Trim or Balance & why

[Vertical Freedom]

I am interested to hear other Rotory Pilots views on flying in trim, or in balance & technically why please? I was trained to fly in trim always.............now after just 20 years of flying I have come across a bunch of Pilots who fly our AS350B3+ in balance & frankly it feels awefull like I am falling off my seat & it looks outside that we are skidding across the sky. Your right a... cheek may be lower in-trim but it feels right, balance ball in the centre feels :confused: :eek: :mad: :ugh: :yuk:

What do the aerodynamics Guru's like Prouty etc have to say on the topic???

Thank You for all your responses & as my ole' mate Guru Instructor who has now passed on to that big Heli Hanger in the sky used to say; 'do everything into wind........except piss' ;)

[Lonewolf_50]

If I could post it, I'd find my copy of Al Ashley's old article on why it is best in the SH-2 to fly with the ball out a quarter ... but it's an old two pages stuck together copy of a copy of a copy that might not scan. I also don't have the rights to it. His explanation was involved enough that I don't want to try to reattempt it from memory.

[Darkhorse30]

Single rotor helicopters will always have a miss match between being in trim with the ball centered and sideslip angle. This is because the fuselage has to balance side force produced by the thrust of the tail rotor. It is particularly apparent at slow speed high power settings when on the back side of the power curve. I fly in trim because there is no way to display side slip angle unless you have special instrumentation. You can put a piece of yarn on the wind screen and with a little practice negate some of this annomolly by playing with pedal position, but I think it is more trouble than it's worth in the Huey I fly. Maybe it might be more usable in a R22.

[haihio]

Do you maintain the helicopter in trim by looking at the trim strings or the ball in the inclinometer?

I'm flying R66's and there is a noticeable difference between the two.

Please give me your views on the matter!

[African Eagle]

Never had to fly a Robinson (thankfully) but in the frog-ships (AS350's) it depends on speed as to which is the most reliable.

[Vertical Freedom]

Fling wing machines should be flown in 'trim' & not in balance :uhoh:

[GoodGrief]

When in compliance with the string the ball is about half way right out of center in my 355.

[[email protected]]

With a tail rotor, the helicopter experiences inherent sideslip (the same as TR drift but in forward flight) so the fuselage doesn't travel along its centre-line with 'wings' level (level on the AI), it drifts slightly to the left or right depending on the MR direction rotation and therefore the anti-torque thrust direction from the TR.

The string will give a reasonably accurate indication of the relative airflow over the fuselage and is often preferred in the slower speed bracket whilst the balance ball will give an indication of skid in the turn but will only remain central on a constant heading if there is a little opposite bank applied to counter the inherent sideslip.

[peeush]

'Ball' in my view must be considered more reliable, especially in low speed regimes where the rotor downwash affects the strings. Further, the 'Ball' also behaves exactly in accordance with 'Seat of pants' flying mechanics for balanced flight..... I would personally go with the 'ball' in contradictory situations any day.

[[email protected]]

The problem is that in slow speed flight you are often out of balance because you have an angle of bank to compensate for TR drift.

[African Eagle]

The string will give a reasonably accurate indication of the relative airflow over the fuselage and is often preferred in the slower speed bracket whilst the balance ball will give an indication of skid in the turn but will only remain central on a constant heading if there is a little opposite bank applied to counter the inherent sideslip.

This is my experience.

'Ball' in my view must be considered more reliable, especially in low speed regimes where the rotor downwash affects the strings.

The string only starts to flutter in very slow speed conditions ie. nearing hover but as crab mentioned is often more reliable at "slower" ie. between 20-50kts than the slip indicator.

[peeush]

Well.... we don't have strings in most helicopters, but 'ball' in all..

[SASless]

I assume I am in "Trim" or "Balance" when something centers....be it string, ball, or pedals....in no particular priority. The best indication is when the wind quits roaring through the cockpit doors in one direction or the other.

Do you guys lay awake at night thinking about answers to such questions?

If so...ponder this one.....

If you maintain the aircraft in "trimmed" flight....how much offset do you add to the formula determining the correct heading to be flown to fly a particular track across the ground from your Takeoff Point to your Destination?

It would seem if this TR drift thing is so significant...then CFS trained Instructors would have a colored pencil designated to depict that bit of the Vector Diagram.

[407 too]

SAS, it depends on which direction you are flying, don't forget the earth is spinning below you once you are skids (wheels) up:rolleyes:

[SASless]

It would seem if one maintained a steady hover relative to the spot in space you happen to occupy....you could get about a Thousand Miles an hour out of an R-22 or Bell 47.

Whew....now that is flying!

[haihio]

Thanks for the replies.

I've noticed that when I keep the trim strings centered the ball is always a little off to the left.

Personally I prefer to determine trim with the trim strings but I've noticed other pilots prefer the ball method

[topendtorque]

Well.... we don't have strings in most helicopters, but 'ball' in all..not quite true, it is only recently we over here to have balls installed, just think what sort of 'real' results we could have done with them. I never knew the install instructions. Ship level on the rigging jacks or in the hover skids hanging low differently for every type, it's a quandary. Remember one day with a high order F/W department person on board; 'Have a go' I says to Barry, next minute I'm nearly a block of ice in the wind draft. Ol' mate is fighting the ball which is only attached one end, but there cos it's the CP's machine and you all know about the blessed rules eh. I cover the ball and tell him fly what the reckons is right, give it a flick with my thumb and praise his accurate flying once I uncover it as now right in the middle.

As I get older yeah it's hard to work out when I lift off, now do I go faster forward to get there or slower backwards with this earth spinning thing.

I was taught without strings or ball, fly for fastest airspeed at the given power setting, that's when it is in best trim and balance.Try it, you may be surprised how comfortable it feels.That is regardless of what you are doing at the time, turning, climbing or cruising.

[SASless]

Are your strings getting "pure" air.....or is there some interference that influences the airflow a bit?

[icedriver]

I generally try and fly where the seat isn't trying to tear my jocks off.

[fostaire]

What is the condition of the ball in a hover? If the aircraft is loaded with an off center lateral CG, the ball will be off center in a hover. The string is NA at this point in flight. Now in forward flight, (cruise) put the ball in the same position as observed in the hover and you should be pretty close to trim. Now see what the string indicates in comparison to the ball position? Must agree with earlier post, if you are loosing sleep on this, you are pretty fortunate with no other higher priority matters at hand.

[ShyTorque]

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.:p

[Shawn Coyle]

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...

[topendtorque]

And I've never found any textbook (fixed or rotary wing) that describes what the slip ball actually measures...
And that's good enough for me I'll quote that.
thanks, tet

[JohnDixson]

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.

[SASless]

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.

[agodesign]

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.:O

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. :eek::D So the string and ball (weight) is one of the first attitude references used in aviation.

[407 too]

I thought the cat and duck were the first used :}

[Devil 49]

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...

[ShyTorque]

And I've never found any textbook (fixed or rotary wing) that describes what the slip ball actually measures...

It's a spirit level.

[[email protected]]

---but the ball, having more mass than the air bubble, is affected by lateral accelerations ie sideslip.

[SASless]

How does that work in FJ's....where high G Turns are common?

[[email protected]]

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:ok:

[BOBAKAT]

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:ok:

[Shawn Coyle]

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).

[[email protected]]

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

[Jack Carson]

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.

[AMDEC]

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.

[SASless]

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.

[Jack Carson]

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.