Va and weight

[MikeOH58]

2 days from the initial, and let the second guessing and forgetting begin...

Why does Va change with weight? (Higher Va higher weight/Lower Va lower weight)...

The only answer I could find is that at lower weight, the aircraft could accelerate faster in a gust or turbulance, but I find myself thinking there is more to it?

help please!


One more question.......RCO's...Specifically the RCO at Bradley. If one wanted to open a VFR flight plan out of Bradley, would you use their on field RCO to contact FSS?

arghhh...less than 48 hours away stress is awful =(

[BuickCFI]

Originally Posted by MikeOH58

2 days from the initial, and let the second guessing and forgetting begin...

Why does Va change with weight? (Higher Va higher weight/Lower Va lower weight)...

The only answer I could find is that at lower weight, the aircraft could accelerate faster in a gust or turbulance, but I find myself thinking there is more to it?

help please!

has to do with AOA and how close you are to critical at each weight

[MikeOH58]

Originally Posted by BuickCFI

has to do with AOA and how close you are to critical at each weight

So lower weight, wings are producing less lift than what would be needed for a heavier airplane, so to compensate, AOA has to be increased, thus reaching critical angle at slower speed? is that correct?

[BuickCFI]

Originally Posted by MikeOH58

So lower weight, wings are producing less lift than what would be needed for a heavier airplane, so to compensate, AOA has to be increased, thus reaching critical angle at slower speed? is that correct?

yup

[deadstick]

Originally Posted by MikeOH58

thus reaching critical angle at slower speed?

Not sure I follow this part...

For a constant airspeed, the wing of a light aircraft is required to produce less lift. Therefore, (per lift equation and CL) it has a lower angle of attack when compared to a heavier aircraft (w/ greater AOA). The difference between the Critical AOA (a design constant) and the AOA of the lighter aircraft is greater that that of the heavier aircraft. The lighter aircraft would have a greater risk of exceeding the load limitation (ie wing falls off) before the wing stalls.

So, if the lighter airplane is slowed down (to the lower Va, and results in a greater AOA than if the plane was going fast) the differences of the AOAs (light @ slow speed Va vs heavy at gross wt/max Va) vs the Critical AOA will be equal...in theory :-)

Clear as mud?

[tgrayson]

The amount of load factor you can generate on an airplane is roughly equal to the ratio of the stall AOA to the current AOA. If your current AOA is 3 and the stall AOA is 18, you can pull about 6's. Increase the AOA to 6 and you can only pull 3 g's.

Since for a given airspeed, you're at a higher AOA than you would be at a lighter weight, the amount of possible load factor you can generate is lower, so you can go faster.


For a more thorough discussion, go here:

http://forums.jetcareers.com/technic...ing-speed.html (Manuevering Speed)

[MikeOH58]

Originally Posted by tgrayson

The amount of load factor you can generate on an airplane is roughly equal to the ratio of the stall AOA to the current AOA. If your current AOA is 3 and the stall AOA is 18, you can pull about 6's. Increase the AOA to 6 and you can only pull 3 g's.

Since for a given airspeed, you're at a higher AOA than you would be at a lighter weight, the amount of possible load factor you can generate is lower, so you can go faster.


For a more thorough discussion, go here:

http://forums.jetcareers.com/technic...ing-speed.html (Manuevering Speed)

I had to re-read those over and over a couple times, but it clicked...good explaination!

[tgrayson]

Originally Posted by MikeOH58

I had to re-read those over and over a couple times

Same for everybody.

If called upon to present this, I would draw airfoils representing the AOA's that I mentioned and show how the heavier airplane (twice as heavy if the AOA is twice) is safe, where as the lighter airplane is not.

Specific examples are always better than generalities.

If you need a sound-bite answer, say "The heavier airplane is flying closer to the stall AOA and can't generate enough lift to exceed its load factor limit."

[deadstick]

Originally Posted by tgrayson

If you need a sound-bite answer, say "The heavier airplane is flying closer to the stall AOA and can't generate enough lift to exceed its load factor limit."

OOOhh. I like this one. K-I-S-S is for me!