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Coordinated turns

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Coordinated turns

Old 31st Jul 2020, 22:31
  #1 (permalink)  
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Join Date: Jul 2020
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Coordinated turns

I own a new to me STOL aircraft. It seems to have a tendency to turn right and skid left which I can’t quite figure out. Any insights welcome.

In slow flight with flaps out and high engine revs to maintain altitude, wings level and with the balance ball centred with some right rudder, it turns to the right.

Also, in a left hand circuit in the base to final left hand turn at slow but safe speed, it requires significant right rudder.

In both situations, it doesn’t feel in the seat of my pants like the aircraft is skidding left before I add right rudder.

In the slow flight, the fact that wings are level and the aircraft is turning right, makes me think the use of right rudder is causing a skid into a right hand turn.

In the left base to final left turn, when I add right rudder to centre the ball, it then feels in the seat of my pants like the aircraft is slipping to the left.

Could it be a faulty balance ball, faulty aircraft design or faulty pilot?

Any insights welcome! Thanks.
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Old 31st Jul 2020, 23:54
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Well, perhaps "fault" is too strong a word, rather not as refined as others...

Sure, some planes require attention to the pedals with other control inputs. The ball indicator in the T&B/TC will always be right (unless it's simply not working at all). So, unless to intend uncoordinated flight, use the pedals to keep the ball in the middle. You can research aileron design, as there is lots to know about adverse yaw, which can occur when ailerons are applied, and yaw the airplane counter to the intended direction of turn. Really well designed ailerons don't do this (much). Read up on "frise ailerons", then observe if your plane has this design feature. In any case you'll get the idea.If the plane is STOL because of the installation of a STOL kit, yes, these do change the relationship of rudder to ailerons, and you'll need to use more rudder all the time. Get use to it, it's a very good habit anyway!

Now, to a more basic point - safety. STOL stands for "Short takeoff and landing". Sure, do that. However, it does not, and should, not stand for, nor inspire needless slow flight, particularly at lower altitudes. Yes, "STOL" planes (and I've flown dozens of types, and owned one for 33 years) are more capable of flying at slower speeds, and higher angles of attack. But they are no more capable of gliding and power off landings at these slower speeds, that is the same as the base plane. So, if you have a STOL kitted Cessna, for example, yes, it'll get off the ground in a shorter distance, and at a slower speed, excellent! But, to maintain the safety you have come to expect, allow it to accelerate in ground effect, and climb away at book speeds. Sure, if you have to clear an obstacle, do that. But understand that the plane can fly so slowly that in the event of an engine failure, it will not be possible to enter a useful glide, and make a power off landing before simply impacting the ground in a stall. Because if the engine stops in a "STOL" climb (or even slow flight), you're already flying more slowly that the ideal glide speed for the plane. You'll have to shove the nose down, and accelerate, just to get to glide speed, then glide to flare and land. By the time that all happens, you may have already reached the ground in a clump. If you have a Robertson STOL Cessna, the Robertson Flight Manual Supplement provides some warning words about this - take them seriously!

So, whatever the published glide speed for the plane is, fly at least that fast as often as you're able, knowing that if you're slower, a safe power off landing is not assured.
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Old 1st Aug 2020, 01:44
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Aileron input

There was one additional factor that occurred to me - with flap out I had to add left aileron to fly straight, which makes me think there is an imbalance between left and right flap, more left then right. I’ll talk to my engineer about adjusting that. Thanks for the moderator’s reply. All slow flying was above 3000 feet except for circuits which I fly at Saftey speed as per the POH.
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Old 1st Aug 2020, 01:46
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Does it fly straight in normal flight ? Any miss rigging will be exacerbated in slow flight.
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Old 1st Aug 2020, 01:57
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Big Pistons

Hi Big Pistons. Yes, flies straight in normal flight but I think it turns right with flap out. I need to go for a flight and check.
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Old 1st Aug 2020, 05:08
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Maybe you ARE skidding.
Is the ball centred when parked on the ground? A quick check with a spirit level could ensure the structure is level, rather than an eyeball estimate.
Quite a few manufacturers place slots around the circumference of the turn co-ordinator/turn & slip for adjustment of the ball/level. l have seen significant errors on several aircraft.
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Old 1st Aug 2020, 09:44
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Some basic facts: Most modern engines such as Lycoming, TCM and Rotax, which are commonly installed in modern aircraft, rotate to the right. Asymmetric thrust and the helical path of the propellor slipstream means that a permanent yaw to the left is always present. Manufacturers deal with this is many ways: offset engine thrust line and/or an offset fin. Cessna also commonly provide a fixed trim tab at the foot of the rudder which is bent so deflecting the rudder as required.

All the above are only going to be effective as desired with a given RPM and airspeed combination. The factory therefore arranges this to suit the RPM/speed for the most common cruise combination: i.e. the period that the pilot is most likely to spend time. When flying more slowly then the offsets begins to dominate and causes a yaw to the right.

When assessing the basic aeroplane it is important that all the references used are not in error. Until the mounting of the balance ball tube, turn Ind./turn Coordinator and the Attitude Indicator is serviceable and proven to be correct they should not be used. You need a stable day with a clear horizon. At the POH recommended cruise RPM and speed then check the wings are absolutely level with the natural horizon and that the nose remains fixed constantly on a distant reference point. It doesn't matter at this stage whether you need to maintain wings level and zero yaw with ailerons and rudder. The important thing is are you able to do so whilst maintaining a constant heading with the wings level. We are establishing by doing this check whether or not the fuselage is twisted or deformed. If it is you will not be able to maintain a constant heading with the wings level and will also have a difficulty with maintaining height. A series of heavy landings overtime can do this to the fuselage or a single heavy landing.

The aircraft logbooks should have recorded any accident damage and repair but ....?. I've lost count of the number of aeroplanes in this condition that I've discovered during pre-purchase flight surveys, This is the first thing I look for because just about everything else can be fixed. With your hands and feet off and the aeroplane flies straight and level the fuselage is good and all variable settings are also correct. Note; some aeroplanes have a fixed trim tab on the right aileron which may need adjusting, by bending as with the rudder fixed trim tab. These fixed trim tabs can only be adjusted on the ground of course and can take a number of flights to get spot on. These above checks need to be completed first before any other checks or you could be chasing your tail, so to speak.

You haven't told us the aeroplane type and model and the type of engine installed. There will be many who can help you knowing the particular characteristics of your aeroplane. There is a lot more to add but we need detail to help further. I don't want to go further without those details.

I'm not sure that you are using the terms "skid" and "slip" correctly so that needs some clarity. Also the term "turn" we need to separate this from "yaw" - in physics they can be the same thing I know but for flight we usually keep them seperate. Yaw then is a swing around the normal axis but turn describes a change of direction of the flight following a roll to an angle of bank. Pilot Dar has already explained "adverse aileron yaw" which causes a swing away from the direction of the intended turn but it is yaw and is only prevented by the proper amount and the correctly timed application of rudder.

Last edited by Fl1ingfrog; 1st Aug 2020 at 18:00.
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