Best glide AOA
- Thread starter dbuds2
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Juvat
Member
Here's a good "Test Pilot" article from the EAA archives that will help with the concept of best glide AOA (also known as L/D max): http://www.oshkosh365.org/saarchive/eaa_articles/2001_05_19.pdf
Juvat
Member
The AOA indicator only reads the angle of attack. It cannot tell you anything about the lift curve or drag curve of your particular airfoil. You will need to do your own flight testing to calculate the L/D max airspeed for a given weight. Then go fly the airplane (at the calculated weight). Fly at the L/D max airspeed and reference the AOA indicator. That's now your max glide, max range, L/D max AOA (they're all the same thing).
Probably easier to just know your L/D max airspeeds for various gross weights and then fly those. I think the airspeed indicator is much more precise than colored squares on the AOA indicator.
Probably easier to just know your L/D max airspeeds for various gross weights and then fly those. I think the airspeed indicator is much more precise than colored squares on the AOA indicator.
Juvat
Member
Jimmie: I don't think I'm seeing what you're referring to on 15-10 and 15-11 of the Skyview 7.0 installation guide. That section only talks about pressure check and calibration of the AOA system. I don't see anything discussing AOA as it relates to best glide speed. Please elaborate. Thanks.
Juvat
Member
Here's another tidbit for everybody. Once you find your L/D max (which is also best glide and max range), you can also calculate the airspeed for minimum power required (also known as max endurance speed). This is .76 x L/D max.
Juvat
Member
Oh, and here's another one for you. Optimum fuel flow or Optimum Cruising Speed (also known as "Carson's Speed"). This is V-L/Dmax times 1.316.
Juvat
Member
Okay, one more trick and then I need to go to bed. An easy way to calculate V-L/Dmax without doing any math is to just use the nm/gal calculation that our Skyviews already do. Assuming no wind, play with the power until you achieve a stabilized maximum nm/gal value. This is the V-L/Dmax for that given weight. Whatever you see on the AOA gauge is the V-L/Dmax...forever...regardless of weight.
One problem though, if I recall correctly, our Skyview AOA lacks enough resolution to be useful in finding V-L/Dmax. I'm not sitting in my airplane right now though, so I could be wrong. Can anybody at Dynon please help elaborate on this?
An easy way to calculate V-L/Dmax without doing any math is to just use the nm/gal calculation that our Skyviews already do. Assuming no wind, play with the power until you achieve a stabilized maximum nm/gal value. This is the V-L/Dmax for that given weight. Whatever you see on the AOA gauge is the V-L/Dmax...forever...regardless of weight. One problem though, if I recall correctly, our Skyview AOA lacks enough resolution to be useful in finding V-L/Dmax. I'm not sitting in my airplane right now though, so I could be wrong. Can anybody at Dynon please help elaborate on this?
Juvat
Member
So I've been thinking about our AOA indicator on the Skyview. It occurred to me today that it's really useful as an indicator of approach to critical AOA (ie: stall). But it seems to lack the resolution needed to be useful as an indicator of your actual angle of attack, measured in degrees.
Here is another technique. If you just measure the difference between the flight path marker (the circle with the three tick marks) and the nose pitch, you get the angle of attack in degrees. It's all right there on the primary flight display. So for example, if my pitch is at 2.5 degrees nose up, and the flight path marker is on the horizon (0 degrees) then my AOA is 2.5 minus 0 = 2.5 degrees of AOA. If I slow the aircraft, pitch will increase to maintain level flight. AOA increases until reaching the critical AOA, at which point the airfoil will stall. This will be displayed on the primary flight display as the distance between pitch angle and the flight path marker increases.
You don't have to be in level flight either to measure your AOA this way. You can be in an inverted dive and it still works. Subtract the flight path marker degrees on the pitch scale from the nose pitch in degrees, and that's your AOA at that moment.
Here is another technique. If you just measure the difference between the flight path marker (the circle with the three tick marks) and the nose pitch, you get the angle of attack in degrees. It's all right there on the primary flight display. So for example, if my pitch is at 2.5 degrees nose up, and the flight path marker is on the horizon (0 degrees) then my AOA is 2.5 minus 0 = 2.5 degrees of AOA. If I slow the aircraft, pitch will increase to maintain level flight. AOA increases until reaching the critical AOA, at which point the airfoil will stall. This will be displayed on the primary flight display as the distance between pitch angle and the flight path marker increases.
You don't have to be in level flight either to measure your AOA this way. You can be in an inverted dive and it still works. Subtract the flight path marker degrees on the pitch scale from the nose pitch in degrees, and that's your AOA at that moment.
The AOA indicator only reads the angle of attack. It cannot tell you anything about the lift curve or drag curve of your particular airfoil. You will need to do your own flight testing to calculate the L/D max airspeed for a given weight. Then go fly the airplane (at the calculated weight). Fly at the L/D max airspeed and reference the AOA indicator. That's now your max glide, max range, L/D max AOA (they're all the same thing).
Probably easier to just know your L/D max airspeeds for various gross weights and then fly those. I think the airspeed indicator is much more precise than colored squares on the AOA indicator.
OK, let me ask this another way. What angle of attack, as indicated on the DYNON AOA after calibration, is close enough to max L/D to be useful in our type of flying?
Our AOA calibration routine and the resulting AOA display was designed with simplicity in mind. You show it what a low AOA looks like, and that's the "full bar". You show it what high (critical) AOA looks like, and that's the other side of the indication where you're getting the flashing chevron. The rest of the AOA gradient is then smoothly interpolated in between those points.
The trade-off for this simplified calibration procedure is that the precise L/D AOA for the airfoil isn't explicitly determined during that procedure. It's surely possible to to have such a calibration procedure, but the procedural and software complexity goes up quite a bit. In fact, our sister company - AFS - has such a procedure for their line of AOA probes (see http://www.advanced-flight-systems.com/Support/AOAsupport/AOA%20Manual%20rev4.pdf for the nitty gritty details).
So without that facility, you can still do the normal Dynon calibration procedure, and then determine best L/D using another more conventional method (of which there are a few available via various EAA articles) to arrive at a best glide speed for your aircraft. Once you have that, simply note the state of the AOA indicator, and that's your best L/D AOA. Note that that particular AOA indication will vary from airplane to airplane.
The trade-off for this simplified calibration procedure is that the precise L/D AOA for the airfoil isn't explicitly determined during that procedure. It's surely possible to to have such a calibration procedure, but the procedural and software complexity goes up quite a bit. In fact, our sister company - AFS - has such a procedure for their line of AOA probes (see http://www.advanced-flight-systems.com/Support/AOAsupport/AOA%20Manual%20rev4.pdf for the nitty gritty details).
So without that facility, you can still do the normal Dynon calibration procedure, and then determine best L/D using another more conventional method (of which there are a few available via various EAA articles) to arrive at a best glide speed for your aircraft. Once you have that, simply note the state of the AOA indicator, and that's your best L/D AOA. Note that that particular AOA indication will vary from airplane to airplane.
Juvat
Member
Folks, I think there's an important point being missed here. When you are flying at V-L/Dmax, the airspeed is so high and the AOA so low that you will just get a "full" green AOA indication on the Skyview. As I stated above, there most likely isn't enough resolution on our AOA indicator to be useful for L/D determination. Please correct me if I'm wrong, but this is my experience so far.
Best to just calculate your own L/D, write it on a piece of paper and memorize it. Add or subtract a couple knots for various fuel weights, and you're good to go.
Best to just calculate your own L/D, write it on a piece of paper and memorize it.
Add or subtract a couple knots for various fuel weights, and you're good to go.Couldn't agree more. The weight variation in these aircraft is not large enough to make more than a few knots difference. I would use the max weight figure once it's been calculated. This gives better penetration into any headwind and avoids ending up on the wrong side of the drag curve - always better to be a few knots fast rather than slow. The theoretical loss of range at lighter weights is minimal.
I'm afraid that the earlier poster who suggested that the AoA is the difference between FPV and pitch attitude has over-simplified the situation and failed to take into account other factors such as angle of incidence.
I'm afraid that the earlier poster who suggested that the AoA is the difference between FPV and pitch attitude has over-simplified the situation and failed to take into account other factors such as angle of incidence.