Am I over-complicating this?

flyTotheSky

flyTotheSky

Well-Known Member
If we have an increased load factor (let's say we have a forward CG and more tail-down force is required to compensate) the airplane's stall speed increases because of the increased wing loading...

I've also heard that the stall speed increases because we are flying at a higher angle of attack (close to critical AOA) and will reach it sooner.

Am I mixing these two concepts up here or are they related? I've always thought they were related to each other.

*Oops..typo...meant to say "complicating"

<fixed it. SteveC>
 
flyTotheSky said:
If we have an increased load factor (let's say we have a forward CG and more tail-down force is required to compensate) the airplane's stall speed increases because of the increased wing loading...
Click to expand...

No load factor increase here. Yeah, I know that load factor is L/W and you have more L because of the tail down force, but you have no vertical acceleration so up forces = down forces. Let's just throw the tail down force into the weight. :)

<<I've also heard that the stall speed increases because we are flying at a higher angle of attack (close to critical AOA) and will reach it sooner.>>

That's true. Pretty much the only reason that stall speed increases is when, for some reason, you have a higher AOA for a given airspeed. Load factor and CG position are the only ones that come to mind.
 
tgrayson said:
No load factor increase here. Yeah, I know that load factor is L/W and you have more L because of the tail down force, but you have no vertical acceleration so up forces = down forces. Let's just throw the tail down force into the weight. :)

<<I've also heard that the stall speed increases because we are flying at a higher angle of attack (close to critical AOA) and will reach it sooner.>>

That's true. Pretty much the only reason that stall speed increases is when, for some reason, you have a higher AOA for a given airspeed. Load factor and CG position are the only ones that come to mind.
Click to expand...

Gross weight is another. Might not be as much of a factor in small airplanes, because you don't change gross weight very much as you burn fuel, but in big planes the difference from the start of the flight to the end can be VERY significant, just due to how much fuel you've burned. A 747 or DC-10 after a 8 hour flight may have burned more than 100,000 lbs.
 
tgrayson said:
No load factor increase here. Yeah, I know that load factor is L/W and you have more L because of the tail down force, but you have no vertical acceleration so up forces = down forces. Let's just throw the tail down force into the weight. :)

<<I've also heard that the stall speed increases because we are flying at a higher angle of attack (close to critical AOA) and will reach it sooner.>>

That's true. Pretty much the only reason that stall speed increases is when, for some reason, you have a higher AOA for a given airspeed. Load factor and CG position are the only ones that come to mind.
Click to expand...


Right, I've got my words mixed up here. No load factor, but an overall increased load on the wings or no, because of no vertical acceleration? I guess my next question is then, what is the term "wing loading" referenced from.
 
flyTotheSky said:
an overall increased load on the wings or no, because of no vertical acceleration?
Click to expand...

I don't quite understand that sentence.

<<I guess my next question is then, what is the term "wing loading" referenced from.>>

Wing loading, in its purest sense, is simply W/S, where W is the weight of the aircraft and S is the square footage of the wing. As a practical matter, you have to multiply it by the load factor to figure out how much force each square foot of wing is providing. Although this latter figure isn't technically "wing loading," it's often what pilots are talking about when they use the word.
 
Wing loading is talked about an awful lot in the field of aircraft design. It is one of the 2 biggest general measures of performance (the other being thrust to weight ratio). It is the ratio of the weight to the surface area of the wing, so the units are always force per unit area (lbs/sq ft for example). Quite a few performance characteristics are based on wing loading, like maneuverability, stability in turbulence, stall speed, etc.

Some typical numbers are:
Sailplanes 6 lbs/sq. ft.
Homebuilts 11
G.A. single eng. 17
G.A. Twin eng. 26
Twin tuboprop 40
Fighter 70
This data from Daniel P. Raymer's Aircraft Design: A conceptual approach.

The pilot speak definition would be, that it's a general term related to the size of the airplane compared to the size of the wing, and it affects performance when you are comparing one airplane to another. Kind of like the thrust to weight ratio can be a performance comparison between airplanes.
 
tgrayson said:
No load factor increase here. Yeah, I know that load factor is L/W and you have more L because of the tail down force, but you have no vertical acceleration so up forces = down forces. Let's just throw the tail down force into the weight. :)

<<I've also heard that the stall speed increases because we are flying at a higher angle of attack (close to critical AOA) and will reach it sooner.>>

That's true. Pretty much the only reason that stall speed increases is when, for some reasonyou have a higher AOA for a given airspeed. Load factor and CG position are the only ones that come to mind.
Click to expand...

So by having a heavier aircraft, the wing will have to fly at a higher AoA to produce sufficient lift (increasing CL) and as this is done, the wing flies at a closer to CLmax. Tail down force is nothing more than added weight.

Tail down force doesnt equate to an increased load factor because lift will equal weight during level flight. But the flip side being if the aircraft is heavier (as a result of the tail down force), it must fly at a higher AoA to produce the necessary lift to overcome weight putting the plane closer to the critical AoA.
 
meritflyer said:
So by having a heavier aircraft, the wing will have to fly at a higher AoA to produce sufficient lift (increasing CL) and as this is done, the wing flies at a closer to CLmax. Tail down force is nothing more than added weight.

Tail down force doesnt equate to an increased load factor because lift will equal weight during level flight. But the flip side being if the aircraft is heavier (as a result of the tail down force), it must fly at a higher AoA to produce the necessary lift to overcome weight putting the plane closer to the critical AoA.
Click to expand...

Yes. Caution, though. Even though we can think of the tail down force as added weight, it isn't really. We often use the weight as having a correspondence to mass, but the taildown force doesn't add any mass to the airplane.
 
meritflyer said:
Right. Wouldnt a high amount of tail down force really be an increase in drag as opposed to "weight"?
Click to expand...

It will certainly increases drag, but it does so via the mechanism of 1) generating its own lift, and 2) requiring an increase in lift on the main wing.
 
tgrayson said:
It will certainly increases drag, but it does so via the mechanism of 1) generating its own lift, and 2) requiring an increase in lift on the main wing.
Click to expand...

I take it that you'll experience an increase in profile drag as a result of your points listed above.

PM sent.
 
You must log in or register to reply here.
Back
Top