Timothy i for one agree with you. I think blue line is clear and simple and will do just fine combined with pilot dars very helpful point about picking a spot (perhaps at the end of a glide) and adjusting speed accordingly. As someone said if nothing else twins do have a habit of running out of fuel 
too often which will do if all else doesnt manage to cause a dual engine failure.
I also think your point about feathering is massively important and not appreciated by many pilots.
For what it’s worth, the SEFA which standardizes pretty much the whole PPL training in France wants instructors to teach using best glide speed plus 5 kts. I guess they also figured that it is rarely a question of just reaching a landing site on your last joule of potential energy, but rather good to have some margin for error in both ways. Also a windmilling prop will make you go down steeper than one with an engine at idle behind it, so you always train slightly differently than what you will see in the real case.
Timothy wrote:
All this theory trying to reach an exact magic number when there is a lovely line already painted on every twin’s ASI which, everyone is agreed, is close enough and will give a very good approximation to best glide range.
Well why not use the red line? Or perhaps the flap limiting top-of-the-white-arc? Anyone for Vne? Or perhaps just a nice round 100 knots: that always has a big tick mark beside it :). The speed you choose matters rather more than whether you gain or maintain altitude.
Airborne_Again wrote:
The minimum sink rate and minimum drag are the same point (as in the absence of power you need sink to overcome drag). Best glide is at the point of maximum L/D, which is always at a higher speed than minimum sink/drag.
Since lift has to equal weight, which is constant, how can minimum drag be anywhere other than maximum L/D?
Your mathematical relationship may be a good rule of thumb, but in practise it depends on the airfoil and the parasitic drag of the airframe.
It’s better than a rule of thumb. It only depends on the parasitic drag coefficient being quadratic across the range of interest. I agree it’s not exact, but it’s a lot more precise than any of the relationships between these speeds and the climb speeds being discussed.
>>The minimum sink rate and minimum drag are the same point (as in the absence of power you need sink to overcome drag).
MInimum sink rate isn’t the same {airspeed, AoA, drag} as minimum drag.
Minimum drag occurs at (Cl / Cd)max.
Minimum sink rate is minimum power, aka minimum (drag x airspeed), and a few seconds of algebra shows it occurs at (Cl^3/2 / Cd)max.
Agree with Jarvis, although the calculus proof may take a bit longer than a few seconds.
Minimum drag is derived from the thrust curves, while minimum sink is derived from the power curves.
Best rate of climb, Vy, is where the difference between power available and power required is at a maximum, which for propeller aircraft is approximately Vmd. See Aerodynamics for Naval Aviators courtesy US taxpayers.
With feathered props there is a slight increase in drag so the drag curve would move up and to the left, hence best glide is slightly less than Vy, in a no wind condition.
Vy tends to be a fair bit higher than blue line, Vyse, because of the drag elements of asymmetric flight. A double engine failure removes these, hence being closer to Vy. At least with a double engine failure a Vmc rollover won’t get you, which remains a factor for multi engine propeller aircraft, the mixmaster excepted.
bookworm wrote:
Well why not use the red line? Or perhaps the flap limiting top-of-the-white-arc? Anyone for Vne?
Because all the actual quoted figures from real POHs seem to come close to Blue Line.
There just seem to be some rules of thumb useful for people who fly different types and don’t have the opportunity to learn all the minutiae from every POH. These rules of thumb seem to be remarkably consistent between designs, and there is probably a mathematical reason for them.
For example, if you don’t know Vref, half way between Blue and Red lines seems to work pretty consistently.
If you fly level at your approach speed in trim, dropping the gear will create a 3° slope with no further intervention (this works from Arrows up to airliners)
It is very helpful to have these rules of thumb in the back of your mind for when things are getting difficult, and I guess they don’t get much more difficult than a double engine failure out to sea. So having a mnemonic that works for such a Black Swan event makes sense, and the one that seems to make sense is Blue Line.
Having dismissed the question about how you get to Blue Line as relatively less important, could you answer it?
Dave_Phillips wrote:
Not it won’t – it may. Been there done it.
Although the engines may not fail, you can at least get a loss of power due to induction icing. Been there, done that. Alternate air is there for a reason.
Indeed, and the AFM is quite clear on the use of alternate air although, in some respects, S03 contradicts the Normal/Abnormal Operating Procedures. However, playing the pedantic game, it is incorrect to say that:
A DA42 will get a dual engine failure in icing conditions, specifically -15C, if the alternate air is not being used.
Checking Commander Timothy’s thesis against the PA34 and DA42, Vy and Vyse are a few knots difference, none in the case of the Seneca 1, so blue line for your garden variety twin would be a good rule of thumb.
