Originally Posted by
Mad (Flt) Scientist
A couple of things.
Re: "no definition of takeoff flap" - there probably is such a definition, explicit or implied, in the Airplane Flight Manual; you can't read the TCDS in isolation.
re Stall speed effect on VMC. There's nothing explicit in the regulations linking VMC and Vs - they both then impact operational speeds independently. But there is an effect of the lifting capability of the wing on VMC - simply, if you can't maintain level flight you can't demonstrate VMC, and it was all about demonstration back then. So even if there is theoretically enough control power at lower speeds, you can't have a VMC below a certain speed, regardless.
re flaps effect on VMC - there's also (sometimes) a directional stability effect of having flaps down (more flaps = more directional stability) - this then can help in a VMC condition as the helping effect of directional stability is why the bank into the live engine is helping. So a higher VMC with less flaps could work.
I specifically mentioned that I do not recall flap 0 being an approved takeoff flap for the G159 setting, I couldn't find an AFM, so we were referring to the TCDS as the only info we had. I would personally be looking at the differences between stability and control tests at flaps takeoff and flaps 0 at various symmetrical power settings before going down the track of pursuing asymmetric drag as a cause of differences in Vmc.
I am not sure what your point is with reference to Vmc and stall, as far as CAR4b is concerned Vmc is not to be greater than 1.2 Vs. I am aware that other takeoff speeds have a Vmc additive, my opinion on Vmc and V2 minimum was that it is not unreasonable to have both limited by 1.2 Vs.
I am also aware that the Vmc determination for the original Fokker F28 was done in a less than 1g flight path hence lower than 1g stall speed. While I don't know why this was done, Fokker and the regulatory authority may have been very conservative about flying around at close to stall speed in an early T tailed jet. The F28 had an artificial stall identification, (also artificial stall warning), there may have been concerns about the stall ident accuracy with sideslip - who knows, but a less than 1g flight path would be an easy test to perform if one worked up to the final test points.
Your point about banking towards the operating engine can use the aircraft directional stability to assist in directional control, is well made - just make sure the bank angle is sufficent to ensure that the sideslip is toward the operating engine.
Extract from a report I wrote many years ago is instructive:
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