Hi Guys,

Has anyone seen this question? It is found in the BT cyber exam and has thrown my thinking (or lack of) Does anyone have any thoughts on this?
7
Marks: 1
If indicated air speed is kept constant and angle of attack is increased
Choose one answer.
induced drag will increase but parasite drag will remain the same
both induced drag and parasite drag will increase
induced drag will increase and parasite drag will decrease
induced drag will remain the same but parasite drag will increase
An increase airspeed at any particular angle of attack will result in an increase in lift. If lift increases, induced drag must also increase. Also, since the aircraft must force its way through the faster airstream, parasite drag will also increase.
Incorrect
Marks for this submission: 0/1.

Also while I am on this. The following question is in relation to a bird strike and broken windscreen / parasitic drag.

My question - Is there a rule of thumb for this or similar situations? To guess a speed slower than that of the best Lift/Drag in order to reduce the new parasitic drag would be difficult. Lets say the bird strike was to the leading edge of the wing. Do you slow down? Test your new stalling speed? Pray for no structural damage? Or just land ASAP? - Confused.....

An aircraft suffers an in-flight bird strike resulting in a broken windscreen. The pilot now wishes to continue the flight while flying for maximum range. If the published maximum range speed is 80 knots, to achieve maximum range in the damaged condition the pilot should -
Choose one answer.
still fly at the published speed for best lift/drag ratio
fly a little slower than the published speed
fly at the speed which requires minimum power
fly faster than the published speed
The aircraft would now suffer an increase in parasite drag caused by the disturbed airflow passing over the broken windscreen. Parasite drag increases rapidly with extra speed. The best range speed in this condition would be achieved by flying slower than the published speed.
Incorrect
Marks for this submission: 0/1.

These are my thoughts only. I'm sure Bob or Richard, or someone more insightful than me will be along to give the correcnt answers.

Question 1.

I would have chosen - "induced drag increases, parasite drag remains the same"

I think you may have misread the question based on your explanation as the questions says the airspeed is kept constant, and angle of attack is increased, whereas you have argued based on airspeed increasing and angle of attack remaining constant.


For the second question, I would have chosen - "keep flying at the published speed for best L/D ratio".

Just remember that even though there is damage suffered and parasite drag may well have increased, this would apply over the entire speed range, so whatever speed you fly at, overall drag at that speed would have increased i.e. the parasite drag curve, and consequently, the total drag curve, would move up. Also it would be too difficult to work out what speed to fly to attain max range if that did happen in real life. More over, flying slower and you can risk control insufficiency and be more proned to unintentionally stalling the aircraft.


Anyways, thats my 2 cents.

I think 'his' argument is actually the worked answer supplied by the cyber exam itself, in saying that, it doesn't sound quite right based on what the question was asking. The answer I would've picked would be "induced drag increases, parasite drag remains the same".

Increasing angle of attack -> increases spanwise flow -> increases wingtip vortices -> increases induced drag.

The explanation does make me ponder something. If you increase airspeed, lift increases, which according to the explanation, induced drag increases. That makes complete sense, but I always thought increasing airspeed actually reduces induced drag thanks to the downwash acting further aft, displacing the molecules less -> less rearward lift component?

I think it relates to the fact that when we normally increase airspeed, we would reduce the angle of attack to maintain S&L flight, hence induced drag reduces in the practical sense.

However, if we increase airspeed with angle of attack kept constant, we would be creating greater lift than required. More air is brought to flow over the wing at the same constant angle of attack due to the increased airspeed, which means more air is available to flow around the tips.

That's my thought, but I also ponder on the fact that with my explanation there is a flaw, it sort of contradicts what Bob has said in his book about the amount of induced drag depends on 2 factors: the pressure differential and the amount of time available for the air to migrate around the tips. In my instance the increased airspeed should decrease the amount of time available for the air to flow around the tips and not what I have explained?

Can get quite niggly and confusing aero! But love it!

Thanks Guys,
I might give Bob a call on this one tomorrow. Just to be sure.
My main problem is that the question clearly states "constant airspeed"

Enough aero's for tonight my brain is getting motion sickness.

James

Good point basketball, hadn't considered that.

Anyway James which answer did you pick?

Hi Guys,

I picked the answer - induced drag will increase but parasite drag will remain the same.

The only reason I picked this answer is the question is very clear in stating constant speed.

So I now have a new question as I can not find this in the texts. If you have a constant speed (I think this would be hard in real life) and you increase AoA then lift will increase. As lift increases the by product has to be induced drag; does parasitic drag also increase?

I had this exact same thought when I looked at your original post!

My initial thought would have been that parasite drag would have increased - as the airfoil (with higher aoa) now is produced to the airflow in a way that makes the airframe have to work harder to move through the air. But then I referred to the very basic principle that parasite drag increases when airspeed increases (based on the parasite drag curve), so if airspeed is constant, then parasite drag must remain constant. Hence my answer.

However I think there is more to it then that, and the real story more complicated.

Have you managed to ring Bob yet?

Hi,
Yes got onto Bob. Thanks Bob.

So it seems that when faced with a question that relates to "constant" airspeed and AoA changes that we should think along the lines of a wind tunnel rather than a real aircraft. So if we had a load cell on the wing mount mount that is in a wind tunnel increasing AoA will produce more lift and the added surface area of the wing will create larger force of parasitic drag.
I am going to think of this as a hand out the window of a car.

I'm into the exam at 2:30 today so fingers crossed.

Remember that when you look at the total drag curve [the sum of induced drag and parasite drag], you must remember that it is describing how drag varies with airspeed IN LEVEL FLIGHT. Once you begin talking about changing angle of attack at a constant airspeed, you are no longer dealing with level flight because clearly, the lift will increase if angle of attack is increased at constant airspeed.

The only way you could increase the angle of attack at a constant airspeed would be in a wind tunnel. In that case, the induced drag would increase even though the airspeed is kept constant because you have an increased pressure gradient between the top and bottom surface of the wing, but no change in the time available for air to flow over the tip since the airspeed is being held constant.

A wing passing through the air at a higher angle of attack has a different effective shape as far as the air is concerned, so parasite drag will increase as well. This increase in parasite drag would occur even if we ignored the spilling of air at the wing tips. So to sum it up, an increase in angle of attack at a constant airspeed would result in an increase in both induced and parasite drag.