Increased drag with weight

[172510]

Originally Posted by bookworm

The algebra/calculus is not too hard for this. Considering only lift-dependent drag and profile drag as a function of speed v and weight W, the drag curve (in arbitrary units) looks like:

D(v, W) = W^2/v^2 + v^2

This formula is not homogeneous, so something must be missing.
Where does it come from? What assumptions are behind?

[Goldenrivett]

This formula is not homogeneous, so something must be missing.

I suspect it is simply stating Total drag is the sum of induced drag + form drag.

For induced drag see:
https://en.wikipedia.org/wiki/Lift-induced_drag
Since Lift = Weight (at 1g).
Induced drag is proportional to W^2/v^2

Form drag is proportional to v^2.

Hence Total drag is proportional to W^2/v^2 + v^2.

[Wizofoz]

One factor that often get's overlooked (though not by glider pilots!!) is that a change in AofA ALSO changes profile drag, as the airframe is presented at a different angle also.

That's why you can actually achieve a better LD ratio in some gliders with a small amount of flap deployed.

[Intruder]

For the glider example, it likely has to do with the basic design philosophy of the builder, as well as nomenclature.

If a glider is actually a high-performance sailplane, focused on competitive long-distance soaring, then the wing will be designed as much for speed as for glide ratio (L/D). A "1 degree" flap setting or similar may be incorporated to get better L/D at lower speeds for thermalling. OTOH, other sailplane mfgrs will instead incorporate a "negative flap" setting for high-speed dashes, while the normal 0 deg setting is used for thermalling.

[Wizofoz]

Originally Posted by Intruder

For the glider example, it likely has to do with the basic design philosophy of the builder, as well as nomenclature.

If a glider is actually a high-performance sailplane, focused on competitive long-distance soaring, then the wing will be designed as much for speed as for glide ratio (L/D). A "1 degree" flap setting or similar may be incorporated to get better L/D at lower speeds for thermalling. OTOH, other sailplane mfgrs will instead incorporate a "negative flap" setting for high-speed dashes, while the normal 0 deg setting is used for thermalling.

You actually go for minimum sink (Min drag) during thermalling, and thus often use a positive flap setting.

This will be at a lower speed and thus higher deck angle than best L/D (which isn't always the most efficient inter-thermal speed) in a glider with no flaps. Thus, if the gliders incidence is set such that it produces minimum profile drag at best L/D, you will present the bottom of the fuselage to the airflow, increasing profile drag.

A positive flap setting will give approximately the same wing performance, while putting the deck-angle back to optimum.

You are correct that then a reflex, negative flap setting will do the same thing at speeds greater than best L/D.

I've often thought variable incidence would be a better solution than flaps, though obviously mechanically more complex.

[BARKINGMAD]

After only 12 years of hibernation, this query raises it's ugly head on the 1st April. What planet am I on??

[Wizofoz]

Originally Posted by BARKINGMAD

After only 12 years of hibernation, this query raises it's ugly head on the 1st April. What planet am I on??

That's between you and your therapist.

[Capt Pit Bull]

Well. it's only been necro-ed the same number of times as Jesus.

[scifi]

I think a lot of the posters who are saying the Parasitic Drag... increases... have forgotten one thing... The Axis of the fuselage is not the axis of the mean chord line. Airplane designers ( model or full size.) include about 3 degrees difference in these two axis.


So it might just be that the fuselage will align more into the prevailing airstream at an increased weight, if the airplane was initially flying somewhat nose-down. There should only be one speed where the fuselage is level, at all other speeds it is either Plus or Minus.
.

[underfire]

Post #21 was 12 years in the making!

[Wizofoz]

Originally Posted by scifi

I think a lot of the posters who are saying the Parasitic Drag... increases... have forgotten one thing... The Axis of the fuselage is not the axis of the mean chord line. Airplane designers ( model or full size.) include about 3 degrees difference in these two axis.


So it might just be that the fuselage will align more into the prevailing airstream at an increased weight, if the airplane was initially flying somewhat nose-down. There should only be one speed where the fuselage is level, at all other speeds it is either Plus or Minus.
.

Yeah- this was exactly my point four posts up....

[scifi]

Hi Wizzy.. I think you mean 6 posts up, not the therapist post...


For Gliders the figures are soooo small. A typical 350kg glider with 35:1 L/D
has a drag of just 10kg... 5kg Induced drag and 5kg Parasitic. So even sticking your hand out of the DV window is likely to double your Parasitic Drag. I also think that the fuselage may not contribute much to this drag, as the tailplane is providing negative lift, so it's Induced drag gets counted towards the parasitic drag. One reason for putting water ballast in the tail.


.

[TURIN]

I never had these sort of complex problems in hang-gliders. Just stuff the bar out in thermals and up she went. Pull in for best glide inbetween. Unless you had that fancy Variable Geometry (Billow) system not much else to worry about. No tailplane either.

[Volume]

I am afraid most information you find through Google will be very simplicstic... Real life is a bit more complex, probably even too complex to discuss it in a forum with the restricted possibility to draw up some graphs.

profile drag should only increase with speed

Unfortunately if we talk transport airplane we can not ignore mach number. In typical cruise the upper wing surface is 30-50% supersonic and profile drag is strongly influenced by this. A little change in lift may change transsonic drag significantly.
If we strictly talk subsonic, turbulent and moderate Cl your simplification is correct. If we go transsonic, consider laminar airflow and go closer to Cl max it becomes quite complex.
As Cl is limited, a weight increase typically means a speed increase (starting already before you even fly with Vr calculation...), this makes the induced lift discussion a bit academic. Nobody would (only) increase Cl to compensate for heigher weight.

[FMSPEED]

Originally Posted by Smells like...

I know that LDD (induced) increases with increased weight, but profile drag should only increase with speed.
So my question is, how will my total drag graph change with increased weight?

Your total drag will increase as well because total drag=profile drag+induced drag. Therefore, even though the change in the weight will only affect your induced drag, it will ultimately affect your total drag, hope it is clear..

[scifi]

Is this a bit of an academic question anyway, there are not too many ways for aircraft to gain weight in flight, but lots of ways to loose weight... Some with disastrous consequences...

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