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Speed and AOA?


animaal
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I was flying around in the F-5 last night, and took some damage - the TACAN and airspeed indicator stopped working.

 

Luckily, I knew the direction to my airbase so lack of TACAN wasn't a big deal. I was worried about my landing speed though - I find it tricky to judge.

 

But then I noticed that the AOA indicator was still working. This was a surprise to me. Without ever really thinking, I had just assumed speed was an input to the AOA, since it has lights to indicate "too fast" and "too slow".

 

Now that I've thought about it, it seems reasonable that the AOA indicator only needs to know the angle of attack and rate of descent in order to function, since landing speed would be different depending on flaps, loadout, and possibly even the condition of the flight surfaces.

 

Is this how it works?

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Now that I've thought about it, it seems reasonable that the AOA indicator only needs to know the angle of attack and rate of descent in order to function

The AoA indicator doesn't need the ROD or anything else. The important item is that it's only showing the 'correct' i.E. approach AoA/speed in unaccelerated flight, which means 1.0G.

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  • 1 year later...

Hello all,

 

Lets ressurect this thread.

Only yesterday I started flying the F-5E.

It is a tricky plane to master, But this is what makes it fun to fly.

 

However, landing is a bit more complicated than usual. And I found out that I need a to stay at slight fast speed to properly land, without sinking in the ground.

 

However, I noticed that in the manual there is a formula you must have in mind when calculating landing speed. And the base line speed is 145 knots, but if you have ammo in the cannon, you have to add 5 knots. And if you have fuel or other loadouts, you have to add more speed.

 

Ok, this is natural.

 

 

But then, my question is: is the AoA indexer on the F-5 calibrated for a "naked" airplane, with no stores, no ammo, and only 1,000 pounds of fuel? So, when coming down to land a bit heavier than that, one must pay more attention to IAS than the AoA indexer? And using it just as a baseline, not as THE reference for landing? i believe that happens because the simple analogue computer on the F-5 does not take in account loadout weight and fuel. Or it does not interfere at all for AoA indication?

This is an amazing sim! 'Nuff said!:pilotfly:

 

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AOA indication will be accurate regardless of the weight of the aircraft, the stores you might or might not have onboard, or their location. Just fly to the green indication and you are correct regardless of aircraft loading or fuel status.

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AOA indication will be accurate regardless of the weight of the aircraft, the stores you might or might not have onboard, or their location. Just fly to the green indication and you are correct regardless of aircraft loading or fuel status.

 

Thanks, Now I understand it is totally not dependant of weight at all. Thanks.

This is an amazing sim! 'Nuff said!:pilotfly:

 

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Thanks, Now I understand it is totally not dependant of weight at all. Thanks.

 

AOA at a given speed will change, flying to AOA will obviously mean higher speed with more stores.

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Thanks, Now I understand it is totally not dependant of weight at all. Thanks.

 

AOA will change with speed , weight , air density,wepons instaled, etc,

AOA gage is showing angle of attack value and accuracy if this is very good in flight and very poor at very low speeds for example when plane is taxing or is parked stacionary.

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This is my understanding of the relationship between the required AoA and aircraft weight and speed.

 

3JwX1gc.jpg


Edited by Ramsay
Correct diagram terminology

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@Ramsay, with all respect for how spot on your analysis typically is, I don't know if I am a fan of that graphic as the coloration and some of the terminology seems misleading to me, but maybe I am missing the point...

 

Angle of attack is the difference between the chord line of wing (not necessarily the "aircraft nose / datum line" as shown by the graphic, since the chord line changes with the configuration of the wing) and the relative wind (aka direction of travel).

 

Now perhaps I am misinterpreting the diagram posted, but with the difference in red and green and their positions on the triangle shown, it seems to be implying that the angle of attack would be different for the two different weights. This is of course not the case; correct angle of attack, as indicated by the green donut of the indexer, is always the desired angle of attack -- in the diagram you show, that angular relationship (the triangle) would always be the same, regardless of speed. The required speed to meet that angle of attack in unaccelerated flight (aka a constant rate of descent) will change, but not the AoA. The diagram is correct in saying that a lighter weight will yield a slower approach and vice versa, but I think the colors might be misleading -- if I am the only one seeing it that way and everyone else just gets it, my apologies.

 

Regardless of all that, the entire point of resurrecting this thread was to answer RodB in post #6 -- and the answer remains; if a correctly functioning and configured F5 is on final with the green donut AoA indicator illuminated, the speed is correct for approach. That speed necessarily will be different as the aircraft loading changes, but it will be correct regardless. Fly the green donut and don't sweat IAS...


Edited by tom_19d
Removed incorrect statement

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@Ramsay,

That diagram is perfect for understanding the AoA/speed relationship during the approach. Great work :)

 

@tom 19d

I don't what makes you think that the angles in these two triangles is different. In both triangles it is obvious that all three angles are identical.

 

I'm curious what atmospheric things would cause the approach speed to change?

 

'Fly the green donut and don't sweat IAS' is a bad advice because you should always crosscheck AoA and IAS.

 

Since you should always know the actual weight of your F-5 you should know the correct approach speed.

 

If the AoA vane gets stuck, sticky, bent etc. and you get a on-speed indication at e.g. 180kts at a noticable flatter attitude than usual, you should immediately realize that the AoA indication is incorrect.

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Angle of attack is the difference between the chord line of wing (not necessarily the "aircraft nose / datum line" as shown by the graphic, since the chord line changes with the configuration of the wing) and the relative wind (aka direction of travel).

 

Yeah, I struggled to find a non-technical description for the wing's chord, I've taken your feedback into account and adjusted the terminology to "wing chord".

 

I'm not sure how well goggle translate will handle it for non-English speakers but hopefully the diagram should be self explanatory.

 

... perhaps I am misinterpreting the diagram posted, but with the difference in red and green and their positions on the triangle shown, it seems to be implying that the angle of attack would be different for the two different weights.

 

In my idealised diagram (CG%-MAC=15), On-speed AoA is the same for the two aircraft weights (the two triangles share a common direction of travel / hypotenuse).


Edited by Ramsay
Spelling

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Yeah, I struggled to find a non-technical description for the wing's chord, I've taken your feedback into account and adjusted the terminology to "wing cord".

There's a typo in the updated diagram and the above text. Cord instead of chord. Makes it difficult to google ;)

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I'm curious what atmospheric things would cause the approach speed to change?

 

I could be a smart**s and say "manufacturer recommended speed increase for gusts" but you are of course correct, nothing atmospheric will change the required IAS, post above edited.

 

'Fly the green donut and don't sweat IAS' is a bad advice because you should always crosscheck AoA and IAS.

 

In real life of course, but DCS doesn't model any damage to the AoA system that won't have an associated caution. That is why I specifically said "correctly functioning F5" in my post; I prefer to save my admittedly limited mental bandwidth to keep my head on a swivel and fly the donut. But, whatever floats your boat...

 

In my idealised diagram (CG%-MAC=15), On-speed AoA is the same for the two aircraft weights (the two triangles share a common direction of travel / hypotenuse).

 

Fair enough, I totally get what you are doing with the AoA and agree with your depiction, like I said something about it just tickles my brain wrong when I try to picture it from the perspective of someone just starting out with the concept -- maybe my thoughts are that someone could incorrectly association the red / green on the diagram with a red indexer. Obviously that would be incorrect, but just trying to see this from a different perspective. Regardless, like I said above, if I am the only moron with an issue, my apologies, so I will let it be. Thanks for the effort and the chord terminology change, cheers.

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In real life of course, but DCS doesn't model any damage to the AoA system that won't have an associated caution. That is why I specifically said "correctly functioning F5" in my post; I prefer to save my admittedly limited mental bandwidth to keep my head on a swivel and fly the donut.

Good and valid point.

I guess that almost 40 years of RW flying and countless proficiency checks prevent me from simply enjoying flying in DCS without constantly calculating something :lol:

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I guess that almost 40 years of RW flying and countless proficiency checks prevent me from simply enjoying flying in DCS without constantly calculating something :lol:

 

Right on, right on.:thumbup:

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It's worth trying to fly on pitch/power using the altimeter as your ( sink rate ) instrument anyway, given how many times I've had to land a damaged aircraft with no avionics...

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  • 4 weeks later...

As I stated in another thread, for those pilots with troubles on AoA -Speed - Attitud during approach, I highly recommend to spend time flying your Tiger on landing configuration. Fly it like a cessna. Learn to fly it on "slow flight".

 

Find the pitch/yoke back and throttle combitaion for the weight you have in that moment in order to keep flying at 200 Kias. Once you feel confortable flying in that condition, fly a traffic pattern and enter into downwind in "slow flight" configuration at 200 Kias.

 

Turn into final with 200 Kias and reduce your throttle a bit in order to lose altitude. Use your speed brakes to slow down to touchdown speed.

 

Once you feel confortable and you are ready to land, cut the throttle and maintain your pitch with a bit more of yoke back. Using speed brakes insted of reducing throttle is a good way to avoid lossing engine power when you probabily need it, ie: during bombing dive or landing, when you don't have enough time to wait for your turbojet to get it hight performance for a recovery or a missed approach.

 

Salute

Gavilan

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