Following a few threads, I realized I don’t know the minimum sink speed of the plane I fly (DA40NG). It’s also not in the aircraft manual.
I could think of a few situations where it could be handy. I might be wrong so please correct me:
1) pretty much every situation where the engine fails, and one has to decide where to land. Then once landing spot decided on, best glide.
2) on EFATO, this could allow different choices of field. it would lower the altitude at which a safe return to runway is possible (one would so something like the first (90 to 180, I think some value in between would be optimal. It would reduce the total distance for the reversal since the radius would be smaller) degrees of the reversal at minimum sink, then the rest at best glide.
3) in case the landing place is right below, it would give more time to try to do restart drills.
4) in some cases where one has to glide, if the tailwind is greater than difference of minimum sink rate and Vbg, then one could go a further distance forward by flying minimum sink.
Is there a reason why it’s not on the manual? Maybe since best glide IS generally more useful one wants to avoid confusion between the two?
Or all these situations are generally under distress and the additional benefit of being slightly more optimal could just lead to more load and overall less optimal flying / things overlooked / more risk?
What, please, is the difference between best glide and minimum sink? Same thing in my mind – am I missing something?
172driver wrote:
What, please, is the difference between best glide and minimum sink? Same thing in my mind – am I missing something?
One gives maximum range, one gives maximum time in the air.
What matters for best glide being feet per nm, what matters for minimum sink is feet per minute.They won’t happen at the same IAS.
On the reversal to the runway, I know in my plane at least that I loose much less altitude by increasing the rate of turn to 2 (as opposed to 1); the risk there obviously is letting your speed decay and stall/spin.
Best glide is equivalent to L/D max, and is the tangent to the origin for the polar drag curve in a no wind condition.
Minimum sink is derived from the power required and power available curves for propeller aircraft, and is less than Vbg, usually closer to the stall speed – it is independent of wind condition.
The calculus for the two is quite elegant, some find it trivial, but I can’t recall it from memory, except that the curve on the back side of the drag curve, and therefore the derived power required curve, is steeper due to the increase in lift dependant drag.
Crudely Vy and Vbg tend to be approximately in the same neighbourhood, and Vx/ VminPR are also quite similar but slower – for propeller aircraft. The thrust line for turbofans is quite different so the relationship is a bit different.
In theory the Super Cub best glide is 60 mph while minimum sink is 45 mph, and this 0.75 or 0.76 relationship can be proved with calculus.
Thanks for the explanations, learned something!
RobertL18C wrote:
Minimum sink is derived from the power required and power available curves for propeller aircraft
No, actually it’s derived from minimising sink speed and is achieved at (C_W)²/(C_A)³
RobertL18C wrote:
The calculus for the two is quite elegant, some find it trivial, but I can’t recall it from memory, except that the curve on the back side of the drag curve, and therefore the derived power required curve, is steeper due to the increase in lift dependant drag.
For an assumed shape of the drag curve (based on a quadratic Cd vs Cl) the drag curve has a characteristic shape D = v^2 + 1/v^2 (for a given weight).
You minimise sink angle by minimising D. You minimise sink rate by minimising D * v.
D = v^2 + 1/v^2 has a minimum at v = 1
D * v = v^3 + 1/v has a minimum at v = 1/(3^0.25) = 0.76
You can put in all the pre-factors but the ratio of minimum sink speed to best glide speed is always 0.76.
RobertL18C wrote:
In theory the Super Cub best glide is 60 mph while minimum sink is 45 mph
In the book for my 150hp model it says the flaps up stall speed is 43 mph, so Ican’r believe min sink can’t be that close to the pwer off stall? I can find reference to best glide being 70mph, but not the min sink. Best Angle of Climb is 45mph but by definition thats at full power.
bookworm many thanks
here is a useful link to minimum sink on the vans site with relation to the power required curve, which is drag converted to work
http://www.vansairforce.com/community/showthread.php?t=134077&page=2
On the SC 60 mph x 0.76 gives you 46 mph
