Aircraft stability

[chx230]

Here is the question I met:
With anhedral and high angle of sweepback, when the airspeed is low and the AOA is high, the aircraft is easy to go into
A. Dutch roll
B. Sprial
C. Spin
Why the answer is A?
The sweepback is helpful for lateral stability, meanwhile anhedral reduces it. Why is it sure that the lateral stability is more than directional stability, which will lead to dutch roll?

[WhyByFlier]

Sweep back causes Dutch roll. High sweep even more so. A large rudder helps counteract this. A large rudder causes spiral dive instability. A swept wing counteracts this.

With the answers offered in my opinion 'A' is the only answer.

[chx230]

What about the anhedral? I think it counteract lateral stability caused by swept wing.

[Oktas8]

My understanding from ATPL theory days, is that anhedral is only used when lateral stability would otherwise be excessive even with the normal dutch roll protection measures in place (multiple yaw dampers etc). So, with swept wing jets, it is generally true to say that dutch roll is a potential problem at low IAS, whether anhedral or dihedral.

This understanding leads me to answer (a), on the basis that the mention of "anhedral" is a distraction that does not change the answer.

Of course, there could be an expert out there with a different & better reasoned answer...

[WhyByFlier]

I'll start by saying that I am not any more qualified in aerodynamics than ATPLs and a bit of experience have taught me

Anhedral in general destabilises the aircraft laterally. On a low wing it would for sure increase the spiral dive tendency and reduce the Dutch roll tendency. On a high wing anhedral is needed to destabilise highly stable wings.


Swept wing: increase Dutch roll tendency
Dihedral: increase Dutch roll tendency
Big rudder: increase spiral dive tendency
Anhedral: increase spiral dive tendency

Some times (often in my experience) aviation theoretical exams such as ATPLs or annual tech exams once on the line contain 'red herrings'. You're not going to find the perfect text book answer and certainly won't in real life. You have to for the most correct answer. It's like pilot maths: 2 x 2 = about 5!