Va changes with Weight
- Thread starter LS1 Life
- Start date
This is exactly why; what happens is there and why it is happening is in the same definition.
Thank you for going into detail in showing the why for weight for Va is because of the AoA gap not because stall speed goes down.
My point earlier with this:
is that Va goes down because of this gap and not for the same reason stall speed goes down. Stall speed goes down because of what I discussed in my original post. This definition while theoretically true and will give you your Va gives no practical understanding of what is happening and if you say this to your FAA guy you had best be ready to back it up with the AoA gap explaination.
Nice explaination Midlife.
Edited: Removed Italic from last paragraph
Maurus
The Great Gazoo
I wasn't aiming for a practical understanding in my post as it was already given earlier in this thread by another poster.
splash
your social justice comic center
A boat in a turn going 25mph loaded down with weights along the outside of the turn will create more force against the outside wall... on the same boat going 25mph with much less weight will create much less force against the outside wall and the structure (boat) will not break up on the outside of the turn. I believe centrifugal force makes this problem or concept easy to understand.
tgrayson
New Member
His explanation is identical to that of the_dmn8tr and AirmetTango.
The dependence of Va on the stall speed is real, direct, and mathematical. To say that it is wrong, is, well, wrong. Va is the stall speed at the load factor limit.
To that that this is wrong, simply because you prefer to focus on the "gap" is a bit silly, IMO. They're both important concepts.
jake.durham
Well-Known Member
Heavier the plane the more airspeed required and a greater angle of attack is needed to generate the same lift. Abrupt control inputs will cause the heavier plane to stall quicker due to it being flown closer to the critical angle of attack. The lighter the plane the greater the lower the angle of attack increasing the possibility if exceeding "Gs" before the wing stalls (greater range of motion).
Hope this helps some. Its what I used on my CFI and was good enough for the examiner. You can also use a Vg diagram as you describe this.
Hope this helps some. Its what I used on my CFI and was good enough for the examiner. You can also use a Vg diagram as you describe this.
My point since apparently I didn't make it clear, is not that these definitions in themselves are wrong. The point is that certain wording in these definitions will likely not be taken properly by your student unless of course you send all your students through flight dynamics ground school. These are the 2 specific phrases that I find discomforting and I can easily see a student rattling off this answer then digging themselves into a big hole when asked to explain it:
These phrases taken literally can send a student down the wrong thought process leaving them with poor conclusions. Example: Va and stall speed change with weight for the same reason, taken literally means exactly what it says they change for the same reason. This is wrong, they vary directly with one another mathematically, but the reason for them changing is not the same.
To see where this can lead to digging a hole, take the student that forgets the reason stall changes with weight but remembers this definition as well as its relation to AOA gap. Trying to link this AOA gap to the reason stall speed goes down because their CFI said "...changes for the same reason," will cause them all kinds of problems in their attempt to explain the change in stall speed with weight.
The gap definition should be used not because it is any more right or wrong then the other definitions but because it isn't subject to any faulty thinking even when taken word for word literally.
tgrayson
New Member
So you keep saying, without any sort of coherent argument.
Doesn't matter. The equation I provided shows you're wrong; the mathematics works because it describes reality. If I were you, I wouldn't fall on my sword on this one.
The reason stall speed decreases with weight is the exact same reason as why Va decreases with weight, as has previously been described. Va = Vs at your limit load factor. Whether you choose to describe it mathematically as opposed to in terms of AoA gap completely depends on your audience. If I were describing it to a Math/Physics/Engineering major I'd do so in mathematical terms, if I was explaining it to the average student (who doesn't appreciate mathematics) I'd be talking about AoA gaps.
You can very easily (and correctly) describe the variation of Vs in terms of AoA gaps. Consider two identical planes maintaining slow flight, one heavier than the other, the AoA of the heavier will be closer to the critical AoA and thus have the smaller AoA gap and less 'room to play with' before the stall.
The reason is the same, but personallyI prefer the mathematical terms, but then I have an Engineering degree. As an instructor, it is all about adjusting our style/technique to suit the student, and not the other way round.
You can very easily (and correctly) describe the variation of Vs in terms of AoA gaps. Consider two identical planes maintaining slow flight, one heavier than the other, the AoA of the heavier will be closer to the critical AoA and thus have the smaller AoA gap and less 'room to play with' before the stall.
The reason is the same, but personallyI prefer the mathematical terms, but then I have an Engineering degree. As an instructor, it is all about adjusting our style/technique to suit the student, and not the other way round.
splash
your social justice comic center
lets keep it simple as this folks...
http://www.youtube.com/watch?v=Vp7P...imoffkeyboardcat.com/&feature=player_embedded
http://www.youtube.com/watch?v=Vp7P...imoffkeyboardcat.com/&feature=player_embedded
This is exactly my point and is what I have claimed throughout this post. The majority of people we are teaching to do not have degrees in engineering and in my experience most of them also cannot stand math. To make matters worse are the erroneous thought processes that could develop from the mathematic description by someone who doesn't appreciate or think in that manner, takes it literally and then you have a problem.
tgrayson said:The equation I provided shows you're wrong; the mathematics works because it describes reality. If I were you, I wouldn't fall on my sword on this one.
There is no sword to fall on, I never argued that your math was wrong. I made an error in my original post, when I said:
"For example, the reason stall goes down is completely unrelated to the reasoning for why Va goes down with weight"
Which you corrected and I agreed with and let it be known that I wasn't referring to the mathematical reason. I was simply looking at what the words in each definition, ignoring the math (like most students will do), and pointed out that the mathematical definition is the wrong way to answer this question, due to the erroneous thought processes that will likely form, for the given scenario. Edit: added bold and changed some spacing
We are here to teach pilots, not engineers, therefore, you give them practical real world definitions and save your equations for the 10 percent of students that actually are interested in them because the other 90 percent will just end up confused later on with the engineering definition. Not to mention if he gave this definition to the FAA guy they might not even know it and what they think is right or wrong is what matters in the long run as anyone knows that has tried to talk to the FAA about aerodynamic force surely knows.
High_Alpha
Well-Known Member
Not to beat what may be a dead horse and I know many qualified people have explained, but here's a qualitative explanation that helped me to understand...
Remember, the wing always stalls at the same angle of attack, blah, blah. The amount of lift being produced when the wing stalls depends on the airspeed. At high airspeeds, the lift produced is probably at the design limit of the wing--say, for example, 8000lb. (Using a hypothetical 2000lb gross wt airplane designed to 4gs).
Now say the same airplane is flying at gross weight Va and hits the hypothetical gust, but the actual aircraft weight, instead of being 2000lb, is now 1400lb. The wing still makes 8000lb of lift when it stalls, but now the airplane, instead of being subjected to 4gs, is now subjected to 5.7gs. The wing is still fine, but what about the engine mounts, seats, instrument mounts, cargo restraints, floors, etc...They've been subjected to loading beyond their design limits. Hence the varying Va.
Remember, the wing always stalls at the same angle of attack, blah, blah. The amount of lift being produced when the wing stalls depends on the airspeed. At high airspeeds, the lift produced is probably at the design limit of the wing--say, for example, 8000lb. (Using a hypothetical 2000lb gross wt airplane designed to 4gs).
Now say the same airplane is flying at gross weight Va and hits the hypothetical gust, but the actual aircraft weight, instead of being 2000lb, is now 1400lb. The wing still makes 8000lb of lift when it stalls, but now the airplane, instead of being subjected to 4gs, is now subjected to 5.7gs. The wing is still fine, but what about the engine mounts, seats, instrument mounts, cargo restraints, floors, etc...They've been subjected to loading beyond their design limits. Hence the varying Va.
OhioStatePilot
Well-Known Member
Sorry to say, but that doesn't make much sense to me. You say at high airspeeds, the lift produced is at the design limit of the wing -- 8000 lbs for a 2000 lb airplane? That's only the case in a 4g load condition, not straight and level flight.
How does the aircraft's actual weight change from 2000 lbs to 1400 lbs?
Maybe I'm just reading this completely wrong.
fish314
Well-Known Member
Yes, he was talking about the lift produced AT STALL. You are absolutely correct that he's not talking about straight and level flight. He's talking about a stall at max-g... an "accelerated" stall.
So for his hypothetical airplane, which was designed with Va based on 2000lbs. Gross Weight and a 4g load limit, when you pull "g" on the airplane you get to 4g (which is 8000lbs of lift) and then it stalls, because that was how he designed the hypothetical airplane. It's got a 4 g load limit, and at Max gross of 2000lbs, so at Va, it stalls at 4g. For sake of argument, let's give it a hypothetical Va also... say 200 KTAS.
Now imagine the same airplane, but it's lighter... 1400lbs. Maybe you've been flying around for a couple of hours and burned off fuel or whatever. So at that same speed, 200 KTAS, you pull back on the stick and you still get 8000lbs of lift before the stall. But since the airplane is lighter, 1400lbs vs. 2000lbs, the g has gone up. 8000lbs./1400lbs.=5.714 G instead of 4 g. So basically you are capable of producing more "g" than the load limit of the airplane at 200 KTAS and at this gross weight, than you were capable of at 2000lbs. If you slowed down, there would be less g available. At some slower speed, (167 KTAS actually) the airplane would stall at 4 g, the limit load factor, and the lift produced would be 5600lbs. This new airspeed (167 KTAS) is the maneuvering airspeed at this gross weight.
Maneuvering airspeed (the speed at which the airplane stalls at the limit load factor) has decreased, because the weight has decreased.
OhioStatePilot
Well-Known Member
Oh man, that makes a lot more sense. I wasn't even thinking accelerated stall but it's clear now! I'm not sure why I never drew the obvious connection...
High_Alpha
Well-Known Member
Thanks, Fish!
This is exactly how i've been understanding it. (lift/weight= Load factor)
i understand that a lighter aircraft is further away from the critical AOA vs the heavier aircraft. But from my point of view that doesnt describe why we can fly faster at a heavier weight.
Great posts guys im loving every bit of this! thanks a ton! :nana2:
The aircraft is not flying faster because of the heavier weight. If you look at almost any aircraft as you increase the weight you bring the CG further aft. This aft CG is the reason for us flying at a faster airspeed because as we know aft CG equals cruising at a lower angle of attack
Now for what you probably didn't know, here is the induced drag formula:
Di = (k + CL2 / A) * Q * S
Where: CL2 = 2 pi * AOA
In this formula you can ignore everything except CL2. In the induced drag formula you can see that our induced drag isn't going up because of lift, if it going up because of AOA. Increase the AOA increases the coefficient of lift which increases induced drag.
You can take a closer look at this when looking at any total drag but make a slight change to it. Here is a link to the actual: http://upload.wikimedia.org/wikipedia/commons/0/04/Drag_Curve_2.jpg
Now along the bottom, for parasite drag leave speed, but for induced drag you can relate it to AOA on the bottom. Lift always has to equal weight right? So at a slower speed we will raise our AOA since our speed has gone down, this is why we have more drag at lower airspeeds.
A higher AOA acts exactly like full wing flaps. Picture a water skier letting go of the rope, they slow only a little at first, but as soon as they have to raise their AOA of their skis they almost immediately stop and sink. Obviously air is less viscous so the effects are weaker but the same concepts still apply.
Lets take a quick look at a practical lift equation now "L = speed + AOA." Obviously this equation will not work, but for all practical purposes it works here. This is why the induced drag equals drag from lift confuses pilots in my opinion. Pilots have two ways they can control lift, with their speed and with the AOA. Yes induced drag is only caused by the AOA and has nothing to do with the speed side of the lift equation. (Note: on the actual lift equation there is also S for density and A for wing area, but there is V which isn't in the induced drag formula.)
I won't take the time to do all the math, but now that you see induced drag is related to AOA and heavier aircraft tend to have rear loading which brings the CG further aft lowering AOA you should see why you can fly faster.
PS If your caught up on why aft CG means lower angle of attack please say so because that is another common confusion among pilots. We think aft CG equals nose up, that is wrong.
We must have read different quotes? He said, "But from my point of view that doesn't describe why we can fly faster at a heavier weight." I bolded more of it, pointing out that we were not talking and I was not talking about Va, we were talking about cruise speed. Cruise speed varies for the reasons I stated and I really am unsure why your talking about Va?
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