Aerobatic Tandem Two-Seater

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christos

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Interesting design.
About airfoil, it is not necessary to choose symmetrically. Symmetrical is NACA 00xx series.

Also, what CFD model did you use? How many boundaries layers?

Best Regards!
Christos, civil engineer
 

Lendo

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Rad-dude 1234,
I'm very interested in your design, I'm aiming for a similar mission with a similar design. Please tell us what Riblett Airfoil you select and why. I've selected the 37A 415, but not aiming for T-34 level of Aerobatics, more mild aerobatics and aiming at providing strength of the 6G and the 415 for stall speed on LS.

What airfoils you using for the Tail feathers, an Aeronautical Engineer friend put me onto a good one NACA 0010-64, Symmetrical airfoil -I like it!
George
 

Radicaldude1234

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Interesting design.
About airfoil, it is not necessary to choose symmetrically. Symmetrical is NACA 00xx series.

Also, what CFD model did you use? How many boundaries layers?

Best Regards!
Christos, civil engineer
CFD was used for stability analysis. The only plan to use CFD is to identify any weird drag spots and to verify control surface loading. Stability analysis was done in Excel and then checked with XFLR5, which is wonderful program...that's also free!

Rad-dude 1234,
I'm very interested in your design, I'm aiming for a similar mission with a similar design. Please tell us what Riblett Airfoil you select and why. I've selected the 37A 415, but not aiming for T-34 level of Aerobatics, more mild aerobatics and aiming at providing strength of the 6G and the 415 for stall speed on LS.

What airfoils you using for the Tail feathers, an Aeronautical Engineer friend put me onto a good one NACA 0010-64, Symmetrical airfoil -I like it!
George
Being a converted Beech Bonanza, the T-34 level of aerobatics is not very high...

From the previous posts, I had more or less come to conclusion that Riblett airfoils are great for a cruising type airplane, but terrible for an aerobatic one due to the high pitching moment. That's why I threw out the Riblett airfoil and went with a more conventional NACA one.

As the professor in my Aerodynamics class once said, airfoil selection doesn't matter as much as you think it does!
 

Lendo

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Rad-Due 1234,
Well I know may newer aircraft have specific airfoils designed for specific applications/ missions to suite a specific aircraft design, so that last statement leaves me somewhat bemused.
Have a good read of Riblett's book and see what to expect from the different NACA designs.
Riblett has developed is designs after the 6 series NACA airfoils, so he thinks their OK. One of his biggest concerns about some airfoils are the sharp stalls.
I know a number of people designing their own aircraft preferring the Riblett, one an Aeronautical Engineer, another engineer who has done much research on Airfoils - however you may be more ambitious, with a more leaning toward Aerobatics, I would still like to know what you select - just out of curiosity.
George
 

Toobuilder

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WRT airfoil selection - the symetrical section used on the Hiperbipe was reportedly eyeballed on the hangar floor. TLAR all the way.

In my experience it performed just fine.
 

ScaleBirdsScott

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Gonna guess if you know the general shape and concept of a wing airfoil curve you can absolutely just sketch one out and go for it. It'll certainly fly.

Crunching numbers and optimizing those last few percentage points is for the brains that can wrap around the maths. Certainly something to gain from those last few points.
 
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christos

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I meant the CFD code. Did you run the k-omega SST model or something else, also what about the grid? how many cells and how many boundaries layers and how much thick?

If you used the auto option of your software, just make sure that it takes the correct values for your problem. For example, if it used k-omega SST, check k, omega, kinematic viscosity, y values etc.
Because of a low-quality grid and incorrect equations will give you a non-realistic cd, cl and the most important, stall behaviour, etc.

I like your project. Be careful with NACA 6 series, it is laminar flow airfoils, something excellent for high efficiency, but on the other hand, some of these have poor stall characteristics.
 

PiperCruisin

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Cherokees have NACA airfoils and have a very mild stall. Did a power on stall in a Warrior last week and it didn't do anything...just mushed along.

I would feel better about the Ribblet airfoils if I saw some actual test data and they had more significant digits in the graphs.

Aero engineers I've talked to all say something different. NACA, Riblett, custom, doesn't matter, use only something with test data...and on and on.
 

christos

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Cherokees have NACA airfoils and have a very mild stall. Did a power on stall in a Warrior last week and it didn't do anything...just mushed along.

I would feel better about the Ribblet airfoils if I saw some actual test data and they had more significant digits in the graphs.

Aero engineers I've talked to all say something different. NACA, Riblett, custom, doesn't matter, use only something with test data...and on and on.

Piper has naca65415, and combinate it with a good chord (the bigger the chord the higher the Reynolds number). As a result, the airfoil is efficiency in aircraft's speed range. It is not only the airfoil of the piper but the overall design.
It needs a more careful design a laminar airfoil aircraft and maybe a good washout (geometric or aerodynamic).
But it is more efficient and faster with better glide ratio (safer in an engine failure).
 

Radicaldude1234

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I meant the CFD code. Did you run the k-omega SST model or something else, also what about the grid? how many cells and how many boundaries layers and how much thick?

If you used the auto option of your software, just make sure that it takes the correct values for your problem. For example, if it used k-omega SST, check k, omega, kinematic viscosity, y values etc.
Because of a low-quality grid and incorrect equations will give you a non-realistic cd, cl and the most important, stall behaviour, etc.

I like your project. Be careful with NACA 6 series, it is laminar flow airfoils, something excellent for high efficiency, but on the other hand, some of these have poor stall characteristics.
I used the Flow Simulation module of Solidworks for some very basic drag estimates. XFLR5 uses the VLM1 method which can be described here: https://engineering.purdue.edu/~aerodyn/AAE333/FALL10/HOMEWORKS/HW13/XFLR5_v6.01_Beta_Win32(2)/Release/Guidelines.pdf

Even then, I only took very basic values like wing moment, drag, etc and plugged it into an excel sheet developed from an aircraft stability textbook. Basic lift and drag are relatively easy to calculate when compared to figuring out control and stability, especially for a super conventional design such as this one...and it's much more reliable and easier to calculate that stuff based on proven equations rather than rely on CFD.

WRT airfoil selection - the symetrical section used on the Hiperbipe was reportedly eyeballed on the hangar floor. TLAR all the way.

In my experience it performed just fine.
That's awesome!

...and on and on.
So back when I was working on my Mech Eng degree, one of the requirements to graduate was to participate in a group design project during my senior year. Mine just happened to be to design an RC airplane. Anyways, our team spent weeks upon weeks doing exhaustive analysis to pick the perfect airfoil....until the advising professor told us to just pick one so we could get on with our lives.

The moral of the story is that unless there are absolute performance goals like speed and range that you must hit, don't lose too much sleep over what airfoil to pick.

For reference, check out the airfoil on this RC model:

 

Lendo

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I see a lot of holes in statements and opinions here, Pipercruisen's statement is the most correct. It would be wonderful if all if all Airfoils could have the correct test date, other than that probably the closest is Eppler program and it's far from perfect.
Naturally very experienced people can draw a Airfoil on the hanger floor and it may be close enough i.e 'fit for purpose'. I suppose anything will fly with enough power, flat panels have been and still are used in tail feathers - however there is the issue of optimum lift with minimum Drag and beware the stall.

I think it's up to the designer to do the research and live with the decision, I'm no expert but I am following the lead of some very smart, very talented and very experience people.
All I can suggest is do the same and start with "Theory of Wing Sections" by Abbott and Von Doenhoff.
George
 

Toobuilder

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My point about the Hiperbipe airfoil was meant to illustrate that there is a lot more design work to do before you pick a specific example. This is not a transport category aircraft that needs to eke out the Nth % of efficiency to make or break it's ROI. We're also not iterating to make a specific range on a bomber or tanker with a fixed powerplant. This is a sport plane with literally dozens of well proven, off the shelf airfoils available that will perform so closely together that even an experienced test pilot would struggle to tell a difference. At this point in the design, airfoil selection should be narrowed down to "yes".
 

Topaz

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My point about the Hiperbipe airfoil was meant to illustrate that there is a lot more design work to do before you pick a specific example. This is not a transport category aircraft that needs to eke out the Nth % of efficiency to make or break it's ROI. We're also not iterating to make a specific range on a bomber or tanker with a fixed powerplant. This is a sport plane with literally dozens of well proven, off the shelf airfoils available that will perform so closely together that even an experienced test pilot would struggle to tell a difference. At this point in the design, airfoil selection should be narrowed down to "yes".
I think Billski's thoughts on inverted performance are well-taken but, otherwise, ... yeah. Don't sweat the airfoil choice on an airplane like this. You can do more to tailor post-stall behavior for a regular positive-G design with wing planform and twist than will be determined by any given airfoil.

If you're really serious about full aerobatic capability, bite the bullet and use a fully symmetrical section. If you're not, you could do a lot worse than the appropriate Ribblet section for your design.
 

christos

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I see a lot of holes in statements and opinions here, Pipercruisen's statement is the most correct. It would be wonderful if all if all Airfoils could have the correct test date, other than that probably the closest is Eppler program and it's far from perfect.
Naturally very experienced people can draw a Airfoil on the hanger floor and it may be close enough i.e 'fit for purpose'. I suppose anything will fly with enough power, flat panels have been and still are used in tail feathers - however there is the issue of optimum lift with minimum Drag and beware the stall.

I think it's up to the designer to do the research and live with the decision, I'm no expert but I am following the lead of some very smart, very talented and very experience people.
All I can suggest is do the same and start with "Theory of Wing Sections" by Abbott and Von Doenhoff.
George

Eppler code isn't interactive. Xfoil is a better choice (and interactive) for 2d analysis. Eppler code includes some empirical calculations (to simulate turbulence flow) and it is prone to errors.
Drella's Xfoil is a better solution.
But wing's analysis is a 3d problem and not a 2d, it is much better to use a real CFD program.
 

christos

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I used the Flow Simulation module of Solidworks for some very basic drag estimates. XFLR5 uses the VLM1 method which can be described here: https://engineering.purdue.edu/~aerodyn/AAE333/FALL10/HOMEWORKS/HW13/XFLR5_v6.01_Beta_Win32(2)/Release/Guidelines.pdf

Even then, I only took very basic values like wing moment, drag, etc and plugged it into an excel sheet developed from an aircraft stability textbook. Basic lift and drag are relatively easy to calculate when compared to figuring out control and stability, especially for a super conventional design such as this one...and it's much more reliable and easier to calculate that stuff based on proven equations rather than rely on CFD.



That's awesome!



So back when I was working on my Mech Eng degree, one of the requirements to graduate was to participate in a group design project during my senior year. Mine just happened to be to design an RC airplane. Anyways, our team spent weeks upon weeks doing exhaustive analysis to pick the perfect airfoil....until the advising professor told us to just pick one so we could get on with our lives.

The moral of the story is that unless there are absolute performance goals like speed and range that you must hit, don't lose too much sleep over what airfoil to pick.

For reference, check out the airfoil on this RC model:


About stability, a VLM program is ok. But it is better to check your aircraft around stall with a real CFD solver like yours, OpenFOAM (i prefer it) because you can see how it will operate after the stall.
If you go with the traditional (old day) way, you have to be more careful throughout the first flight test.
An RC model of the prototype is a good idea also.

Are you an engineer? I am also a civil engineer Mintg. How old are you?
Are you a pilot?
 

Radicaldude1234

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So I finally got around to punching in the numbers after switching the airfoil to the NACA 1415, which yielded these results:

Trim Forces Comparison.jpg

So the conclusion is that because of the lower moment on the NACA airfoil, the maximum tail lift loads (and therefore loads on the fuselage tail boom) are reduced by ~1500lbf and enables reduction of area for the horizontal stabilizer.

Plugging in numbers into the stability spreadsheet:

2020_0702_Case2_SP_200kts.jpg2020_0702_Case2_PG_200kts.jpg

Shows that the lower airfoil moment and the reduction of "decalage" (longitudinal incidence) still results in positive short period stability. As with before, the Phugoid mode is still mildly unstable but manageable, which is typical for light GA aircraft.

I'm debating whether going through the effort of analyzing roll stability, given the dihedral and rather large tail. Would be useful to find the Dutch roll characteristics though...

On the performance end, current estimates are:

O-470 (230 HP): Max Level: Sea Level 195kts (10K ft 250kts) Climb: Sea Level 2800ft/min (10K ft 2750 ft/min)
O-540 (300 HP): Max Level: Sea Level 205kts (10K ft 275kts) Climb: Sea Level 3850ft/min (10K ft 3790 ft/min)
IO-720 (230 HP): Max Level: Sea Level 230kts (10K ft 305kts) Climb: Sea Level 5330ft/min (10K ft 5270ft/min)

Rather conventional performance for an airplane of this power and size. As they say, conventional methods yield conventional results!
This is the best case scenario, results could vary ±10%; probably on the - side...

And as usual, a beauty shot:

2020_0703_BowlessCanopy.JPG

See y'all in another 8 months....maybe!
 
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Lendo

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Rad-dude123, Everyone I know is using Riblett mainly for docile stall behavior, I'm using 37A 415, others are using 35A415 as Orion (Designer) apparently liked that one. I believe Billski is using one as well, can't remember which one. Their a modified NACA 6 Series, which is pretty good in itself.
Attached is my take on a Tandem Aerobatic. The design is a constant state of flux and the drawing is an older one.
 

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Radicaldude1234

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Oh my God, it's been less than 8 months...what's going on?!

Anyhow, was working on the cockpit ergonomics since that's what the airplane is designed around.

If it looks looks cramped, that's because the figures are of 99 percentile man, who is 6'3.5". The adjustment scheme is also designed to accommodate down to a 1 percentile woman, or someone who is 4'10".

2020_0714_SeatingArrangement.JPG
 

Scheny

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Vienna, Austria
I have tried around in Solidworks FlowSim and the flow trajectories beautifully show how the air behaves, but I neither managed to get meaningful lift/drag values (2800N drag and 1300N lift seems strange for cruise condition), nor could I get this separation bubbles.

What did you use for the visualization to show this separation? Can you elaborate on that RadicalDude?
I would also be glad if you said how you set up the mesh parameters.

BR, Andreas
 
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