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Author Topic: Some videos of a tufted SV11  (Read 846 times)

Offline Mark wood

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Some videos of a tufted SV11
« on: November 07, 2021, 12:14:54 PM »
I put some cameras on an SV11 to see what is going on. As it turns out some interesting flow field things are happening. Lots of questions can be answered in these. The conditions of the day were a wind of 8 - 10 MPH existed. The impact can be seen on the fuselage up by the propeller. The yarns can be seen to move as the airplane goes around the circle in response to the wind. The yarns I used are a bit heavy as they detach during negative G operations. I have some 280 denier thread which I will replace them with.

What drove me to do this is I was interested in a couple of things. First the flow field on the airplane which I make the primary assumption that the airplane is flying tangent to the circular flight path. This is the assumption I made in my fastest jet models when I calculated the required wing sweep using the assumption that the fastest speed would be achieved when the engine is tangent to the circle creating the greatest thrust condition. Same assumption for the stunt model, it will behave the best when flying tangent to the circle.

The second thing that drove me to do these tests is the flap deflection. My intuition is that the flaps are being deflected too much and separation is occurring causing high drag during maneuvers. This is confirmable by watching the tail cam videos. The first order fix is to reduce the flap deflection as Igor does with the log flap drive device. The question then becomes how much is too much which is to say at what angle does the onset of separation occur? It's clear in the videos that there is significant separation.

In these first videos a number of things can be seen besides the separation. Things like how much spiral slipstream is there in flight? That question comes up a lot. Watch the end of the flight when the motor shuts off. There a yarn up front right behind the propeller. How much impact does hinge gap have and how well does sealing help? There are yarns that bridge the gap. It is clear that the there is leakage through the gap in the A flight and in the B flight the leakage is reduced. Flying wise I hardly notice the difference but maybe a master pilot can notice it. Watch the separation of the flaps between the flights to see if you can tell the difference, that's where you could tell. Turbulators could help maybe. Or vortex generators, I know there are some videos of those which Howard told me about. I may give them a try.

I am working a different avenue based on some analysis that I have done with larger flaps with reduced deflection to achieve a similar Cl at a baseline AOA. Yeah, I know, there is a debate over what the meaning of AOA is with flaps deflected is but I use the term the same way we would collect data in the wind tunnel is done. Set the base airfoil in and deflect the flap which offsets the lift curve.

The wing I'm working has a slightly larger flap of approximately 22% total chord and has an offset hinge line in order to reduce the increased hinge moment. The hinges are made in a manor that they can be readily exchange for longer or short ones which can accommodate a different hinge offset and / or larger or smaller flaps. From some previous analysis I performed 25% flaps would be a good target and their deflection would be fairly low. In order to accommodate this requirement I have designed a variation of Igor's log flap drive that the output ratio can be modified relatively easily.

Here are the video links:














Life is good AMA 1488
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Offline Tim Wescott

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Re: Some videos of a tufted SV11
« Reply #1 on: November 07, 2021, 01:06:25 PM »
Most important question first -- how do you keep your dog from walking through your lines?  I always keep dogs away from my CL planes, for fear that they'll get tangled in the lines and cut up their hocks.
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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #2 on: November 07, 2021, 05:26:01 PM »
Most important question first -- how do you keep your dog from walking through your lines?  I always keep dogs away from my CL planes, for fear that they'll get tangled in the lines and cut up their hocks.

Well, there's an assumption built in to that question which may or may not be correct. She does occasionally tangle in the lines but not horribly usually when we're just getting on the way out, no worse than us humans do. I keep a fairly close eye on her when we are going out and she does a good job of healing to the side I tell her. Callie is a mini Australian Shepard AKA Velcro dog and generally stays close by. Sometimes known as a PITA dog. The real trouble is she loves to go flying with me and it's really tough to leave her behind. Even when I fly out back of the house. After we have been out for a while she will sit and wait outside of the circle.
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Offline Tim Wescott

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Re: Some videos of a tufted SV11
« Reply #3 on: November 07, 2021, 06:50:26 PM »
Some general thoughts mixed with a question or two.

How do you know there's separation?  I'm seeing significant span-wise flow, with the tufts pointing in toward the fuselage on outside corners -- is that what you're talking about?  How do you know it's not air "bleeding" off the fuselage end of the flap?

My understanding of the point of sealing your surfaces isn't so much that they do something dramatically good, as they go a long way toward making all the surfaces act the same.  Just a little bit of difference between gaps makes is felt to make a lot of differences between the lift generated by the associated surfaces.  I haven't gone out and tested this myself -- this is one of those areas where I just do what the "big boys" say is right, and I don't ask questions.

I've seen the suggestion -- and, I think, pictures -- that to combat this you can make really big hinge gaps.  Like, 1/8" or so.  The theory is that 0.01" of precision is hard to achieve with wood, and if your overall gap is 0.02", then 0.01" is a huge percentage of that, while 0.01" out of 0.125" isn't much at all.  You may want to consider this with your balanced surfaces if you run into funny trim problems.
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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #4 on: November 07, 2021, 09:19:27 PM »
Some general thoughts mixed with a question or two.

How do you know there's separation?  I'm seeing significant span-wise flow, with the tufts pointing in toward the fuselage on outside corners -- is that what you're talking about?  How do you know it's not air "bleeding" off the fuselage end of the flap?

My understanding of the point of sealing your surfaces isn't so much that they do something dramatically good, as they go a long way toward making all the surfaces act the same.  Just a little bit of difference between gaps makes is felt to make a lot of differences between the lift generated by the associated surfaces.  I haven't gone out and tested this myself -- this is one of those areas where I just do what the "big boys" say is right, and I don't ask questions.

I've seen the suggestion -- and, I think, pictures -- that to combat this you can make really big hinge gaps.  Like, 1/8" or so.  The theory is that 0.01" of precision is hard to achieve with wood, and if your overall gap is 0.02", then 0.01" is a huge percentage of that, while 0.01" out of 0.125" isn't much at all.  You may want to consider this with your balanced surfaces if you run into funny trim problems.

That is a very good question which some of the answer is having done a lot of watching. The yarns in this application are too heavy. In the outside corners they pull away from the wing under heavy G's. That's not normal and part of why I am changing the yarn to lighter heavy thread. I would disregard most of the negative G as the yarns have exited the boundary layer, well it's interesting. The inward flow in high positive G is another interesting thing and it appears on both sides of the fuselage and I'm not sure why. I haven't seen that before, at least to such a degree but then this is a first time I've done this on a model. 

The place where I see what would be indicative of separation is when you see the yarns turn their tails around and flutter backwards. That occurs when the surface goes turbulent which is the separation and there is inflow from the TE. The bottom line is that when the flow is attached and flowing smoothly the yarns are pretty much straight. When they start bending is because there is turbulence.

In some of the maneuvers I did after the pattern sequence I really pulled hard but wasn't hard enough to get the full wing to separate. That's probably a good thing. I've moved the CG forward and the elevator no longer has the authority to drive the wing to a full stall consistently. Have a look at my full size Laser snap roll video that shows a full on deep stall and watch how the yarns respond. In that situation almost all of the yarns are pointing forward or up.

Your input on gap sealing is probably correct. I'm actually very fastidious about my hinge gaps and that I don't notice a difference is likely because of that. Depending on the model I'd say my average gap when done the way I want is .01 or less, certainly less than .02. For the new wing the gap seal is a 3D printed part. The LE of the flap will likely be a molded balsa or CF nose cone. I have some existing D tube tools that may work or a new one can be created easy enough. I don't know yet as I'm still doing my engineering geek thing where I totally over think every single detail and fix every single bit until I get frustrated and just start building. In the mean time I'm making interesting data.
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Offline Dennis Toth

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Re: Some videos of a tufted SV11
« Reply #5 on: November 08, 2021, 06:14:40 AM »
We have a local flier that had an old Chief that had 1/8" flap gap and the ship flew and turned fine. It seems that the biggest impact for unsealed hinge lines is when the gap is tight. As Tim points out a small surface  variation is a big % of these tight gaps. What the leakages can do on some ships is cause roll as the flaps are engaged were one side leaks more than the other. Its like having to big a wing offset (like the All American OTS ship) where the ship flops with the inboard wind high as it goes through various parts of the pattern. This was the best reason I have heard for sealing the hinge lines apart from getting better control response for less surface deflection.

Best,     DennisT

Online Ken Culbertson

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Re: Some videos of a tufted SV11
« Reply #6 on: November 08, 2021, 09:17:36 AM »
This was the best reason I have heard for sealing the hinge lines apart from getting better control response for less surface deflection.
I would say those are two really good reasons.  If the plane has perfect gaps then I doubt it will matter much.  I seal mine because I use a hinge rod which forces the gap to be wide enough for the wire.

Still working on sealing imbedded flaps.  I am going to try a "squeege" at the centerline next.  No contact at neutral, slowly closing with deflection.  Sticking is only an issue when there is no control load and leaking only occurs when the flaps are deflected so a slightly elongated flap bay should work - or not.

I still am not sure that recessed flaps need to be sealed.  Right now my recessed elevators seal at about 15 degrees mechanically and the flex of the very thin monocoated TE keeps them sealed through 45 degrees with no apparent added pressure.   I would think that a very thin flap on the top and bottom could work.  I think this has been tried but I am not sure it was with recessed flaps.

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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #7 on: November 08, 2021, 12:15:25 PM »
Here's a picture of what I am planning to do. The seals are 3D printed polycarbonate. I haven't yet decided what the hinges themselves will be but polycarbonate would work as well. The LE of the flap will be something like a fiberglass D Tube. The hinge post will be pinned in place and the pivot wire will be full length of the flap. The hinge posts can be made in differing lengths which can then be fitted with different flap configurations. In this variation the hinge offset is a constant dimension which works out to be 25% of the flap chord at the tip and 17% at the root. The flap deflection will be limited by the log crank to less than 27 degrees and the nose of the flap won't un-port. I haven't yet integrated the log crank assembly into my airplane model as of yet.
Life is good AMA 1488
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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #8 on: November 08, 2021, 12:27:45 PM »
Another view of the log flap crank detail. It was too large to include in the previous post. My iteration of the log crank turns the flap drive in to a new lever and puts turnbuckles in the system to drive the flaps. I have a functioning version made from CF Nylon and should be super capable. Both levers can be replaced to change the ratios as desired.
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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #9 on: November 13, 2021, 07:16:19 AM »
Some general thoughts mixed with a question or two.

How do you know there's separation?  I'm seeing significant span-wise flow, with the tufts pointing in toward the fuselage on outside corners -- is that what you're talking about?  How do you know it's not air "bleeding" off the fuselage end of the flap?


Tim

I meant to respond to this and took some time to ponder more deeply. Here's what I believe is happening. I too saw that inward flow and it is very curious. So if you go and watch some of my 4/4 Laser videos looking out at the wing you'll notice that the ailerons don't billow as we would expect if the pressure were lower outside than inside of the aileron. Instead the fabric is pressed inward indicating the pressure is lower inside and higher outside. This is because the ailerons are driven by torque tubes that are connected to the Cassutt style "monkey motion" mixer inside the fuselage. The fuselage is at a lower pressure than the surrounding static pressure so the pressure on the ailerons cause the compression. People are often surprised to find that the wind in a leaky canopy hits them in the back of the head. It's also the primary source of pitot static system errors and why, during flight test, they use a trailing instrumentation "bomb" on a very long tether behind the aircraft.

What my assessment of the SV11 tufts moving inward is that pressure field around the fuselage creates a lower static pressure than the TE of the wing. This makes sense although it is counter intuitive. At some lower angles the flow inward towards the lower pressure region is what can be expected. Admittedly, I haven't seen such a great amount of this flow on other testing but then I've never specifically looked for it either. I'm fairly certain I've never run tufts along the TE either and without them there I wouldn't have seen it.

One part of me initially said it is part of the fact that the airplane is traveling in a curved path and that is the driving cause. However the flow inward is span wise symmetric and both sides indicate inward flow especially since all of the other tufts indicate the flow field being straight inline with the flight path. One of my reasons for doing this testing was that I wanted to verify the flight orientation which I believed to be basically tangent to the circle. It appears that this assumption is correct and that it would be interesting maybe to cause the airplane to skid and repeat the flight videos for skidding flight.

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“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online frank williams

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Re: Some videos of a tufted SV11
« Reply #10 on: November 13, 2021, 08:20:11 PM »
Hi Mark

I just bumped into your threads here in Stunt Design.  I don't get down here too often, but I should for sure from now on.
I really like what you are doing, here and in the thread on precession.  We can guess what things are doing, but any experiment at all will shed valuable light on the subject of interest.  Keep it up.

A couple of things ..... the spanwise flow, inboard on inside maneuvers and outboard on outside maneuvers that you see is to be expected.  Its an "artifact' or result of the tip vortex strength influencing the overall flow field. Pull a maneuver and the vortex increases in strength and begins to show up on the wing flow.

I do think i can see less stall on the sealed video.  I don't know if it was enough that you could feel it..

You can see the wind effect on flow direction over the wing surface.  A couple of degrees as you go around the circle.  Interesting.

The cameras you are using look kinda  .... shall we say .. heavy.  Have you ever seen a "key fob" camera?

https://www.ebay.com/sch/i.html?_nkw=key%20fob%20camera%20hidden&norover=1&mkevt=1&mkrid=711-34002-13078-0&mkcid=2&keyword=key%20fob%20camera%20hidden&crlp=_5087&MT_ID=&geo_id=&rlsatarget=kwd-77172152592899:loc-190&adpos=&device=c&mktype=&loc=73428&poi=&abcId=&cmpgn=395437759&sitelnk=&adgroupid=1234751765500201&network=s&matchtype=p&msclkid=a8dc473190f6120411be26a1eca6ce48

They have an iphone camera I think and only weigh about an ounce.  Excellent video and high quality for no weight penalty.  ... and inexpensive.

I'll study these videos a bit more and see if anything stands out.

But ...... keep it up .... glad to have more people looking for answers to why it works like it does.  (I'll comment on the precession videos over on that thread tomorrow.)

Oh yes ..... you left out the triangles on the second video .... sorry no pattern points1



« Last Edit: November 13, 2021, 09:07:45 PM by frank williams »

Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #11 on: November 14, 2021, 09:58:54 AM »


A couple of things ..... the spanwise flow, inboard on inside maneuvers and outboard on outside maneuvers that you see is to be expected.  Its an "artifact' or result of the tip vortex strength influencing the overall flow field. Pull a maneuver and the vortex increases in strength and begins to show up on the wing flow.

I do think i can see less stall on the sealed video.  I don't know if it was enough that you could feel it..



You are probably correct about the vortex flow driving the spanwise motion. Lighter tufts would help seeing the negative G conditions which I plan on doing. The tip vortex is very clear in the outer tuft. However I still think the flow field along the TE is influenced by the total pressure distribution.

My jury is out on the sealing aspect but there may be some delayed separation. The "stall" isn't really a full stall and more separation on the surface which creates lots of drag. That isn't so noticeable in behavior but in energy on the exit. The take away is that a larger flap deflected less for a given Cl would be less likely to become turbulent thus has less drag and loose less energy.

The plot you have is interesting but 1/32 gap is a lot, mine are more like 0.01" and less on the stab where I can run the hinge pin full length. Honestly that most likely accounts for why I don't see much difference on models. We have to be careful when looking at charts like this one. It's really interesting to see the whole thing but generally we don't operate quite that way, especially on the tail plane. For a given maneuver we fly at a given Cl which on your chart would be moving horizontally between curves which results in just a couple degrees difference in elevator deflection and likely less for a smaller gap. As you say, probably not noticeable but maybe. BTW, what was the source? Nice work.


You can see the wind effect on flow direction over the wing surface.  A couple of degrees as you go around the circle.  Interesting.


Yeah, probably the most enlightening part of the whole video. Surely not what I would have anticipated but after watching, the "oh, I get it, of course.." falls on your brain. What I find particularly interesting about that is the airplane doesn't seem to yaw much in response. Of course, that is difficult to pick out but this has occupied a lot of my thinking time. I think that this is a practical demonstration of how significant the line placement is and unless the fin is very large the line tension is the dominating factor. Add a test case for the future, move the line around and fly.


The cameras you are using look kinda  .... shall we say .. heavy.  Have you ever seen a "key fob" camera?


Looks are deceiving. I took the wife's Keyfob out and compared it's larger and almost twice as heavy, 45g vs 27 g. I paid $19.95 for the cameras I'm using which we bought for when we were doing gun shows to monitor our wares as such for my use now they are basically free. We have smaller ones but they required a separate battery and don't have the resolution.


Oh yes ..... you left out the triangles on the second video .... sorry no pattern points1


I did that intentionally to see if anyone was paying attention. ;) In reality, I am super good at leaving maneuvers out.
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Offline Dave_Trible

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Re: Some videos of a tufted SV11
« Reply #12 on: November 14, 2021, 06:12:39 PM »
This is fascinating.  Something I'm curious about:  A few years ago Chris Rud did something similar with the tuffs and a camera.  Something was different though in those tests that stick in my mind that wasn't really mentioned in those tests or since but is very different here.  In Chris' test the tuffs that passed over the hinge line were sort of sucked into the hinge line, traveled a short distance toward the tips then jumped back out for the last bit to follow the slip stream again off the TE of the flaps.  The tuffs looked like a 'Z' .  I assumed there was a vacuum formed in the hinge line drawing the air (and the tuff) down in.  Maybe those tuffs were more fine and lighter-don't know.  Here, the trailing edge appears rather square.  I'm wondering if this is creating a shear rather than a vacuum.  It would make me think a vacuum might help re-attach the boundary layer to the flaps and make them more effective.....a guess.  I think Chris's trailing edge was more rounded.

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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #13 on: November 14, 2021, 06:28:32 PM »
This is fascinating.  Something I'm curious about:  A few years ago Chris Rud did something similar with the tuffs and a camera.  Something was different though in those tests that stick in my mind that wasn't really mentioned in those tests or since but is very different here.  In Chris' test the tuffs that passed over the hinge line were sort of sucked into the hinge line, traveled a short distance toward the tips then jumped back out for the last bit to follow the slip stream again off the TE of the flaps.  The tuffs looked like a 'Z' .  I assumed there was a vacuum formed in the hinge line drawing the air (and the tuff) down in.  Maybe those tuffs were more fine and lighter-don't know.  Here, the trailing edge appears rather square.  I'm wondering if this is creating a shear rather than a vacuum.  It would make me think a vacuum might help re-attach the boundary layer to the flaps and make them more effective.....a guess.  I think Chris's trailing edge was more rounded.

Dave

I think what you are noticing is the reason why the squared off edge has become so prevalent as opposed to how we did it as kids. Back then we made our hinge line round because we used either cloth or thread to make hinges with. I can't speak to how his videos looked directly but the coanda effect will cause the airflow to follow the curve and draw the air downwards.  What may be happening in the previous video is transitions between the hinges. My yarns are definitely to heavy and stiff for the application. I have some 280 denier thread which will follow better. I haven't had the opportunity to do some checking with the camera to see how well it picks them up but they should be able to provide more insights. My inner regular guy is being totally overthrown by the geek child.
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“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online frank williams

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Re: Some videos of a tufted SV11
« Reply #14 on: November 14, 2021, 07:38:48 PM »
Interesting .... the thread that Chris used was thread that I sent him .... he initially used heavy yarn too.

The data came from my garage wind tunnel.  Frank McMillan published it in MA.  The data says that for the same force from the unsealed elevator / stab will require slightly more elevator deflection (drag).  A 12 inch diameter section that I can partition off with some vertical plates for two dimensional foils.  Top speed about 50 mph, turbulence  not too bad.  Two axis force measurements lift and drag.  No moment.

You win on the camera.  I'll look into those.

Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #15 on: November 14, 2021, 09:08:01 PM »
Interesting .... the thread that Chris used was thread that I sent him .... he initially used heavy yarn too.

The data came from my garage wind tunnel.  Frank McMillan published it in MA.  The data says that for the same force from the unsealed elevator / stab will require slightly more elevator deflection (drag).  A 12 inch diameter section that I can partition off with some vertical plates for two dimensional foils.  Top speed about 50 mph, turbulence  not too bad.  Two axis force measurements lift and drag.  No moment.

You win on the camera.  I'll look into those.

That's way better than the one I built in my apartment when I was young. I would probably never sleep if I had one around here in my shop. I already do too many things.
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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #16 on: November 18, 2021, 11:57:17 PM »
Hi Mark

A couple of things ..... the spanwise flow, inboard on inside maneuvers and outboard on outside maneuvers that you see is to be expected.  Its an "artifact' or result of the tip vortex strength influencing the overall flow field. Pull a maneuver and the vortex increases in strength and begins to show up on the wing flow.


Spent some more watching and I'm in basic agreement the vortex flow is definitely at play here. Particularly at the fuselage junction. The inside tufts can be seen rolling in it at times clearly showing why fillets work. Watch the spade flight videos they're interesting.
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Offline Mark wood

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Re: Some videos of a tufted SV11
« Reply #17 on: December 01, 2021, 08:40:17 AM »
I've been working on getting my new Smoothie ready to fly. Well, and attempting to reduce the massive chaos in my shop.. Those efforts have taken me away from the aero tinkering, well, not entirely. I was reviewing video for instances of yawing, which I found and will work on fixing with a rudder device as of yet to be determined. The airplane yaws a lot more than I thought it did on the outsides and not so much on the insides. Interesting and definitely a subject for discussion as to why this would be. So, I began with thinking about a sighting device which could be used to quantify the yaw. That lead to this idea, why not get Angle Of Attack as well. So, here ya go. It's on the way to the printer and will likely be installed before I head out to fly today. We have another nice day in KS today. Should be interesting to see the results.

Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman


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