S

Sig600

Well Known Member
How do you guys with AOA setups, calibrate them? Does Van's publish L/Dmax, CLmax, Critical AOA, etc? Basically, how does the AOA setup know when I'm at best glide AOA, max range, max endurance, etc?
 
Not sure how the other systems do it, but Dynon has a "set-up" menu. You enter the menu and do a series of power-off and power-on stalls with flaps both retracted and extended and it calibrates the AOA bar.
 
You are sort of thinking in reverse on this issue.

AOA really doesn't need calibration per se as it is a physical measurement of the angle between relative wind and by convention the cordline of the wing.

Whatever device you are using (wind vane, or Dynon Pitot probe) the measurement is direct. You don't even need to know the true angle you could call it units if you like.

AOA doesn't map consistently to airspeeds across the range of possible Gross Weights and Density Altitudes for your airplane. The airspeed values of Vs, Vs1, Vx, Vy will be different for each GW and DA combination but those performance indicators occur at the same AOA independent of GW and DA- hence the benefit of an AOA instrument. If AOA mapped to Airspeed across the spectrum of GW and DA then what would be the benefit of an AOA indicator as it would simply be the the same as your airspeed indicator with a converstion to degrees or units?

The process of noting when the stall occurs is merely to provide the set point for the warning. You'd have to do the same thing for Vx and Vy by flying those profiles and noting (mentally or electronically) the AOA in which they occur. But the AOA itself needs no calibration rather you just need to determine what AOA results in Vs, Vs1, Vx and Vy and fly those AOA's when you need to.

Make sense?

Ken
 
The GRT AOA indication is very different from all the rest - it has no sensor, but is "derived" with data from the attitude platform. While I am not convinced that it is useful in aerobatic flight (things happen too quickly for the computational cycle), in the kind of (relatively) steady state flight you do in the pattern, it works quite well. You do fly calibration routines (slow flight, decelerating flights to a nibble) to get it to "know" your airplane.

Paul
 
We have been toying with calculated AOA for some time, you know the story: "Should we, should we not, should we, should we not...".
Of course we support the real thing and have had that ever since the Enigma came out long ago. A calculated AOA is very tempting as this means it does not cost us any hardware, no extra pressure sensors and electronics and of course it is easy to install - you don't do anything...

The comments related to aerobatics are true - calculated AOA is not very useful in those cases. Even a steep turn could pose problems for the calculations.
But that does not mean it is useless, it can and does work well in the straight and more or less level scenario (like in an approach).
Using a simplified calculation, you take the pitch angle as measured by the AHRS, take the actual path through the air based on true airspeed and vertical speed and the difference is your relative AOA. To get an absolute AOA you match this to at least two calibration points, one close to stall and the other at the lowest AOA you can fly your aircraft at.
Your aircraft works in much the same way as an old fashioned AOA vane that you still see sometimes at the nose of older airliners and the calculated AOA pretty much mimiks this (if you keep the blue up and brown down).
You can start taking bank angle into account as well to sensitise the reading as bank angle increases. The laws are well known.

Should we add this as an option to our instruments ? What do you think ?

Rainier
CEO MGL Avionics


Ironflight said:
The GRT AOA indication is very different from all the rest - it has no sensor, but is "derived" with data from the attitude platform. While I am not convinced that it is useful in aerobatic flight (things happen too quickly for the computational cycle), in the kind of (relatively) steady state flight you do in the pattern, it works quite well. You do fly calibration routines (slow flight, decelerating flights to a nibble) to get it to "know" your airplane.

Paul
Click to expand...
 
Anyway, back to the original question (I just happened to have my AFS manual handy), Mel pretty much nailed it. For the AFS, you do a 0 wind calibration (in a hangar or similar), a 0 load calibration (in flight, do a 0-g maneuver, and a calibration at stall plus 15% (full flap and no flap). The AFS computer uses those data points to calculate AOA at any flight condition, given the caveats previously discussed. I believe the AFS manual is available for download as a PDF from their site; it has a good discussion of how it works, how to calibrate, and how to use it to aid flight.
 
Rainier Lamers said:
A calculated AOA is very tempting as this means it does not cost us any hardware, no extra pressure sensors and electronics and of course it is easy to install - you don't do anything...

Rainier
CEO MGL Avionics
Click to expand...


And everybody KNOWS that software is free, right Rainier?? :D:eek:
 
RV8R999 said:
You are sort of thinking in reverse on this issue.

AOA really doesn't need calibration per se as it is a physical measurement of the angle between relative wind and by convention the cordline of the wing.

Whatever device you are using (wind vane, or Dynon Pitot probe) the measurement is direct. You don't even need to know the true angle you could call it units if you like.

AOA doesn't map consistently to airspeeds across the range of possible Gross Weights and Density Altitudes for your airplane. The airspeed values of Vs, Vs1, Vx, Vy will be different for each GW and DA combination but those performance indicators occur at the same AOA independent of GW and DA- hence the benefit of an AOA instrument. If AOA mapped to Airspeed across the spectrum of GW and DA then what would be the benefit of an AOA indicator as it would simply be the the same as your airspeed indicator with a converstion to degrees or units?

The process of noting when the stall occurs is merely to provide the set point for the warning. You'd have to do the same thing for Vx and Vy by flying those profiles and noting (mentally or electronically) the AOA in which they occur. But the AOA itself needs no calibration rather you just need to determine what AOA results in Vs, Vs1, Vx and Vy and fly those AOA's when you need to.

Make sense?

Ken
Click to expand...

<---- Naval Aviator. Got it. :)

My question was in reference to how it's setup. The AOA for L/Dmax, CLmax, and critical AOA never change for a given wing. Like you said, the speeds for those AOAs change given the host of variables. AOA is a more accurate way of flying to optimize performance, it's just about all we use in the F-18. I know what AOA gives me best range, endurance, lift, etc. Given the fuel weight I know ABOUT what a/s I should see. For instance I know that at 35 degrees AOA I'm at CLmax, I can get there from any airspeed, resulting in different G loading for each airspeed up to Va or "corner speed" (the point at which max G is attained before critical AOA). 4.6 Alpha gives me max range, again no idea what airspeed that is for a given condition. I was wondering if these AOA systems I see on guys planes can be used to fly an RV the same way. i.e. rather than climbing out at Vy of Vx which is probably within the ball park, can I climb out at the AOA that corresponds with CLmax? How do you calibrate these systems to your given wing design? That's more what I was getting at.
 
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I get what you are asking now..

Yes, you can fly AOA for all those points just like in the F-18 once you know what they are.

Are you familiar with Sawtooth patterns for climb performance determination?

nothing special really but you'll use your altimeter and a stop watch and not the VSI to determine best climb/descent profiles since the VSI has too much lag and is not accurate enough (generally). You'll establish a steady-state climb and descent throughout a range of airspeeds (5 mph increments works well)

Set a reasonable altitude band of about 2000' (say from 2000-4000ft PA). Start about 500 ft below the target altitude band and trim at an airspeed about 5 mph above your Vs1 speed and initiate a full power climb and trim to maintain A/S +- 1mph. As you hit the lower altitude start your stop watch. Record: MP,RPM,A/S,AOA,VSI,OAT,ALT Band, Time to climb, fuel. Pitch Attitude.

Do same thing for the descent and start over at next higher A/S. Repeat until you've covered the A/S range you are interested in. With all the data you can use excel to calc to plot or calculate the data for the parameters you are interested in. The one caveat though is fuel burn changes your gross weight and will skew your data as you move up the A/S range. If you do it quickly it will be in the noise. In an F18 though you'd have to use what is called the referred weight/sigma method and use math to eliminate weight and DA as variables...its not hard but requires a lot more work.

If you do this method throughout the entire A/S range at very light weight, mid weight and max GW (under similar test conditions) and at low, med and high altitude bands you can then develop climb/descent charts through interpolation. Takes some time but is worth it.
 
Yes, this should work well and give a meaningfull result.
To make this easier, use any of the EFIS systems that can record the required data you are interested in for later analyses. This makes it much easier as you can concentrate on holding your airspeed rather than jotting down numbers.

Rainier
CEO MGL Avionics

RV8R999 said:
I get what you are asking now..

Yes, you can fly AOA for all those points just like in the F-18 once you know what they are.

Are you familiar with Sawtooth patterns for climb performance determination?

nothing special really but you'll use your altimeter and a stop watch and not the VSI to determine best climb/descent profiles since the VSI has too much lag and is not accurate enough (generally). You'll establish a steady-state climb and descent throughout a range of airspeeds (5 mph increments works well)

Set a reasonable altitude band of about 2000' (say from 2000-4000ft PA). Start about 500 ft below the target altitude band and trim at an airspeed about 5 mph above your Vs1 speed and initiate a full power climb and trim to maintain A/S +- 1mph. As you hit the lower altitude start your stop watch. Record: MP,RPM,A/S,AOA,VSI,OAT,ALT Band, Time to climb, fuel. Pitch Attitude.

Do same thing for the descent and start over at next higher A/S. Repeat until you've covered the A/S range you are interested in. With all the data you can use excel to calc to plot or calculate the data for the parameters you are interested in. The one caveat though is fuel burn changes your gross weight and will skew your data as you move up the A/S range. If you do it quickly it will be in the noise. In an F18 though you'd have to use what is called the referred weight/sigma method and use math to eliminate weight and DA as variables...its not hard but requires a lot more work.

If you do this method throughout the entire A/S range at very light weight, mid weight and max GW (under similar test conditions) and at low, med and high altitude bands you can then develop climb/descent charts through interpolation. Takes some time but is worth it.
Click to expand...
 
How do you calibrate these systems to your given wing design? That's more what I was getting at.
Click to expand...

It would depend on the actual system. Most in the General Aviation market use some sort of differential air pressure, such as a hole in the top of the wing & the bottom, or a pitot with another hole at a different angle. I have seen an RV-10 with a wingtip vane, but I don't know any details on that one.
Each manufacturer has instructions for calibration, but it's obvious that they only consider it a useful tool during approach - because they blank out the info at the faster speeds. There are ones out there that use a mechanical gauge to show the pressure differential all the time. This is much more useful for "us Navy guys" who want all the data range.
Once calibrated, you're on your way. As far as calculating points, I think that the wing's design is well known and you should be able to find all the critical angles in the description of the wing. At least that's my intention when I get this thing flying.
John
 
johngoodman said:
Each manufacturer has instructions for calibration, but it's obvious that they only consider it a useful tool during approach - because they blank out the info at the faster speeds.
Click to expand...

Dynon does not blank out AoA at any time or speed. It works equally as well at 40 knots as it does at 160 in an accelerated stall.

There are some EFIS units that hide AoA when you are at low AoA and it's not a concern, but I am not aware of any that will not calculate above a given speed.
 
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