Lift versus drag conversation

[Bear Cub]

I recently overheard a conversation between two (student?) pilots - they were discussing the gliding configuration to be used during a forced landing.

They agreed with each other that, according to the books, "one stage of flap" (PA28 pilots) provides more lift than it does drag.

Their argument then revolved around "Would the immediate application of one stage of flap at the time of engine failure give you more time in the air?"

I'd be interested in an aerodynamicists point of view.

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[DABHAND]

When in a power off configuration, the aircraft will glide best at the Vmd (Minimum drag speed). This is the point at which lift dependant and form drag meet on a typical drag curve. Therefore, if flap is lowered the form drag will increase, as will total drag. This is not the best configyration and a clean aircraft flown at the correct speed will give the best glide range. By lowering flap, you could achieve a higher glide angle to clear obstacles, but this would lead to lower range.

[Cuban_8]

Dabhand,

You seem to be somewhat confused with regards to your theory. In a power-off forced landing situation, i would suggest that Vmd is NOT what you are looking for!

For the greatest range over the ground during such a glide, any aircraft must fly at a speed which gives the best lift/drag ratio - a value for which is suggested by the manufacturer. In terms of remaining in the air for the longest period, then the aircraft must fly at a speed that gives Cl^1.5/Cd maximum.

Your argument is even more incorrect considering that when flying at maximum endurance configuration, Cd=4Cdo, whereas for a maximum range case, Cd=2Cdo. Therefore, in a maximum flight duration situation, the drag coefficient is even greater. Hence, your suggestion that Vmd is the desirable speed is false.

[little red train]

***All those who read, plase note fundamental mistakes have been made in this post, I'd like to leave it here so the rest of the string make sence & we can all learn***

Far to thick for the maths above. But my understanding is.

Vmd is minimum drag, at this speed there is the best Lift to drag ratio, this determines Angle of impact.. sorry, Glide angle. not time in the air.

Vmp is minimum Power. Power is the amount Thrust over Time. no engine results in no thrust, but theres still time!.

Using the least power to keep the A/C in the air, will result in more time to panic in the air.

Vmp is lower than Vmd, Vmd is at the lowest point on the total Drag to Speed curve, giving the best ratio. Vmp is the point hit by a line tangential ***Perpendicular (Right angles), always been crap at maths)*** to the curve from the origin.

these are with no power, & should be applicable to both prop & Jet, however, powerplant differences result in differences when the noisy things still whine away (Engines not Passengers).

As for putting flaps down, I'd be thinking how fast you want the student to hit the ground, & how familiar the attitude to look.

[This message has been edited by little red train (edited 04 November 2000).]

[Cuban_8]

Little red train,

Vmd and Vmp are different things to Cl/Cd max, and Vmd certainly doesnt coincide with Cl/Cd max!! For best glide range, let me assure you that an aircraft must be flown at Cl/Cd max! In a typical PFL situation, this is the most prudent speed to select as it allows a greater range of landing areas to be considered.

[John Farley]

Well said Cuban

And on the subject of Vmd in general, flying around at Vmd, while the theoretical way to fly for max endurance, is best not done either. (min drag equates to min power which for the given atmos conditions is bound to be min fuel consumption) You need to add a few kts above Vmd to cater for the turbs around that day and your accuracy of flying the ASI. Loose any kts from Vmd and by definition (since you are now speed unstable) you will need a shovel full of power to get said knots back, or if gliding you will need to stuff the nose down and toss away a lot of glide angle to have your weight component along the flight path accelerate you back to Vmd.

JF

[Vmu]

Speed for L/Dmax is equal to speed for minimum drag. For best glide angle (no wind) fly speed for minimum drag. Using flaps increases drag, thus decreasing the L/D ratio, and gives a steeper glide.

For min rate of decent fly speed for min power required.