Trim or Balance & why
Avoid imitations
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Ball v, string....,
Years ago we had to teach instrument flying in the RAF Gazelle. The aircraft drifted off heading to the right if the ball was centred, quite noticeable. However, if an En Route Supplement (20 mm thick book) was jammed under the right hand end of the instrument panel, it lifted it up on its rubber mounts and the aircraft stayed on heading. Problem was caused by the heavy extension on the right hand end of the IF panel, pulling it off level. The Army versions had a smaller panel and didn't have the problem.
Answer is to know your aircraft.
Years ago we had to teach instrument flying in the RAF Gazelle. The aircraft drifted off heading to the right if the ball was centred, quite noticeable. However, if an En Route Supplement (20 mm thick book) was jammed under the right hand end of the instrument panel, it lifted it up on its rubber mounts and the aircraft stayed on heading. Problem was caused by the heavy extension on the right hand end of the IF panel, pulling it off level. The Army versions had a smaller panel and didn't have the problem.
Answer is to know your aircraft.
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Fostaire has a very good anwer.
When flying an Alouette in the Alps many years ago, we were trying to get up to a mountain hut. I was dutifully flying with the ball in the center and thinking that, given the pathetic rate of climb (about 300fpm if memory is still good) that we were never going to get there. The Swiss test pilot suggested that I put the the slip string in the middle instead, and we tripled the rate of climb to just over 1,000fpm.
In all my flight testing, I found the slip ball to be useless below 60 knots - there's just not enough side force for it to react properly.
And I've never found any textbook (fixed or rotary wing) that describes what the slip ball actually measures...
When flying an Alouette in the Alps many years ago, we were trying to get up to a mountain hut. I was dutifully flying with the ball in the center and thinking that, given the pathetic rate of climb (about 300fpm if memory is still good) that we were never going to get there. The Swiss test pilot suggested that I put the the slip string in the middle instead, and we tripled the rate of climb to just over 1,000fpm.
In all my flight testing, I found the slip ball to be useless below 60 knots - there's just not enough side force for it to react properly.
And I've never found any textbook (fixed or rotary wing) that describes what the slip ball actually measures...
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And I've never found any textbook (fixed or rotary wing) that describes what the slip ball actually measures...
thanks, tet
Ball
Very good question, Shawn. When the Army decided to do the MH -60K/MH-47 and IBM Federal Systems, Owego put the avionics system together, one of the basic problems it surfaced had to do with how they synthesized the mechanical ball. They used a filtered lateral acceleration signal, if I recall, and had a tough time with adjusting the signal processing to make it smooth enough to use.
I do not remember if the system stuck with the electronic derivation, or whether they simply added a mechanical ball at the bottom of the MFD.
I do not remember if the system stuck with the electronic derivation, or whether they simply added a mechanical ball at the bottom of the MFD.
Last edited by JohnDixson; 2nd Aug 2013 at 12:20. Reason: Additional thought
Shawn....in the Alouette....you do recall the Mast Tilt that was built into the thing. Something like five degrees was it? (Thinking Alouette III here....). I recall it was noticeable compared to other helicopters I have flown.
I always relied upon the String and recall the ball being about a half marble out to one side.
I always relied upon the String and recall the ball being about a half marble out to one side.
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Nice question VF. I love how everyone can have such different opinion and techniques, what great machines.
I like the basic three points mentioned above:
1. Best performance per power setting is most likely in trim.
2. If using the ball, check in hover and use that as your "basic reference" but understand that things like cross winds (hover), fuel burn, and speed all effect and can change your CG (which way is down).
3. When in doubt use the strings, if they don't exist nothing like some scotch tape and yarn.
I believe the first pilots to fly in clouds many years ago (before "modern" instruments) achieved this by taping a single strand of string, with small weight, on the ceiling of the cockpit.
So the string and ball (weight) is one of the first attitude references used in aviation.
I like the basic three points mentioned above:
1. Best performance per power setting is most likely in trim.
2. If using the ball, check in hover and use that as your "basic reference" but understand that things like cross winds (hover), fuel burn, and speed all effect and can change your CG (which way is down).
3. When in doubt use the strings, if they don't exist nothing like some scotch tape and yarn.
I believe the first pilots to fly in clouds many years ago (before "modern" instruments) achieved this by taping a single strand of string, with small weight, on the ceiling of the cockpit.
So the string and ball (weight) is one of the first attitude references used in aviation.
"Just a pilot"
I fly whatever keeps the heading constant
in the cruise, at lest. Start with the seat of the pants, check the string and ball, average them out. Scan, scan, check everything again, back on heading and a little pedal, look at string and ball, and continue to average everything out until heading starts to wander either side. This works no matter how skewed the trim ball and how everything is configured on that particular flight.
Never much thought about it in other regimes...
Never much thought about it in other regimes...
Last edited by Devil 49; 3rd Aug 2013 at 05:14.
Avoid imitations
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And I've never found any textbook (fixed or rotary wing) that describes what the slip ball actually measures...
---but the ball, having more mass than the air bubble, is affected by lateral accelerations ie sideslip.
How does that work in FJ's....where high G Turns are common?
Most FJ pilots use the pedals as foot rests except for takeoff and landing in crosswinds - after basic jet training a helicopter was a nightmare because you actually had to use your feet all the time
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At 30 kts, the string is very helpful to find (or confirm )the wind direction when you do a low pass above your landing point...
between 30 and 60 kts i use the string,
above 60 kts i use the ball.... that's all for me
between 30 and 60 kts i use the string,
above 60 kts i use the ball.... that's all for me
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crab:
hate to disagree with you, but there is absolutely no relationship between sideslip (an aerodynamic effect) and the slip ball (a purely mechanical device).
hate to disagree with you, but there is absolutely no relationship between sideslip (an aerodynamic effect) and the slip ball (a purely mechanical device).
Shawn - the Light Aircraft Association might disagree with you
SO WHAT IS SIDE-SLIPPING?
An aircraft which is side-slipping is, in fact,
flying slightly sideways. A side-slip is defined
as “a combination of forward movement
and sideward (with respect to the longitudinal
axis of the aircraft) movement.” Simply, the
nose is not pointing towards the relative
airflow [see fi g 1].
Unintended side-slip occurs when the aircraft
is placed into a turn with insufficient rudder
applied; this results in the aircraft being out of
balance, and the relative airflow and direction
of movement through the air being offset from
the longitudinal axis of the aircraft. Indication
of this condition in the cockpit is the slip
indicator (which in most modern aircraft
is ‘the ball’) being off-centre. A common
error seen in the flying of low-hour PPLs after
take-off is that often insufficient rudder is
applied to balance the effect of slipstream,
so the pilot then incorrectly applies bank in
order to maintain direction. A check of the slip
indication, ‘the ball’, shows that the aircraft
is side-slipping while they are maintaining a
constant direction
An aircraft which is side-slipping is, in fact,
flying slightly sideways. A side-slip is defined
as “a combination of forward movement
and sideward (with respect to the longitudinal
axis of the aircraft) movement.” Simply, the
nose is not pointing towards the relative
airflow [see fi g 1].
Unintended side-slip occurs when the aircraft
is placed into a turn with insufficient rudder
applied; this results in the aircraft being out of
balance, and the relative airflow and direction
of movement through the air being offset from
the longitudinal axis of the aircraft. Indication
of this condition in the cockpit is the slip
indicator (which in most modern aircraft
is ‘the ball’) being off-centre. A common
error seen in the flying of low-hour PPLs after
take-off is that often insufficient rudder is
applied to balance the effect of slipstream,
so the pilot then incorrectly applies bank in
order to maintain direction. A check of the slip
indication, ‘the ball’, shows that the aircraft
is side-slipping while they are maintaining a
constant direction
The Butt Knows
I believe that much of the confusion comes from lack of understanding what the slip ball and yarn actually measure. The slip ball measures side forces at the location of the instrument in the aircraft. Side forces are a result of the sum of all dynamic forces on the machine. The yarn/slip string measures sideslip airflow at the location of the string not necessarily the actual sideslip of the machine. In flight test a boom is utilized to locate the pitot system clear of the rotor system thus measuring actual side slip. Like many who have responded on this site I have experienced the conflicts between what my butt says and what the instruments display. A good example is the EC-135. If the ball is in the middle my butt was hanging out and vice versa. The AS-350 was even more confusing. There you have two inputs (string and slip ball) versus your butt. In the end I allowed my butt to prevail and it has served me well.
Last edited by Jack Carson; 4th Aug 2013 at 22:44.
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The ball is a sideslip indicator on a symmetrical aircraft only
In the helicopter axes (where the ball is actually installed) aerodynamic forces are balanced by inertia forces. The ball only sees inertia forces, but they are a view of the opposing aerodynamic forces.
A centered ball indicates that there is no inertia sideforce, and therefore no aerodynamic sideforce. On a symmetrical airplane, the sideforce only comes with sideslip. Flying with a zero centered ball guarantees flying with zero sideslip.
When the airplane is no longer symmetrical, a twin engine aircraft with an engine failure fo example, there is no such obvious link. The remaining engine provides a yawing moment that has to be counterbalanced by a lateral force on the rudder. Flying with zero sideslip means no aerodynamic sideforce on the fuselage. The resulting aerodynamic sideforce is the rudder force and is not zero. It has thus to be compensated by a lateral inertia force and the ball is not centered. Flying with a centered ball means that there is no lateral aerodynamic force. The rudder sideforce needs to be compensated by a fuselage aerodynamic sideforce that can only come from a sideslip angle.
The classical single rotor helicopter is not symmetrical. In zero sideslip conditions (no aerodynamic fuselage sideforce) there is still a lateral aerodynamic sideforce coming from the tail rotor thrust. The ball is therefore not centered. A centered ball means that there is some sideslip, inducing a fuselage lateral sideforce opposing the tail rotor thust.
In high speed conditions a small sideslip angle can generate a significant fuselage side force. The sideslip angle with a centered ball is therefore limited. In low speed conditions a much larger sideslip angle is needed. Close to hover there is not enough speed to compensate for the (high) tail rotor thrust and it is no longer possible to fly with a centered ball.
Using the string in low speed conditions and the ball in the high speed range makes sense.
A centered ball indicates that there is no inertia sideforce, and therefore no aerodynamic sideforce. On a symmetrical airplane, the sideforce only comes with sideslip. Flying with a zero centered ball guarantees flying with zero sideslip.
When the airplane is no longer symmetrical, a twin engine aircraft with an engine failure fo example, there is no such obvious link. The remaining engine provides a yawing moment that has to be counterbalanced by a lateral force on the rudder. Flying with zero sideslip means no aerodynamic sideforce on the fuselage. The resulting aerodynamic sideforce is the rudder force and is not zero. It has thus to be compensated by a lateral inertia force and the ball is not centered. Flying with a centered ball means that there is no lateral aerodynamic force. The rudder sideforce needs to be compensated by a fuselage aerodynamic sideforce that can only come from a sideslip angle.
The classical single rotor helicopter is not symmetrical. In zero sideslip conditions (no aerodynamic fuselage sideforce) there is still a lateral aerodynamic sideforce coming from the tail rotor thrust. The ball is therefore not centered. A centered ball means that there is some sideslip, inducing a fuselage lateral sideforce opposing the tail rotor thust.
In high speed conditions a small sideslip angle can generate a significant fuselage side force. The sideslip angle with a centered ball is therefore limited. In low speed conditions a much larger sideslip angle is needed. Close to hover there is not enough speed to compensate for the (high) tail rotor thrust and it is no longer possible to fly with a centered ball.
Using the string in low speed conditions and the ball in the high speed range makes sense.
Amdec,
In modern helicopters that have tail fins designed to off load the Tail Rotor in cruise flight.....how does that affect your idea? I can see what you are describing as being pretty accurate....up to the point the Tail Rotor is exerting Zero side force....and thus there is no force that requires balancing which would allow the aircraft to fly wings level with a centered Ball.
In modern helicopters that have tail fins designed to off load the Tail Rotor in cruise flight.....how does that affect your idea? I can see what you are describing as being pretty accurate....up to the point the Tail Rotor is exerting Zero side force....and thus there is no force that requires balancing which would allow the aircraft to fly wings level with a centered Ball.
Irrespective of whether the tail rotor or the fin is doing the work the, tail is proving a countering force to react the torque from the main rotor. As a result, some sideslip is required to counter this lateral force. Similarly, the lateral forces for most single propeller engine airplanes do not completely balance out due to the propeller’s “P” factor. This factor is really obvious in aircraft with very high power engines. The T-28 is a good example with its 1550 hp R-1820 Wright Cyclone engine. Significant right rudder was required to keep the aircraft in trim during high power settings.
