Pilot_DAR wrote:
Try a power off approach at this speed, followed through to a power off landing. You may find that once you have the spot made, you’d like to carry an extra 10 knots into the flare, to allow you time to flare.
On the aircraft I fly, the best glide speed is well above the recommended threshold crossing speed and I usually reduce power to idle when crossing the threshold. I don’t have any problems with the flare. Indeed, when I practise power-off landings, I still aim for the proper threshold crossing speed and the flares work just fine.
It could be more of a problem if the aircraft has higher wing loadings, but even the TB20, which has quite high wing loading, has a best glide about 20 kt above threshold speed.
(I certainly agree that it is better to come in high and fast and risk overrunning the runway than low and slow and risk undershooting, but that’s a different issue.)
While the increase in camber increases the coefficient of lift for a given AofA, it will worsen the L/D ratio. Your aiming point on a forced landing is the middle of the field for an aircraft with flaps, by adding full flaps the aiming point will move towards the beginning of the field, and yes full flaps leading to reduced Vat reduces kinetic energy. In a typical training type the L/D is around 10:1 clean, and around 4:1 with full flaps.
More modern types are more efficient, a DA-20 might be 13:1 clean? The SET class (ex Caravan or PC6) comes in at 15:1.
If the circuit is empty why not carry out a constant aspect practice forced landing every time you return to base? Some folk, usually if they have been MEP instructors, have faith in the intrinsic safety advantage of the single engine, but only if you work at being current on your PFL skills.
On a slight tangent, would one not go to full flap at the last moment, to convert some kinetic energy into height?
Obviously this won’t work on a type on which the full flap is mostly just drag – e.g. the C150/152 from vague memory.
Noe wrote:
Then once landing spot decided on, best glide.
Well, yes but….
Try a power off approach at this speed, followed through to a power off landing. You may find that once you have the spot made, you’d like to carry an extra 10 knots into the flare, to allow you time to flare. The “best glide” speed is a certification requirement for maximum distance per altitude lost. Yes, you can flare and land from this speed, but great skill and precision must be applied – flare too high, and you’re dropping on. If you have arrived to the flare at best glide speed, and are too slow/low, you’re stuck, and headed into the hedge. If I have to get it wrong, I would rather be wrong too fast than too slow, as I would rather go off the far end of my spot at 10 knots unable to stop, than slam into the near hedge at 50 knots, unable to squeak over it.
I am a strong advocate for forced landing practice to a full stop landing, to practice the entire event. The go around at 200 feet ’cause you had the spot made, is woefully inadequate. Forced landings are a perishable skill, and should be practiced regularly.
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
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.
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:
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
Thanks for the explanations, learned something!
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.
