induced drag

[ciurpita]

my understanding of induced drag is that the lift vector is tilted aft at low-speed/high-lift and hence generates a slight rearward force, drag.

my question -- is the drag vector also tilted and generating a slight downward force perpendicular to the direction of travel?

[Brandano]

This is a bit of a can of worms. Both the lif vector and the drag vector are what you make them. You could well say: "in my system lift is perpendicular to the path of flight, and drag is parallel and opposed to it". It would require the rework of a few formulas and still work. This incidentally is the system that I tend to use instinctively, and that's why I don't really like your specific definition of induced drag, but it's a question of taste.

[vintage1]

lift and drag ar what you define them to be: I the end all vectors aum to one vector.

Its a bit like saying 'what is the weight of an airship?" do you count the weight of the gas inside it? or not? or is the answer what you get when you put it on the scales?

[Gannet AME]

my understanding of induced drag is that the lift vector is tilted aft at low-speed/high-lift and hence generates a slight rearward force, drag.

my question -- is the drag vector also tilted and generating a slight downward force perpendicular to the direction of travel?

I suggest you get a copy of Martin Simons' Model Aircraft Aerodynamics Chapters 5 & 6 which in my opinion provides an excellent explanation of Induced Drag with a lot of illustrations and not a lot of math.

And if you are a math freak like myself, then you may also want to investigate Prandtl's lifting line theory, which enables accurate calculations of induced drag and its effect on lift.

[MarkusN]

my question -- is the drag vector also tilted and generating a slight downward force perpendicular to the direction of travel?
As the whole "tilting" thing is a result of the airflow being redirected by downwash, yes, that would be the case. (Basically the vector of the force on the wing is rotated, so that would have an influence on drag also.)

[ciurpita]

i see equations for the total drag based on the aspect ratio. would anyone know how to calculate the induced drag over a subsection of the wing?

for example, while circling the lift isn't symetrical. how would you calculate the induced drag at the outer wing tip which has a greater airspeed and produces more lift, than the inner tip, so that you can determine the yaw moment due to the drag.

[Tom Harper]

Check out Alex Strojnik's web site and his books. He has more detail than most.

The usual equations for Induced Drag are general forms that demonstrate the effect. Actual calculations have a number of added terms. Calculating the instantaneous ID for a portion of a model wing during a turn could constitute a new career.

[MarkusN]

for example, while circling the lift isn't symetrical. how would you calculate the induced drag at the outer wing tip which has a greater airspeed and produces more lift.
If the outer tip would indeed produce more lift, a rolling movement would be the result. Which means that in a steady curve the speed effect is compensated either by corresponding aileron deflection or a slight sideslip.

There are programs available that can calculate local induced drag (I only know the one by Frank Ranis, which is in German.)
Any method calculating induced drag of a non-ideal wing must do this by integrating the local components. So that calculation would yield the desired results as well.

[biber]

Mark Drelas AVL does such calculations and i think XFLR5 also does them.
Just goggle the names, it's out there.

biber

[Tom Harper]

This does pose an interesting issue. For a constant rate of turn there cannot be a lift differential on the two wings. A lift differential would result in a roll rate rather than a roll angle.

[ciurpita]

This does pose an interesting issue. For a constant rate of turn there cannot be a lift differential on the two wings. A lift differential would result in a roll rate rather than a roll angle.

this is why reverse aileron is required to hold a turn, or outside yaw is required in a polyhedral to increase the Cl of the inner wing. my interest is if the greater drag, both induced and parasitic, can achieve a balance and maintain some stable bank angle.

[BMatthews]

this is why reverse aileron is required to hold a turn, or outside yaw is required in a polyhedral to increase the Cl of the inner wing. my interest is if the greater drag, both induced and parasitic, can achieve a balance and maintain some stable bank angle.

Then your experience is counter to my own for the most part. Only one poly model I've ever flown had a very slight tendency to require outside rudder in only steeper turns. And even that went away when I moved the CG forward very slightly to counter an elevator response that was a little too hairy for flying effectively at longer distances. By far the vast majority of properly trimmed poly models need a hair of pro-turn rudder to stay in a turn. If I neutralize the rudder the models slowly straighten up and fly away.

I've also flown a few flat winged models with ailerons that needed very little or no reverse aileron. But they all had a couple of degrees of dihedral or used a multi panel wing with small dihedral angles.

The inference being that despite our efforts there is still some side slip towards the low wing in the turns and by using some amount of dihedral we can reduce or avoid the amount of "top stick" needed to avoid tightening the turn.

Real gliders often used to have (and likely still do) a small tuft of wool yarn somewhere in the pilot's view where they could judge the side slip during a turn and coordinate the rudder, ailerons and elevator all at the same time to shift the side slip action towards zero. The times I've been in gliders I noticed that it was a rather delicate balance of the controls needed to achieve that. As ground based pilots we don't have that option so need to try to use other things to induce some coordination in the turns. In my experience a little dihedral helps a lot with reducing the pilot's workload when in a thermal turn.

[MarkusN]

Bruce, your statement and ciurpita's are not necessarily contradictory. He didn't state how that outside yaw comes into play. May very well be the general layout of the plane causing this, without any control input.

And, yup, the string is still there, and a pesky thing ist is for a beginner like me.

[BMatthews]

And, yup, the string is still there, and a pesky thing ist is for a beginner like me.

Sorta like patting your head and rubbing your belly while singing "We all live in a yellow submarine, a yellow submarine, a yellow submarine. We all live in a........." :D