AOA, Can someone explain this to me?

[Purdue]

Well, from what I remember from API... the AOA gauge is valid at all airspeeds/altitudes/weights.

So why do the Air force instructors keep telling me things that don't seem to jive with that? I had the third instrucor (these are retired O-3's and above) tell me that Max Range on the AOA indicator doesn't do what I think it does.

Here's an almost exact quote:
"If you want to stay in the air for as long a spossible, use the Max Endurance AOA. which for the T-6 is 8.8 units, or the White Diamond. So, if you were on a low level and didn't know if you could make it to the field, would you use the Max Range White triangle?"
When I answered Yes, in front of the class... I was quite perturbed to be told:
"Actually No. See, the AOA gauge gives you an airspeed that will give you best range at 4.9 AOA at 31,000 feet. So, if you were on a low level... you'd need to fly somewhere between zero and Two AOA because you need to fly faster to get max range. 4.9 AOA works out to be about 148 knots at 31,000 feet... which is not the airspeed you want to fly when you're this low."

Am I completely confused here? I'm no engineer... but I distinctly remember being taught the AOA gauge was a pretty good indication at ANY time... and in our DASH 1 (kinda like a NATOPS) it does in fact say the AOA gauge is valid at any altitude and airspeed, but the values are "optimized for 31,000 feet MSL" What exactly does that mean? And why does the AF keep telling me that Max Range is unreliable, Max Endurance is always reliable... but to just fly at a set airspeed of 125 knots in any emergency, and never to referance the AOA indicator?

I've had several discussions with instructors trying to either straighten myself out, or straighten them out... and I always end up confused in the end.

 

[MasterBates]

Aggh. Head hurts. You owe me a beer....

I honestly don't know/remember. I would think L/D max would be found at a certain AOA, which would give min power for level flight, and max endurance. Max range is almost always faster than that.

 

[wink]

Some one better come up with a very good explanation for why Purdue is not right, along with everything I have been taught in the Navy and in college aerodynamics.

The AOA gage is just that, Angle of Attack on the wing. The AOA determines the relative performance of the wing, not airspeed. You only fly airspeed during critical phases of flight when an AOA is not available. When AOA is available you fly that.

 

[Purdue]

Some one better come up with a very good explanation for why Purdue is not right, along with everything I have been taught in the Navy and in college aerodynamics.

Well like I said, i'm no engineer... but if someone could just spell it out relatively simply for me... I would love to be able to prove these people wrong. Maybe I just pressed the "I Believe" button too much while in API... but it's hard to be told you're wrong about something when you've accepted it as fact.

 

[wink]

Here is something that will make you lose less sleep:
play their stupid game, know what you know, and don't fight their BS. Wait until you select, if you get orders to Corpus, Whiting, Meridian, or Kingsville (ie NOT VANCE) go into each and every one of their offices and tell them to F*** Off!

Yeah, I wouldn't go tell an instructor he is AFU (hey, AF, no surprise). Still, I hope for some more validation on the academic merits for the benefit of AW members.

 

[Sly1978]

Purdue, I can't help you with your math. I always thought that AOA indicators were a/s independent. But thank you for helping me remember why I'm glad I fly helos.

 

[squeeze]

1) If I were on a low-level and didn't know if I could make the field, I already f_cked it away and should not continue flying low. Lower = higher fuel burn rate.

2) Set a fuel ladder and stick to it. If you piss it away, adhere to your bingo.

3) There is no reason to be flying AOA down low (except in a terminal phase of flight). Low and slow is a good way to limit your options (relight, ejection, etc..) very quickly. Low levels are for tactical flying... fly tactical airspeeds.

4) AOA is independent of A/S.

5) I'd be surprised if an Air Force pilot knew what AOA was exactly.

 

[MasterBates]

Squeeze,

What is a "Fuel Ladder" ?

 

[insanebikerboy]

Well, I have no clue about the T-6 but here goes my interpretation.

On the T-34 the AOA only gives proper indications if gear is down and flaps are either full up or full down. That would mean that there is an associated L/Dmax regarding those different configurations which in turn allows proper indication on the AOA indicator in the cockpit.

I'd think that'd be what they may be talking about.

In addition, the coefficients of drag and lift, which determine L/Dmax, are affected by density, plus a few other things, which is possibly why the AOA at 31,000 is different than the AOA needed at a low level.

Finally, and this is coming from a guy with limited flight experience, I could kind of understand why you should fly at max endurance on a low level if you don't know if you can make the field. My explanation why would be that the sink rate is a lot higher for max range than max endurance, so if you in fact couldn't make the field and were at AOA max range, by the time you realize it you'd be a lot closer to the Big Dirty than if you were flying AOA max endurance. So, accordingly, you'd have a bit more time to perhaps find yet another place to land if not the field.

Comparing that to AOA at 31,000 feet, if you had an engine failure there then I'd fly AOA max range since the altitude would correspondingly give you plenty of time to figure out your field to land at. I'd think it'd be kind of pointless to fly AOA max endurance there since you'll be spending even more time airborne when you didn't really need to.

Your post reminds me why I'm glad I didn't have to go to Vance. Have fun with that, you'll be outta there soon enough.

 

[nittany03]

He might not be trying to say that max range is unreliable. He may be making the argument that until you know where you are, it is better to fly max endurance than max range because it gives you more time until flameout. But max range AOA is max range AOA. The airspeed associated with it may change depending on altitude or gross weight, but that airspeed is still the best range performance you're going to get.

 

[squeeze]

Squeeze,

What is a "Fuel Ladder" ?

It's a time/fuel-based table you make starting from the end of your flight (ie. your set state at the boat -- what the LSO wants you crossing the deck with) and working backwards with known points and fuel flows. For example... a Harrier has a set state of 1.2 (1200 lbs) and your charlie time is 1200. You know it takes 3 min and 400# from the initial to the cross roughly. So your initial state is 1.2+400 = 1.6 at time 1157. From there you work back with know fuel flows and times. Basically, we'll use 600# every 10 min for non-tactical flying. So just build backwards and every 10 minutes, add 600#. Eg: 2.2/1151, 2.8/1145, 3.4/1139 etc... Once you're on you're ladder, it's time to honor it and fly your planned FF basically.

Clear as mud?

/crappy new guy explanation

 

[scoober78]

On the T-34 the AOA only gives proper indications if gear is down and flaps are either full up or full down.

Are you sure about the gear here? I know that you are correct about the flap positions but neither the NATOPS or the infamous systems workbook say anything about the gear.

But max range AOA is max range AOA. The airspeed associated with it may change depending on altitude or gross weight, but that airspeed is still the best range performance you're going to get.

Right...AOA performance has nothing to do with either weight or altitude. (density or any other kind...)

 

[MasterBates]

Ahh. We just plan on 1000 pph, and pray you get a green deck without undue delay. 3-3.5 is the standard at sea flight duration for HSL.

We just keep on taking # of gas -600# (NATOPS min on deck) and divide by actual fuel flow, (normally 1000, but some are different) and keep on mentally cross checking it.

We have a "mental" one but don't work backwards like that.

 

[SemperGumbi]

Adding fuel to the fire…

A true range AOA is based on no wind. Want proof? If you are flying into a 150kt headwind in a T-34 (max power was like 180-190kias, right?) and fly max range AOA…you will go backward. You would need a lot closer to max power to get a true max range in that situation. That is an extreme example, but it can be interpolated down and is valid for all head and tail winds. It annoyed me in primary when I was told I was wrong about this. “Roger that, sir.”

 

[A4sForever]

Well, from what I remember from API... the AOA gauge is valid at all airspeeds/altitudes/weights......

True ... but you need to review Max Endurance (M/E) and Max Range (M/R) to answer your Instructor's "trap" question ... i.e., what are they (M/E and M/R) and where are they "appropriate" .... to put it really, really simple (so I can understand it):

Crossing the Pacific at altitude, Max Range (specific range ... i.e., how many miles of groundspeed can I get per # of fuel) is all that is important. That's why "cruise control" was so important. Props generally never fly M/R ... that's for altitude and the jet age.

On arrival @ Tokyo and I'm told to "HOLD" ... Max Endurance (specific endurance ... i.e., how many hours/minutes can I get per # of fuel before I have to BINGO or flame out) is all that is important.

The greater the M/R ... the farther I can get. The greater the M/E ... the longer I can stay in the air ... the two are not the same (obviously).

Isn't that why P-3's shut down one while on patrol??? To maximize S/E ??? But we would NOT do that with a 747 @ 39,000 feet ... even if told to HOLD .... 'cause you couldn't maintain altitude without all engines and thus M/R and would very soon be converted to M/E and a drift-down (@ max glide speed in this case = M/E) to an altitude you could hold .... and then back to M/R airspeed for that altitude.

Clear??? Of course not ... just remember, the AOA gauge is always good ... it just might not be "appropriate" for all flight regimes.

Anyway .... after many years ... I could always count on 25,000#/hr to get me there .... had to live by the gouge as we had no AOA indexer nor gauge. It would have been nice to have on approach ... but that's not the way the airlines did it .... no constant A/S constant AOA approaches on the 18-Wheeler Big Iron.