PDA

View Full Version : V Speeds on the Drag Curve

blah blah blah
18th Feb 2010, 06:53
Hi all,

Is anyone able to give a fairly concise answer regarding how and where the various V speeds fit onto the Drag curve?

For example where does Vs fit on the curve?

In the other sense as well, what is the practical indication that I can use to fly Vmin drag?

I might have missed something, but I cant find a relation between these figures.

Cheers.

Blah

capt. solipsist
19th Feb 2010, 14:34
Will try my best, see if you can make sense of this. Let me just talk about Total Drag to keep things simple.

Imagine the Total Drag Curve as an italicized upper case letter J. If you draw the hook part of the J at about an inch of distance from the vertical axis, the distance of the bottom of the J to the horizontal axis is about 1.5 of that. The vertical axis is drag (pounds as unit, for example) and the horizontal is speed.

Typical Vs (Stall Speed) is also about an inch from zero (along the horizontal, of course). If you draw a line upwards from Vs, it will intersect with the top part of the hook of the curve (or letter J). From there on, any incremental increase in speed (movement to the right along the horizontal or speed axis) results in lesser total drag. Makes sense so far?

This happens up to the point where speed intersects the bottom portion of the letter J, where it then begins going up again.

You familiar with Improved Climb computations? Where field length is not limiting, the take-off chart tries to give you a higher speed than usual to get you to that low point in the drag curve.

My 2 cents' worth. Cheers :)

3 Point
19th Feb 2010, 18:08
Excellent description of a graph without using a graph!!

QJB
20th Feb 2010, 01:03
hey blah blah blah,

The drag curve indicates how much thrust the engine has to produce to maintain level flight at various airspeeds. Because a constant amount of lift is required to overcome weight for level flight, at slower speeds the wing must be kept at a higher angle of attack. With this increase in angle of attack comes an increase in drag, this is induced drag, responsible for the back side of the curve. One might expect then that as the aircraft flies faster (with an ever decreasing angle of attack) that drag would reduce. This is true of induced drag, but air resistance increases with speed and this again increases drag. Termed parasite drag. If you plot the two curves together you can see the total drag curve for the aircraft, and thus the thrust required curve. The various speeds are as follows:

minimum drag speed: the lowest point on the total drag graph. Also corresponds to the point where induced drag and parasite drag are equal. Anything that requires an increase in angle of attack for a given speed (like an increase in weight) will create an increased amount of induced drag for a given airspeed and thus the airspeed for minimum drag will increase. On the other hand, anything that creates more parasite drag, such as an increase in the parasite area of the aircraft will move the speed lower. Minimum drag speed corresponds to the best glide speed, although for many aircraft this is a lower figure due to the increased parasite drag from a windmilling prop.

Vs: Theoretically the minimum speed for level flight would occur on the low end of the curve where thrust available meets thrust required, but in reality the angle of attack required at this speed is too great and the wing stalls first.

Check out the power required curve which is just the drag required x velocity. Minimum power required corresponds to best endurance, maximum level flight speed is where the power available meets power required at the top end of the speed range. Best rate of climb occurs at max excess power and best angle at max excess thrust.

Sorry for rambling, the best way to find speeds is to look in the flight manual for your aircraft. I'm sure they will be there. If you want to be really precise they are normally given for MTOW and could be reduced in proportion with the square root of weight decrease, but this is really unnecessary for light aircraft.

J

plugster
20th Feb 2010, 09:54
Min FF on a jet does not occur at V min power, as on a jet FF changes with thrust instead of power like on a prop aircraft.
So min drag equals min thrust on any aircraft and min FF thus highest endurance on a jet.
Min FF on a prop equals min power required and does not coincide with Vmin drag/min thrust. Remember: Power required= drag * speed hence if your drag curve is "flat" chances are you will find yourself on the unstable side of the drag curve for min power i.e. increasing power on that side of the power curve doesn't necessecarily mean an increase in airspeed opposite to the other side of the drag/power curve where an increase in thrust/ will always lead to increasing airspeed.

Hope this helped

blah blah blah
24th Feb 2010, 03:23
Hi guys,

Thanks for the responses.

Capt Solipsist - great explanation of the drag curve. Thanks for pointing out how Vs fits on to it.

What Im still trying to find though is the practical use of the drag curve. For example I have a double engine failure. I have indication on the EADI of fast/slow. What do I fly? I want to be flying at min drag for best glide performance, but how do I know what this is? With an aircraft mass that various by many tonnes it isnt practical to know a speed for each case. I am hoping someone can say that a good ballpark is to fly 'x' on the fast/slow indicator.

Any help?

Blah

Microburst2002
24th Feb 2010, 08:05
The fast-slow indicator information comes from AoA directly. No matter what the weight is. But I don't know if it varies with configuration or it is for landing configuration only. I the latter case, it is not valid for an all eng out glide from cruising level.

Thanks EGh

I used to call that the reverse command region (Naval Aviator, I guess)
It is funny how, if you add just al little thrust when speed decays a few knots below target VAPP it won't suffice. It keeps slowing down, so you add a little more thurst. And you can be doing that until you have max climb! That is the back side of the curve. And sometimes actual GW is heavier than the one calculated by the loadsheet, and then you are deeper in the backside. However, usually acutal GW is, more often than not, lighter. The Fast-Slow indicator is a hint, in that case.
Another factor to take into account is the VREF additives for wind component, so you will usually fly somewhat faster than the neutral point iln the fast-slow scale.

blah blah blah
24th Feb 2010, 08:24
Fast/Slow indicator gives indications relative to 1.3Vs and is applicable at all times during configuration changes. ie. if flying with the indicator in the middle (1.3Vs) you have the same margin over the stall whether you are clean or have full flap.

Putting all that aside in case I am just going up the wrong path, what profile would people fly following failure of all engines? Talking large aircraft here, not a little piston that probably just has a standard glide speed.

Anyone?

bookworm
24th Feb 2010, 09:24
Some V-speeds related to performance are at speeds determined by the shape of the drag curve. Vs is not one of them -- it's related only to lift, not drag -- thus it doesn't appear at a particular point on the drag curve. It's convenient that your EADI gives you Vref (or Vs) for any configuration and weight, but unfortunately you can't use that to get a minimum drag speed that is similarly corrected, because Vs and minimum drag speed are not related.

If you know the minimum drag speed for a particular configuration and weight, that speed will vary with the square root of the weight. So a 2% change in weight causes a 1% change in the minimum drag speed. There's no easy way of assessing how minimum drag speed varies with configuration.

Microburst2002
25th Feb 2010, 08:21
It is a err, indeed

Max L/D and min Drag are the same, for a given weight.
Green dot, in airbus, is very nice. However you donīt have min drag speed with flaps.
By the way, Why Airbus don't have fast-slow? Is it because it has VLS?