Aerodynamics

[akafrank07]

Could someone please explain to my the difference between effective weight and actual weight?
Thanks

[Genghis the Engineer]

In what context?

G

[akafrank07]

Hi
I've been reading the aerodynamics section on 'the technical pilot interview' book and it came up a few times in the answers, example;
The properties that affect an aircrafts stall speed:
Weight
a.Actual Weight
b.Load factor, g in a turn
c.Effective weight/center of gravity position
How does the stall speed vary with weight:
"The stall speed will increase with an increase in the aircraft's autual or effective weight"
Thanks

[Genghis the Engineer]

At that level I'd say two factors.

(1) How much g you're pulling. In a 2 g turn, effective weight has doubled, so stall speed goes up with square root of weight.

(2) With CG, further aft CG reduces tail downforce, which reduces lift required from the mainplane, which reduces stall speed.

G

[Fitter2]

Mass x g + tail downforce.

Tail downforce, which adds to actual weight to increase lift (and hence lift co-efficient) is affected by c of g (forward c of g requires more tail downforce).

Genghis can give the long complicated answer if required...........

(edit - I see he has, although in the original post load factor was separated out, and I think that book considers effective weight as lift required in straight and level flight).

[akafrank07]

Cheers lads them explanations have helped

[Meikleour]

akafrank07 (http://www.pprune.org/members/330390-akafrank07): Just a thought. Try reading your theory textbooks thoroughly rather than trying to bone up on typical questions. That way you will be equipped to answer ANY question! Perhaps this is too "old school" for the internet generation?!

[Lightning Mate]

so stall speed goes up with square root of weight.

Tut tut Genghis.

Surely you mean the square root of load factor.

[Genghis the Engineer]

Well if you want me to get picky, stall speed goes up with (weight x load factor / MTOW)^aeroelastic coefficient.

For a rigid wing, aeroelastic coefficient is 0.5. For a non-rigid wing, the coefficient varies according to deformation with loading - a hang-glider can well be in the range 0.6-0.8.

That form of the equation is the Venton-Walters equation.

And you're unlikely to find that in many groundschool notes.

G

[Genghis the Engineer]

akafrank07 (http://www.pprune.org/members/330390-akafrank07): Just a thought. Try reading your theory textbooks thoroughly rather than trying to bone up on typical questions. That way you will be equipped to answer ANY question! Perhaps this is too "old school" for the internet generation?!

Also four different one-line question threads the same week, each with the title "aerodynamics" is both clumsy and lazy.

G

[Lightning Mate]

That form of the equation is the Venton-Walters equation.
And you're unlikely
to find that in many groundschool notes.

It's in my own CFS notes though. ;)

[Genghis the Engineer]

It's in my own CFS notes though. ;)

Very commendable.

This reference (http://bura.brunel.ac.uk/bitstream/2438/2139/5/Stalling%20speeds%20and%20determination%20of%20manoeuver%20s peed%20for%20Rogallo-winged%20microlight%20airplanes.pdf) covers it in a bit more detail if you're interested.

G

[akafrank07]

Ok i'll keep that in mind to elborate more on the heading, thanks for your help guys