Sunday, October 24, 2010

DCS: A-10C Warthog - Angle of attack: watch it, is not just a stall indicator - Part 2

Back when I was teaching biochemistry to medical students in Argentina, there was a veteran professor who used to say: "when you are young you teach what you don't know, when you mature you teach what you know, when you are about to retire you teach what the students really need". After a week long struggle to come out with a nice way to explain this angle of attack thing in the blog, I have thrown the towel. I can fly straight and level without the autopilot. I can trim my A-10 really pretty to fly as I want it to fly. But, I just can't explain how it works to somebody else. Worryingly, they say you don't really know what you can't explain and that's exactly the spot where this angle of attack got me. Maybe I don't know it that well ... Maybe I'm too young of a virtual pilot ... Who knows? :(

So, instead of boring you with lame attempts to explain the importance of angle of attack, I will just point you to where I got this viral idea. My only hope is to raise awareness of this way of flying and how it will affect your virtual missions, from cruising to the objective through the final gun run.

The book "Stick and Rudder" by Wolfgang Langewiesche is the premier source for all this angle of attack mambo-jambo. Ed Kolano has a nice article about the basics of angle of attack here.

The basics are as follows:

  1. You select an angle of attack to fly by using the horizontal stabilizer
  2. The angle of attack determines your airspeed
  3. From a selected angle of attack (and thereby airspeed) you can climb, descend or fly level by adjusting power
In my previous entry I made some big claims about this method of flying. Now that I got your attention with such claims please make sure that you understand the following: at the beginning you may notice an improving in your flying just for the boring parts of the mission (cruising, loitering and landing). At a later time, you will find out that your selected angle of attack actually affects any type of flying. For the time being, if you are dodging SAMs downtown Georgia you can continue doing whatever worked for you.

So, let's fly the Hog above the Nevada desert and let's see what it can do without touching the stick. The airplane will point its nose down or up depending on your throttle setting. At a certain throttle setting, the aircraft achieves steady flight. In this case is around 270 knots, but that varies depending on loading and altitude.

Without touching the stick and the throttle fairly open, the plain plane achieves straight and level flight at around 270 knots. From this state of steady flight, if you increase the throttle the aircraft's reaction will be to climb and if you decrease the throttle, you aircraft will descend. In this example, I just nailed a throttle setting that results in level flight.
The angle of attack (AoA) indicator reads 14. This is an indication of the angle of attack that the aircraft reaches without any input from the stick.

A quick glance at the instruments reveals that each engine of the Hog is consuming around 2,200 pounds of fuel per hour. Assuming a total fuel load of 9,600 pounds, we have around 2 hours of flight left.

So, let's now increase our angle of attack a little bit. That's done by pushing the stick backwards. For long periods of time, it's kind of difficult (and exhausting) to keep your stick input consistent and even. That's why we have the trim! Yeah! Let's trim the nose up one or two notches. I know that as I do this, the aircraft will be trimmed for a larger angle of attack and a lower airspeed, so I don't need that much power now to keep flying straight and level and thereby reduce my throttle a bit.

I have trimmed the aircraft's nose one or two notches up and reduced throttle until achieving level flight. I am now flying at around 200 knots. Nose up -> more angle of attack -> less airspeed.
The angle of attack (AoA) indicator now reads 16. Angles of attack between 15 and 16 are used to do some cruising in the Hog.
Just with two notches of nose up trim, we decreased the fuel consumption and increased our flying time by one hour. Calculation above assumes a total of 9600 pounds of fuel available.

Let's go loco and trim the nose up one or two notches more. I have the Hog now trimmed for a higher angle of attack and thus a lower airspeed, so I ease the throttle a bit until I get into level flight again.

More nose up trimming. My aircraft is now trimmed for an airspeed around the 170's.

The angle of attack of indicator (AoA) now reads 18. Angles of attack of 17 and 18 are used for endurance or maximum loitering time. See next screenshot.
We may be flying slower now, but look at our endurance!

The moral of this blog entry: watch for your tim/angle of attack! After a long commute to the kill zone, you may be arriving with a Hog trimmed for something else than a gun run. This may not kill you but the aircraft will be heavy to maneuver. In case of doubt, neutralize trim before the bomb/gun run.

Some useful values (please remember that the Hog's AoA indicator has values in arbitrary units, not real/physical angles):

  • AoA for cruising: 15-16
  • AoA for maximum endurance: 17-18 (may come in handy for loitering)
  • AoA for landing approaches: 19-20-21

This blog was never in the tutorial business, but my inability to properly explain this angle of attack-based form of flying has me very frustrated. I can only wish that this blog entry will spark some interest in the topic among you virtual pilots.



pirimeister said...

Great post! I don't think you need to worry about being unable to explain the importance of AoA.You're previous posts on the subject were already elucidative. This one, with the extra pictures, worked even better. If I hadn't bought "Stick and Rudder" that time, I would do it now. :-)

Anonymous said...

Don't excuse yourself too much, you made what you wanted to tell quite clear to me with your example...keep up with such posts, I really like them!

GregP said...

Agree with previous posters, this is a very helpful series of posts, even for veteran flyers. I almost never think about AoA, but seeing your examples is making me think twice now, especially as I get more and more into DCS: A-10C. Thanks!

JC said...

Thanks for the kind words, gents.

More DCS A-10 coming soon!


Anonymous said...

Brilliant post, thankyou for taking the time to explain it. Well Done.

JC said...

Thanks for your comment.

Installing the second beta right now. More DCS A-10 coming soon!


Anonymous said...

Thanks for the explanation, I'll got try it now!

Unknown said...

Why this doesn’t happen with propeller airplanes ?