within tolerance, but just occasionally you can get an airframe where all of the manufacturing tolerances can add up to give you something quite undesirable.

That said, this is pure guesswork on the basis of very little evidence. If you can post some detail of what specifically the pilot didn't like about it, and at what conditions, it would be helpful.

G

[John Farley]

We could do with rather more specific info than you have posted

Sluggish to respond to controls or sluggish in performance I wonder? Weight?

In the end only three things give bad handling with an established type - a wrong shape, a wrong CG and wrong controls.

A set of weight and balance checks that are repeatable must be your first aim. The DC3 is not enhanced by an extended aft CG. If the eventual mission you want to use the aircraft for will allow you to keep the CG fairly well forward it would be sensible to do so.

[stressmerchant]

Hi

Thanks for the responses. I kow the information is sketchy, but this is partly due to the availability of instrumentation. The aircraft is not specifically configured with data acquisition equipment, as the flight is merely meant to be a post-maintenance rather than experimental / development flight.

Due to the circumstances I was not able to have detailed discussions with the pilot. In a phone call he told me that:
- the aircraft felt "sluggish", as though the engines were not developing full power or there was an additional source of drag
- the aircraft had a definite tendency to roll to the left.

The aircraft was weighed prior to flight, and the CG was definitely within limits. The pilot trimmed the aircraft for level flight, and this required aileron trim (right wing down), and left rudder trim.

My immediate thought was that the controls were incorrectly rigged. These were checked against the specifications, and found to be correct. We also checked the engine and flight instrumentation, to ensure that the engines were developing the power reflected. All checked out correct.

That left me with structural deformities, which I have described above.

What concerns me is that the fin misalignment was not very large. I'm not sure that it could have accounted for the reported problems.

[KZ8]

The trailing edge shape of control surfaces can have a large effect on trim. One check would be to make sure that the aileron trailing edges have not been damaged or deformed during storage. Also that the covering is in good condition and is not pulling and distorting the trailing edge.

If the aircraft feels one-wing low, it is important to determine whether it is a control trim problem (force is required to keep the wings level but no stick movement) or a wing rigging problem (the controls need to be deflected one way to keep the wings level).

Just a thought.

KZ8

[Genghis the Engineer]

Sluggishness
Have you checked the setup of the props, and done a check-calibration of the RPM indicated in the cockpit against an optical tacho? I've seen this on a few singles where performance was well down, and it turned out that the prop pitch (or occasionally profile) was wrong, but this was being masked by a misreading cockpit tacho.

Rolling
I wouldn't be surprised if a small amount of offset in the fin did that in roll, it's a big fin! But, could you remind us how the yaw and (if it's got one) roll trimmers mechanise on the Dak?

G

[DFC]

Did the landing gear retract fully on both sides?

Having a partially retracted gear on one side would cause drag and yaw.

Regards,

DFC

[sycamore]

S-M, just a few thoughts to go along with what ever has been said.Was it a left w/t that was replaced, as a friend says its usually a sign it may have been g/looped,,repaired/ not recorded etc.Have you done a full symmetry check ,ie w.tip- tail-nose-w.tip, as if its bent ,it should show-up. Are the engine thrust -lines correct iaw the manufacturers spec.? Are the flaps up correctly, not sagging on one side or drooping, same with any engine intake ducts/cooler outlets.
Are the clearances between control surfaces and wing/tailplane /fin structues correct, as if not this can lead to poor control etc.In level flight are the positions of the ailerons noticeably different, looking out of the windows? Does the angle of the aileron trim agree with cockpit indicator? Is there any lost-motion on the tabs due to wear, as in flight this will give a false trim position?
And now you probably need the test-pilots highly expensive piece of test instumentation; a piece of cord( parachute) ,about 10-12",knotted at both ends/sealed, and some speed tape.
Fix one end on the centre-line of the nose(aircraft), about 15-18" in front of the w/screen centre, using the speed-tape; at the end nearest the w/s, mark-off 5 and 10 deg. increments, left and right, but visible to the pilots/observer, up to 20-25 deg.

You now have a sideslip indicator, more accurate than the t&s, and I would suggest you look at progressive sideslips, left/right, to determine if the aircraft is normal. Forces and displacement should be progressive with increasing sideslip-angle, and also with increase in airspeed.Fixed power should be used at each speed, and speed maintained, by slight climb/descent.
Any serious non-linearity should be cause for concern and further investigation.
As you dont say where you are in SA, I have an ex-student in SAAF who may be able to advise.Go carefully !!

[stressmerchant]

Thanks for all the replies, the info is all useful.

Basically everything checked out (flap retracts, gear retracts, trim angle indicators, thrust lines etc) except the fin.

The aircraft is due to be flown again in February, I'll report back on the results of the flight.

Thanks again for the info

[ICT_SLB]

Stress,
Flight controls are not my specialty but I note you said it's a turbine Dak. Is your pilot familiar with the turbine version? If radial piston engines are replaced by turbines, a lot of gyro stabilisation from all that rotating machinery is lost and handling effected. As an example, look at the CL415 versus the old 215 - most of all that extra ironmongery on the wings & tail is just adding back in the stability lost in the engine change.

[taperlok]

Get hold of SAAF pilots who recently flew for 44 squadron, or are flying for 35 ? in Cape Town. They have been flying turbo daks for a few years now

[ShyTorque]

One more thing to confirm. Check the wing fuel tanks are actually taking the amount of fuel that is supposed to be in them.

I once experienced an aircraft that had a damaged fuel tank (it had imploded due to a blocked fuel vent). The gauge read full and it was, but the capacity was reduced by at least a third. The lateral c of g was quite a long way out and it gave some of the roll symptoms your pilot reported.

It also backfired like hell during PFLs when the throttle was opened again due to partial fuel starvation, and the engine then developed the habit of stopping in a spin but that, along with how the vent got blocked....is another story.

This fault probably won't be picked up when the aircraft is weighed, due to the tanks not being full for that purpose.

[stressmerchant]

Feedback:

The aircraft flew this morning. The flight crew report that the handling problems appears to have been resolved. The cause of the problem would appear to be the offset in the vertical fin. Apart from a few minor snags, the aircraft appears to be acceptable. Although the fin offset was small, it appears to have had a major impact on the handling. I’m very relieved that the problem has now been resolved.

We’ve certainly learned a bit from this exercise.

An interesting side point was the effect of temperature on the aircraft symmetry. The aircrew carried out a few last minute checks before flight, including checking the position of the elevators relative to the sides of the tailcone. For those not familiar with the Dak, the rear tailcone has short horizontal stubs adjacent to the elevators. The check suggested that the one elevator was set at a slightly different angle to the other. Then we realized that the aircraft was standing in the sun, with the vertical fin casting a shadow on the one stub. With the temp at about 30 deg celcius, the one surface was noticeably hotter than the other. We poured water on the stubs, and the elevators then lined up. I guess there’s a reason why we do the rigging checks in the hanger, and very early in the morning ;-)

Thanks to all the forumites who shared their experience. It was certainly reassuring to see that our initial ideas were sound.

Blue skies