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The yaw/slip thread (merged) aka Aerodynamics 101

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The yaw/slip thread (merged) aka Aerodynamics 101

Old 10th Oct 2004, 13:55
  #121 (permalink)  
 
Join Date: Oct 2004
Location: australia
Posts: 173
Wow!!!
Extract of my Flight Test Report Canberra Asymmetric Handling ETPS 1970.
Results of Tests
Turning During Asymmetric Flight
a. Turns at 1.1 Vmca
Turns at 30 degrees of bank were made in both directions starting from 4000 feet with the left engine set at idle and the right at 7850 RPM and 187k IAS. The rate of climb during these tests was 1800fpm. Turns were easy to make with trimmer settings of; rudder 4 units right and aileron 1 unit right. The sideslip angle was zero and the slip ball position varied between 1/2 and 1 ball width displacement right. The slipball could be centred using full right rudder trim, 3" right rudder pedal travel and a pedal force of 150lbf. This resulted in a sideslip angle of 5 degrees left during left turns and 2degrees left during right turns. These turns were uncomfortable to fly because of relatively high rudder forces and also because the aileron trimmer setting for zero force changed from 3-4 units left during left turns to neutral during right turns. During these tests the yoke position for zero roll rate varied between neutral (right turns) and 10-15 degrees rotation left (left turns).

Discussion of Results and Conclusions
Turns During Asymmetric Flight
Turns at both safety speed (160k IAS) and 1.1 Vmca (187k IAS) with 7850 RPM set were very easy with zero sideslip. When the slip ball was centred (1.1 Vmca) it was necessary to hold high pedal forces, retrim the aileron and accept aileron positions other than neutral for zero roll rate. It is recommended that the aircraft be manoeuvred with zero sideslip using the trim technique described in para 4.7.1 and co-ordinated rudder, if necessary, to keep the slip ball approximately 1/2 ball width to the right
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Old 11th Oct 2004, 02:59
  #122 (permalink)  
 
Join Date: Oct 2003
Location: Canberra Australia
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ft and zzuf

OK ft I'm back and inserting balanced into the statement of fact.

Here is the statement again.

Any multi having engines outside of centre line will be sideslipping during straight flight, rudder free, when there is any imbalance of thrust.

or in other words

Any aircraft uniformally shaped around its fuselage centre line which has its line of total thrust offset from that fuselage centre line will be unable to achieve balanced flight, rudder free, in a straight line without sideslip.

Confusion continues to arise because of two differing concepts of balanced flight.

The common concept is that balanced flight is achieved when the balance ball/instrument is centred. All OK when the forces on the aircraft are symmetric and our aim is to fly the aircraft most efficiently in the precise direction of its centre line..

One could say that it is a whole new ball game when the forces are asymmetric.

In the asymmetric case we start with an imbalance of forces and seek ways to bring that imbalance into balance or equilibrium so that we can make progress through the air in a straight line. The only means we have to counter an engine thrust imbalance is to sideslip or to use rudder which creates a counter balancing force at the tail.

zzuf

Wish I'd been your tutor!!

Your review of the above will be fascinating considering your report at ETPS.

Last edited by Milt; 12th Oct 2004 at 00:55.
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Old 11th Oct 2004, 05:14
  #123 (permalink)  
 
Join Date: Oct 2004
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Further from my Canberra Asymmetric Handling Report
Determination of Minimum Control Speed
4.3 Minimim control speed was determined by reducing speed from the minimum trim speed in a straight climb at between 4,000 and 5,000 feet (first cut). The minimum control speed was the lowest speed at which a straight climb could be maintained. These test were made with power settings of 7600 RPM and 7850 RPM on the "live" engine and at bank angles of 0 degrees and 5 degrees towards this engine. Rudder pedal forces measured during these tests were very high and difficult to hold for even a short period.

4.3.3 Minimum Control Speed 7850 RPM Wings Level

Using the technique described in para 4.3 the minimum control speed with the right engine set at 7850 RPM was 175k IAS. The rudder pedal position was full right rudder held by a force of 250lbf. At 175kIAS the slipball was about 1/2 width to the right and the sideslip angle was 3 degrees left. Aileron forces were zero with the yoke position at 10 degrees left and the aileron trimmer position at 2 unit left.

4.3.4 Minimum Control Speed 7850 RPM 5 Degrees Right Bank

With 5 degrees of bank towards the live engine, the speed was reduced to 150k IAS before the aircraft commenced a slow turn to the left. The rudder force was still 250 lbf right rudder and a slight rudder buffet became apparent. The yoke and aileron trimmer positions were 5 degrees and 4 units right respectively.
The sideslip angle was 5 degrees right and the slip ball was at 1/2 the total available travel to the right.

Comment
This is clearly not Vmc - rudder forces way beyond the then BCARD limits.
No CAS quoted - no PEC info with sideslip.
At some bank anle between 0 degrees and 5 degrees right the sideslip direction changed from left to right - at some small angle of bank the sideslip angle was zero.
Bank angle was extremely powerful in reducing the minimum control speed


Of the many jet aircraft types I have flown most have had fairly strong lateral static stability( low speed, low altitude), so during asymmetric flight using a small bank angle towards the operating engine and co-ordinating rudder so the roll rate with neutral aileron is close to zero. Sideslip will be close to zero and the aircraft close to optimum for both handling and performance.
(Not true for most Prop/Turboprop aircraft and not true for all jet aircraft).

Milt, sorry gave up on FTSA years ago.
I will now disappear back into cyberspace.
Cheers
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Old 11th Oct 2004, 07:25
  #124 (permalink)  
 
Join Date: Oct 2003
Location: Canberra Australia
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zzuf

Just before you disappear into cyberspace spare a little time to explain the source of your magically produced counterforce produced without sideslip to counter that being produced by the asymmetric thrust.

Surprised that you can't ??
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Old 11th Oct 2004, 08:39
  #125 (permalink)  
 
Join Date: Oct 2004
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Milt,
At zero sideslip the yawing moment due to asymmetric power is exactly balanced by the yawing moment due to rudder deflection.
This leaves the sideforce due to rudder deflection unbalanced until bank is applied.
The magic force is the same force which would normally cause the aircraft to turn once bank is applied except that in this (zero sideslip) case it simply balances the sideforce due to rudder deflection.

I am off to determine maximum bank angle for my Triumph Speed Triple and the variation of tyre sideslip angle with co-efficient of friction.
Cheers All
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