As a continuation of the series of videos I have been making I put together a vane type Angle Of Attack indicator and filmed it. It is interesting to see. I have seen some speculation and some calculations predicting what he AOA is which has a number of dependencies' which make it a bit tough to know precisely. As has been said many times before one good test is worth a thousand calculated opinions.

If the markings are degrees I am both surprised and not surprised at how small a change in AOA creates enough lift to do, well everything.  Quite honestly, I was expecting more and given the number of markings on the gauge, so were you.  The SV is a really good plane.  Is yours powered enough to maneuver cross wind?  I am curious how much difference air speed makes.  I would suggest putting an airspeed gauge on that mount you cooked up *in a day* but you might just do it, so I won't.

Great work - Ken

I need  to make a point here that the AOA indicator is not in the perfect location and does have some influence from the flow field of the wing airfoil.

Yup.  It's hard to find a good place for one, isn't it?  You might be able to calibrate that location by testing it mounted on a car or something. 

One thing the AoA indicator doesn't measure is the actual angle of attack of the cambered wing with flap deflected. So you have that, the tip effects, and inertial effects with which to contend. I think you can accommodate these somewhat with different placement (it can be inboard) and addition of a flap indicator not aligned with the A0A indicator's span-wise position.

You are confounding the measurement by using an unconventional definition of angle of attack as if it has some physical significance, which it doesn’t.

if it were say a few inches out from the tip , there might be no influence from the wings airflow .

It would need to be about two wing chords away to be fully clear of the influence of the wing in forward or lateral directions. Or I can simply run an aerodynamic model and calculate the flow field and make a correction. Which I've done. The correction turns out to be about 1.5 to 1.8 degrees.

What its really recording is vertical yaw , or  mush . To be technical , or not . We could invent a word if there isnt one .

We have one, it's called angle of attack.

And yes , WING A.o.A. is entry ( seperation point ) to departure point ( Flap T E ) so a few more fings visable'd give more data .
A vertcal comb at the L E ( in front off ) with say fluro coton ' teeth ' might capture ' the Split ' of airflow forward .

No, AOA is measured With Respect To chord line of the parent airfoil by definition and this is how I use the term. Anything else is gobbledie gook gibberish.

Nother thought is the olde Pitot Tube placement & correction ( pressure ) factors , as relevant . But good effort there , thanks .

Pitot tube systems are used on airplane to measure AOA this is true and they require installation calibration which is also true. Both are more complicated than necessary and beyond the level of effort I wish to embark on. I do encourage you to take on such a task as it would be very interesting. I once made a device to record the altitude change of my FF models. It wasn't very useful as the data was very tough to adequately evaluate.

You are confounding the measurement by using an unconventional definition of angle of attack as if it has some physical significance, which it doesn’t.

Thanks.

Mark - I see better what you are trying to do and will be interested to see what the  placement on the "stick"  yields, assuming that you have a good model of the flow ahead and radius of turn doesn't create a measurable difference.

SK

Thanks.

Mark - I see better what you are trying to do and will be interested to see what the  placement on the "stick"  yields, assuming that you have a good model of the flow ahead and radius of turn doesn't create a measurable difference.

SK

I guess you could balance the vane aft of the pivot point to compensate for Nz, then servo the scale to compensate for pitch rate.

I guess you could balance the vane aft of the pivot point to compensate for Nz, then servo the scale to compensate for pitch rate.

Possibly. Good input. I'm not sure about the magnitude though. For the time being simply having a good look at it is good. The air data AOA / Pitot probes are balanced on the pivot. Of course the majority of my FT experience is with Helicopters and only two of those were aerobatic and part 25 jets which aren't. The turbine conversion T34 work I did was to sit in the back seat and take hand data for our inverted flight capability on the Allison B17 engine.

....11 G's which is what is required to make a 15 foot radius corner.

And you want me to do a 10 footer? 

Seriously, this is some really good stuff.

Ken

Mark, thanks for showing that. It's very helpful. Neat software!

SK

I made modifications to the AOA probe today. I also made a flap position indicator so I could sort out the flap position as well as someone suggested and I was figuring as well.

Dunno if this is one of the mentioned methods, but you could always film the plane then calculate the acceleration & speed from the video record.  That plus the weight of the plane and the wing area would be enough to work backwards to CL.

Or, if anyone happens to have or is actively working* on a device to record acceleration in flight, you could use that to get lift, then a speed estimate + wing area to get CL.

* Unlike me -- I got started on such a project with the "big TUT", but it's currently on hold 'cuz I'm working at a startup and I'm busy.

Thanks Tim

Filming has been done quite a few times and it doesn't really yield good AOA results although it does put a good value on turn radius. Acceleration is easy to calculate from that and get a maneuvering Cl. It's simply V^2/(r*G) x Cl for level flight which yields around 1.3 for an 18' corner. Or I could plug it in to my software and get 1.23. Both are useful to find a value for a section to design. The next criteria would be at which AOA do I design to. Measuring the acceleration does not yield that answer. The reason this is important is that there are a thousand plus solutions. Ultimately, I could constrain the design criteria and decide that I would use 8 degrees, the angle at which a NACA 0020 with a 20% flap deflected 27 degrees needs to turn that corner. This will result in the -1 sigma current PA model. That's not what I am doing as I already have one of those left and I have plans for several really good airplanes which fit that criteria. I could use any one of those and graft a new tail and canopy on and call it my new design. That's not what I'm doing.

I have a couple GY521 boards and some code to interpret them but I'm not really very good at that stuff. I could get that data if I thought I really needed to. Howard suggested that the rate could be accounted for but the video I have indicates the vane is following fairly well and is catching up within a very short period, see the few oscillations in the transients.

Ultimately too much effort on this isn't truly worth it, reason being is that the design criteria I'm looking for does not need to come from empirical data although some does. This effort was almost a side curiosity. Again my sight device didn't start out with any AOA intent. I just had the "while I'm at it, wouldn't it be interesting to..." insert whatever. I just wanted to confirm what I could already calculate. 

There have been several discussions on why flaps on PA airplanes. Well the best reason is that an airplane with flaps presents better to the judges. An airplane without flaps has to fly at a  higher AOA, 14 vs 8 to get around the same corner and will have a greater angular distance to pitch in order to orient with the flight path trajectory. This results in the non flapped airplane having the appearance of bobbling on the exit if the airplane can even make the corner. Suffice it to say with flaps that delta between trajectory and alignment is less for a flapped airplane. I am working on a design to reduce that 8 degree AOA in the maneuver to 0-1 degrees which will result in the airplane flying through the corner tangent to the radius and have zero trajectory pitch differential and hopefully crisper exits.

The current design airplanes use smallish 20%-25% flaps and there have been some deviations to this generalization. A wing with such a configuration cannot achieve the result I am working on. This is why I did the tufting to determine empirically what the airfoil analysis tells me, the flaps begin separating around 25 degrees. There are two benefits to using a large flap deflected shallowly, first being it can be designed to achieve the stated objective and two, it will have some reduction in drag. Not a huge reduction but within the realm of noticeable 0.021 vs 0.016 kind of thing.

There is a drawback to making flaps larger and that is the hinge moment becomes quite significant. There are methods to address this. See my video on the spades. I didn't do that randomly and it was an effort to see if I had a mitigation I could employ if I were to build this machine I am considering. The answer to that is yes and I scrapped a wing design with a 25% chord flap as result and I' simply going to move to a significantly greater chord flap. This impacts the structure and consequently the airfoil selection. Therein lies the basis of motivation in this effort. I basically need to understand the operating conditions to evaluate in under to design an airfoil section which can host the necessary wing spar for this beasty.


 

Science marches on.

So when you say "camera testing" you mean you're checking how the camera does with rough handling, not that you're testing with a camera?

I'm not certain I understand the question. The first flight video I made with the wing tufts, the camera flew off and impacted the ground. The video is on here some place. It came off in the last quarter of a looping maneuver and hit the ground fairly soon after that. It was a good impact and certainly tested the damage tolerance of it. This is the camera I referred to and it seems to work now. The likely reason it didn't on last nights flight was me inadvertently activating the off button when I put it on the mount. In the flight the other day a different camera fell off, like the result of older command strip and not cleaning the new mount I made of well enough. That camera remains in the field and likely will be as it was the only camera onboard and I couldn't make an accurate determination when it flew off. This one also came off in level flight and likely had a time of flight of 1.5 seconds doing 78 fps. A pretty big search area. My testing is not to test the camera but to gain a better understanding of the airflow on the wing.

For instance, when I do the evaluation of a NACA 0020 with 20% flaps deflected to 27 degrees I get the result that separation is occurring around the 80% chord position at a Cl of 1.3 and 8 degrees angle of attack. Great to know mathematically.  However, how accurate is that? Well, put tufts on the wing, strategically place some cameras onboard to watch and fly some maneuvers to see. The answer is without VGs, pretty much. VGs do seem to help with that part although handling qualities wise I can't say for sure. That needs some A-B-A tests on the same day preferably back to back which is not what my method was. The only thing I noticed there was a slight reduction in line tension, probably from the inside VGs having different alignment than the outboard.

So the effort of all of these videos is one, it's interesting and two, I'm using it for information and guidance on the next airplane I'm going to build. I'd probably build an Impact or a Crossfire but I have some ideas and experience to bring to the effort. The knowledge I've gained in all of this has helped me understand some really interesting aspects. One of them is a fuse mounted camera is a great tool to understand the airplane trim. With it I've made a couple very small trim adjustments which have helped a lot and I have another mechanism in the idea chain.