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  #21  
Old 02-09-2012, 09:11 AM
luddite42 luddite42 is offline
 
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Quote:
Originally Posted by Jetlinkin View Post
One interesting tidbit I learned from this training, if you unload your aircraft from 1G to 1/2G, your stall speed decreases by 42% ( it's a good thing to know when you are in a stall).
I'm not going to comment on the accuracy, validity, or value of this statement, BUT...exactly how does one stall an airplane without more than 0.5G showing up on the G-meter?
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  #22  
Old 02-09-2012, 11:19 AM
Phlyan Pan Phlyan Pan is offline
 
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Originally Posted by luddite42 View Post
I'm not going to comment on the accuracy, validity, or value of this statement, BUT...exactly how does one stall an airplane without more than 0.5G showing up on the G-meter?
Have a very slow airspeed?
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  #23  
Old 02-09-2012, 11:36 AM
gereed75 gereed75 is offline
 
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Originally Posted by Phlyan Pan View Post
Have a very slow airspeed?
and exactly why knowing how to properly unload (reduce AOA) is a valuable skill for recovering from extremis situations.
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  #24  
Old 02-09-2012, 12:25 PM
luddite42 luddite42 is offline
 
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Originally Posted by Phlyan Pan View Post
Have a very slow airspeed?
Airspeed is unrelated to AOA. You can have zero airspeed and also zero AOA. I'm just curious exactly how you would exceed critical AOA (stall) with 0.5G on the meter. This whole 1/2G thing doesn't make much sense to me. It's kinda like saying that at 2G, your stall speed doubles. It doesn't. Stall speed is unrelated to load. It's a combination of airspeed AND load. You can't separate the two. You could pull 2G at 75 mph, or you could pull 2G at 175 mph. Again, I don't understand how load alone relates to stall speed, since at 75 mph, you won't stall at much more than 2G, but at 175 it might take more G-load than the airframe can handle in order to stall. I don't see how 0.5G can be anything but a shallow parabolic arc (no stall).
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  #25  
Old 02-09-2012, 02:15 PM
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walkman walkman is offline
 
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Totally agree.

The limited acro training I had saved my bacon once when CMH tower decided to squeeze out a 7x7 between me and the DC-9 I was following (hazy memory) without advice to me and I rolled inverted practically over the numbers.

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Originally Posted by N355DW View Post
Yes indeed. When I ride with students who do this, I can always tell when this is going to happen. They are almost always surprised when it does.

For those of you who are frightened by the very thought of unusual attitudes, I will say this:

When I was first getting my CFI back in the early eighties, I thought it might be nice to take an aerobatic course in case a student "did something crazy."

I signed up for a ten hour course with Duane Cole. For the first two hours I was terrified, everything was a blur.

After five hours I was more relaxed, and at the end of the course I was loving it so much I went home and bought a Citabria! (Paid $7500 for it, those were the days!)

My point is, if you can get past the initial nervousness and fear, the rewards of learning aerobatics are numerous. You will have a much better feel for your aircraft, your confidence level will go up quite a bit, and you will simply be a much better "stick".

Plus, as others have said, it's a lot of fun, and if you become interested in competition, the IAC is filled with really cool people!

If you just aren't up to the G's, and some folks aren't, even after trying for a while, even an unusual attitude course as the OP talks about can be invaluable, building both skill and confidence, and feel for your aircraft.
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  #26  
Old 02-09-2012, 02:48 PM
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AZtailwind AZtailwind is offline
 
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Quote:
Originally Posted by luddite42 View Post
I'm not going to comment on the accuracy, validity, or value of this statement, BUT...exactly how does one stall an airplane without more than 0.5G showing up on the G-meter?
One way in vertical maneuvers- After a hammerhead turn where you are at a extremely low airspeed in it?s beginning down hill acceleration and pull the stick past the critical AOA and a little rudder- bingo, a stall/spin with maybe .5 Gs on the meter
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  #27  
Old 02-09-2012, 02:58 PM
gereed75 gereed75 is offline
 
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I don't think any thing you are saying contradicts anything Vac said. What ever you want to call it - reduced AOA, 1/2 G, flying a ballastic parabolic, or combinations there of - they are all forms of unloading. And a way to avoid a stall (and the attendent post stall gyrations which are much harder to recover from, and eat up lot's more altitude, and bend airplanes, and break aviators).

All same same, I think.

In Brad's example if going up, the stall can occur somewhere around nearly 0 AOA and nearly zero airspeed and near 0 G. In the case of extremely nose high attitudes, no amount of un-load will save it and avoid the post stall gyration, but at least the velocity vector will soon be transiting the horizon!!
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Last edited by gereed75 : 02-09-2012 at 03:13 PM.
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  #28  
Old 02-09-2012, 03:12 PM
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N355DW N355DW is offline
 
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Quote:
Originally Posted by luddite42 View Post
Airspeed is unrelated to AOA. You can have zero airspeed and also zero AOA. I'm just curious exactly how you would exceed critical AOA (stall) with 0.5G on the meter. This whole 1/2G thing doesn't make much sense to me. It's kinda like saying that at 2G, your stall speed doubles. It doesn't. Stall speed is unrelated to load. It's a combination of airspeed AND load. You can't separate the two. You could pull 2G at 75 mph, or you could pull 2G at 175 mph. Again, I don't understand how load alone relates to stall speed, since at 75 mph, you won't stall at much more than 2G, but at 175 it might take more G-load than the airframe can handle in order to stall. I don't see how 0.5G can be anything but a shallow parabolic arc (no stall).
Hard to talk about, easy to show with models.

First off, it's important to understand that the critical angle of attack changes with changes in airspeed. In other words the angle of the wing in relation to airflow at which the air quits flowing smoothly over the wing changes as the airspeed changes, it is not the same angle at all airspeeds. At higher airspeed the angle it takes to exceed the critical AOA is a much larger angle than at low airspeeds.

So, at zero G, the angle of attack is zero. At anything more than zero G, there is an AOA, however slight it might be. Another way to think about it is, if you feel pushed into your seat or against the seatbelt, that means either the direction the aircraft is traveling is changing, or gravity is pulling on the aircraft, or both. In all these cases, either with gravity or change of direction, you will have an AOA on the wing. Anytime you feel G's, however slight, that means the wings have an AOA. (Except maybe in a really nice tailslide or torque roll, but that's another discussion!)

If the airspeed is, say ten mph, over the wing, it won't take much of an angle to reach critical AOA. This is the same as saying it won't take much G force to exceed the critical AOA. A nice example of this might be a vanilla humpty bump (pull up, pull over the top, pull out). If, when nearing the top of the humpty, I pull back on the stick to do my half loop at about sixty mph, by the time I reach the apogee I might be only indicating twenty mph. At this point I am still flying even though the stall speed of my plane might be up around 60 or better. I am also close to zero G with a very low angle of attack. But I could easily add a 1/2G, or less possibly, pull on the stick because maybe I am in a hurry to get back to the vertical down line at the top of the humpty, and exceed the critical angle of attack and feel the wing buffet. I know, because I have done this many times.

So G is more an indication of the AOA of the wing to the airflow. At low G, the AOA is less, and at high G it is much greater. If your airspeed is low, then at low G, or low AOA, you can stall the wing. If your airspeed is high, you can pull more G, or have a greater AOA without stalling

Hope this helps, I might be confusing myself here!
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Last edited by N355DW : 02-09-2012 at 03:22 PM.
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  #29  
Old 02-09-2012, 03:33 PM
RV8R999 RV8R999 is offline
 
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Quote:
Originally Posted by N355DW View Post
Hard to talk about, easy to show with models.

First off, it's important to understand that the critical angle of attack changes with changes in airspeed. In other words the angle of the wing in relation to airflow at which the air quits flowing smoothly over the wing changes as the airspeed changes, it is not the same angle at all airspeeds. At higher airspeed the angle it takes to exceed the critical AOA is a much larger angle than at low airspeeds.

So, at zero G, the angle of attack is zero. At anything more than zero G, there is an AOA, however slight it might be. Another way to think about it is, if you feel pushed into your seat or against the seatbelt, that means either the direction the aircraft is traveling is changing, or gravity is pulling on the aircraft, or both. In all these cases, either with gravity or change of direction, you will have an AOA on the wing. Anytime you feel G's, however slight, that means the wings have an AOA. (Except maybe in a really nice tailslide or torque roll, but that's another discussion!)

If the airspeed is, say ten mph, over the wing, it won't take much of an angle to reach critical AOA. This is the same as saying it won't take much G force to exceed the critical AOA. A nice example of this might be a vanilla humpty bump (pull up, pull over the top, pull out). If, when nearing the top of the humpty, I pull back on the stick to do my half loop at about sixty mph, by the time I reach the apogee I might be only indicating twenty mph. At this point I am still flying even though the stall speed of my plane might be up around 60 or better. I am also close to zero G with a very low angle of attack. But I could easily add a 1/2G, or less possibly, pull on the stick because maybe I am in a hurry to get back to the vertical down line at the top of the humpty, and exceed the critical angle of attack and feel the wing buffet. I know, because I have done this many times.

So G is more an indication of the AOA of the wing to the airflow. At low G, the AOA is less, and at high G it is much greater. If your airspeed is low, then at low G, or low AOA, you can stall the wing. If your airspeed is high, you can pull more G, or have a greater AOA without stalling

Hope this helps, I might be confusing myself here!
an airfoil only has one lift curve slope with a critical AOA somewhere near the point where the lift curve slope starts to turn negative. This is independent of airspeed. Doesn't matter if you have 1mph or 1000mph the critical AOA will be the same regardless. With any velocity above (or below) zero an AOA exists - by definition. AOA which provides zero lift is the x intercept of this curve... symmetrical airfoils will have zero lift at zero AOA. Any other airfoil will still have lift at zero degrees AOA due to camber and would require negative AOA to achieve a zero lift condition (if positively cambered).
AOA is the angle formed between the cord line and the relative wind, simple as that.
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  #30  
Old 02-09-2012, 04:08 PM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by RVbySDI View Post
Mike,
Thank you for the clarifications. Since I am not into aerobatics (hey I fly a non-aerobatic 9) I am not really looking to go experience this in my plane but I am interested in understanding these concepts. The truth is I am not really interested in intentionally doing these maneuvers for the pleasure of aerobatic flying like many RV fliers out there. However, I would like to be experienced in the theory and would like to have an opportunity to practice them for the purpose of emergency recovery.
http://www.amazon.com/Better-Aerobat...8828807&sr=8-1

This is the best book on acro I've ever read and will explain a lot of the concepts being described in this thread. Zero g can get you out of trouble in a lot of situations and even if you don't do acro, it is invaluable to know in a situation such as an engine out where you have no choice but to turn around.
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Last edited by rocketbob : 02-09-2012 at 04:10 PM.
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