Hi everyone,

Just abit confused with the answers for a couple of questions on pages 7.10 and 7.11 of Bob Taits Aerodynamics Book.

Question 1:
An aircraft of a certain weight is flown at 150kt IAS at 1000ft. If the same aircraft was flown at 150kt IAS at 20000ft, the angle of attack required to maintain level flight would be
(a) greater because of the reduced density at 20000ft
(b) less because of the reduced drag in the thinner air
(c) greater because the TAS would have to be higher
(d) the same

the answer is (d) but i don't really see why (a) isn't the answer. Due to the reduced density to maintain the same altitude and airspeed wouldnt a larger AoA be needed?

Question 10
For an aircraft at a particular gross weight to maintain level flight at a particular height and IAS, a particular power setting is required. If the gross weight increased
(a) a larger AoA and more power would be required
(b) a larger AoA would be required but no change in power
(c) the same AoA could be used along with increased power
(d) a smaller AoA and more power is required

the answer is (a) but I seem to think (c) suits the answer more. If gross weight was increased in level flight, to maintain it wouldnt it be possible that an increase in power will be enough along with the same AoA to maintain straight and level? This question has answers where more than one seems right to me which is why I seem to be struggling with it.

Question 13:
Consider an aircraft maintaining straight and level flight at the speed which produces maximum endurance. If level flight is to be maintained,
(a) more power will be required if speed is reduced
(b) less power will be required if speed is reduced
(c) less power wll be required if speed is increased
(d) less power will be required if speed is changed to the speed of minimum drag

The answer is (a). Honestly i wouldnt really know what answer to pick, I was leaning towards (c). I tried reading the explanation which says "Since the maximum endurance occurs at the spped which requireds minimum power, any other speed, higher or lower, will require more power", but dont really understand how any other speed, higher or lower, will require more power.



I know it's abit to ask buy any help would be appreciated

1) The key to remember is IAS is dynamic pressure, which, when you relate it to the lift equation is 1/2pV2. The aircraft is sensitive to the dynamic pressure being exerted over the surfaces. Whilst you would be flying at a higher true airspeed at 20000ft, there is still the same dynamic pressure being exerted on the surface i.e. 150kt IAS, hence the angle of attack will remain the same. If you increase the angle of the attack at the same IAS, you would get an increase in the CL, and this would mean the Lift equation would not be in balance anymore and the plane will climb. Remember, for SL flight, we are trying to maintain the same Lift value by varying the factors that make up Lift i.e. L=CLx1/2pV2xS. If you think in terms of the lift equation for straight and level flight, it will be easier to work out what is happening.

10) Again, relate this back to the lift equation and you should be able to work out the answer. The key here is that the IAS and height remain constant. Also, for the heavier weight, the value of Lift must be increased to equal the higher weight so Straight and Level can be maintained. Since 1/2pV2 and S of the lift equation remain constant, since airspeed is said to be constant and the wing area is assumed to remain constant, then the only value we can alter to increase the Lift equation is the CL. Remember from your previous thread, that the CL is dependent on the AOA in the simplest sense, so hence we must increase AOA and hence CL for the Lift equation to increase and balance the new higher weight. By increasing AOA, there will be more induced drag produced, and to overcome this induced drag so the airspeed does not drop, more power is required. Hence the answer is A.

13)If you remember that max endurance relates to the min point on the power required curve, then the answer is fairly straight forward. If max endurance is the min point on the power required curve, then this must mean that any speed either side of this min point will require more power. Just look at the graph and this point is obvious. We need more power to fly faster, and need more power when we fly slower due to the increased induced drag, and to prevent the airspeed from dropping further.

Hope thats helps.

Hi Guys,

I got tripped up by question 10 too. I don't think it is a particularly well-worded question. The requirement to maintain the same IAS after the gross weight increase is implied but not actually stated. I originally picked answer c) as well, deciding that it was the most correct answer because it used the word could instead of would - perfectly valid if the requirement to maintain the same IAS is not a factor. But of course as that is the implied requirement, the only correct answer is a).

I sat my Aerodynamics exam a few weeks ago and was very happy with a 100% result. Thank you Bob for your text and for the practice exams within - they set you up very well for the exam.

There was one question that I spent some time pondering, and it related to the effect of increased airspeed on the longitudinal stability of the aircraft. It is not something that I had considered as a factor in stability before and after much consideration I decided that increasing the IAS would have no impact on longitudinal stability. Just thought others might like to consider this one too.

Ray

That is an absolutely spectular result! As far as I am aware, you are the first student I know of who has got the magic 100% for Aerodynamics. If anyone else out there has, please let me know. Congratulations mate - you're a legend..

Bob

Wow! Thanks for that Bob. Naturally I was chuffed with a 100%, but to hear that makes me thrilled that I managed it.

I agree Ray. Simply outstanding performance mate. You sure deserve a pat on the back mate for that one. Well done

Cheers,

Rich

Hi everyone

can anybody help me with this question? any help will be much appreciated

A pilot suspects that he is undershooting a forced landing in calm conditions. The best technique to employ would be -

A)maintain the speed that produces the best lift drag ratio
B)raise the nose to decrease the gliding speed
C)increase the gliding speed to allow for a longer float after round out
D)increase the flap extension

The correct answer is [a]. The best lift/drag ratio in no wind is also the best distance/height ratio. You will never glide further in no wind at any speed other than the best lift/drag ratio speed. If you're not making it to the field at the best lift/drag ratio speed, I'm afraid you're having a bad day! No other speed (and therefore nose attitude) will make any improvement. Remember also that the best lift/drag ratio will always be achieved with NO flap.