# Aerobatic Tandem Two-Seater

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##### Well-Known Member
So after writing the Excel file from hell based on Robert Nelson's Flight Stability and Control and writing a Matlab script to solve the 2nd ODE for dynamic stability, I was thoroughly burned out and sat on the study for about a year and a half....I also started a new career in the Federal government where I spent months getting yelled at, but that's besides the point.

Waking up today after the holiday food coma, I decided to get on with it and ran the study again. Was also wondering if I could get a sanity check before finally proceeding to designing the structure?

Methods used: 3 different approaches

1. Algebraic equations in Nelson's book
2. Matrix based on Nelson's book and MATLAB ODE
3. X-Plane Simulation (ABSOLUTELY NOT PRIMARY)

Without rehashing what's in the pictures, conclusions are:
- Overall generally stable
-Positive Static Stability
-Manageable Dynamic Stability

- Fairly stable for short period disturbances
-The oscillations are damped

- UNSTABLE for phugoid
-Slow phugoid, but increasing over time
-Fairly normal for light aircraft
-Large horz tail might be contributor

- Probable dutch roll tendencies
- Due to long vert tail moment and large area (spin recovery authority)

All this fairly verified in X-Plane.

#### BoKu

##### Pundit
Next to an RV-8...
Wow, that's quite the sizable airframe. For perspective, here's a comparison between RV-8, Tsunami, and Mustang that I prepared a few years ago:

#### Saville

##### Well-Known Member
Wow, that's quite the sizable airframe. For perspective, here's a comparison between RV-8, Tsunami, and Mustang that I prepared a few years ago:

View attachment 89599
Personally I love the tight fit of the RV-8. Feels like you are strapping on the airplane.

Also handy in that it fits into a shared hangar better. More hangar options.

I wouldn't want a larger plane (unless it was a Thunder Mustang) but that's a personal opinion.

#### Lendo

##### Well-Known Member
Ricaldude123. Nice design, what weight category? Light Aircraft I assume.
Looks similar to my Light Sport Tandem design, I've allowed more up-sweep in the rear fuselage and tail, suggested 12° (Roncz) and (12° to 15°) Howe, to avoid tail strikes. I understand your approach to the Vertical Rudder hinge line but in reality the lift curve slope of a slightly swept VT, say approx 20° at 25% chord (see Howe or Roncz) , would not add much more to the VT Area and be a little more Aesthetically pleasing to some. I pushed the VT back to clear the Elevator from the Rudder, eliminating Elevator cut-outs. This would provide more unblanked Rudder area in a Spin, with more tail up-sweep you could consider some Ventral Fin, increasing your unblanked rudder area, which is considered to need from one third to 50% Rudder area - I aim for 50% but accept a little less. I use 40% chord for Rudder and Elevator.
The wing spar placement seems a little forward considering 2 place CG, unless your using a heavy Motor.
Finally I would consider Riblett Airfoils, the thinner the Airfoil (I.e. 12%) selected for the faster the plane, the thicker the Airfoil for the slower the landing speed - needed for 45 knots.
George

##### Well-Known Member
what weight category? Light Aircraft I assume...unless your using a heavy Motor.
It'll only be an LSA if that new 3600lb gross weight rule gets approved. Aiming for 1800lb empty and ~2700lb max gross. Spec-ed out for using a flat-6 as a minimum (O-470/O-540) and up to an IO-720.

...Finally I would consider Riblett Airfoils
The aerodynamics were designed with a Riblett airfoil for the wing.

#### Lendo

##### Well-Known Member
Have you considered Carbon Cloth 40% lighter and 60% more strength, so Carbon cloth then being half the weight of glass cloth comparing weight and strength - with considerable savings on Resin. I know it's more expensive but the weight savings are there. Also Carbon Rods for Spar Caps, the best for weight to strength.
George

#### ScaleBirdsPaul

##### Active Member
Man now that’s a cabin built for big guys. I’m sold.

I’m a fan of the Riblett airfoils as well, although I’m not sure the perils of the NACA foils are as pronounced as he makes them seem
In his book.

##### Well-Known Member
...it's more expensive but the weight savings are there.
Price is pretty much why aluminum is the intended medium. Wet layups don't do much better than metal structure weight-wise and have much lower temperature resistance than simple sheet aluminum. Pre-preg would be competitive in all but price...especially for tooling.

I’m a fan of the Riblett airfoils as well, although I’m not sure the perils of the NACA foils are as pronounced as he makes them seem
In his book.
The thing with Riblett airfoils is that although they have pleasant stalling characteristics compared to the much maligned NACA 23000 series, their pitching moment is much higher. While this is fine for a cross country cruising type airplane, the required tail moment does add up when you're designing for a load factor of 6+.

So for an identical wing where the only difference is either a Riblett or NACA 23000 airfoil, the tail boom for the Riblett wing would need to be 2 or 3 times stronger. The tail would also generate 2 to 3 times the downward lift, which also subtracts from the total lift of the aircraft and forces the wing to work even harder, and induced drag.

Now a Riblett airfoil was selected for this airplane, but with the understanding that I would be getting pleasant stall characteristics at the expense of increased rear fuselage weight and reduced top speed.

Man now that’s a cabin built for big guys. I’m sold.
The cockpit is designed with adjustable seats and rudder pedals for the maximum of a 6'5" man and the minimum of a 4"10 woman, which represents 99% of the American population. The front cockpit could accommodate someone taller with the some discomfort in shins hitting the panel, but the back would be limited by the canopy. Width is about 28".

#### ScaleBirdsScott

##### Well-Known Member
Looks like there's tons more room below the feet, is there something under the floor of the cockpit or just the wings?

#### wsimpso1

##### Super Moderator
Staff member
Log Member
I am concerned about your desire to mix aerobatic primacy with docile stall:
• Riblett airfoils are not usually thought to need washout to get docile stall behavior, so 3 degrees of washout is way into belt and suspenders for docile stalls;
• Riblett airfoils have a bit of leading edge droop in the camber to give docile stall behavior in positive g, but that comes at a price - negative g stall can be particularly sharp with this leading edge;
• A bunch of washout and Harry Riblett's drooped leading edge, I would be REALLY concerned about any negative g maneuvers - get even close to stall in negative g, and the tips are likely to stall, initiating a strong snap roll.
Unlimited aerobatic birds tend to have symmetric foils, no washout, sometimes thinner foils and smaller radius leading edges at the tips to help them do a couple things:
• Have the same behavior in both positive and negative g;
• Make them enter and exit stalls ( and thus spins) cleanly.
Now maybe you are intending to do positive g aerobatic work with no spins. But if this bird does do any negative g and the AOA gets up there, I would expect it to be scary indeed. If you do not think that can happen, I reference the second astronaut flight of SpaceShipOne by Mike Melvill and any spin recovery that ended in a spin the other way....

Billski

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##### Well-Known Member
Yeah, I am a bit concerned that the airfoil selection is more suited to a cruising platform than even a mild aerobatic machine...

I was wondering if you had an airfoil recommendation? At this point I'm strongly inclined to go use a NACA 23000 series or NACA 4 digit airfoil.

Part of my apprehension on using the 23000 series is it's reputation (probably exaggerated by Riblett). Comparing lift curves, the closest thing I've flown that's probably similar was the Decathlon with the NACA 1412 and the Citabria with the NACA 2412.

Primarily focused in the positive G regime.

Looks like there's tons more room below the feet, is there something under the floor of the cockpit or just the wings?
Landing gear wheels and retraction mechanism. I also wanted a thru-spar that didn't intrude into the cockpit.

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#### daveklingler

##### Well-Known Member
Quoting from Orion's paper on airfoil selection:

"Above 130 mph, I start looking at the laminar sections. My favorite has been the 747A315, which I have used with great success on several configurations. Although it does not have a high unflapped Cl, it does have a very low pitching moment, good stall characteristics, and some of the lowest drag numbers in the table. It also doesn't seem to have the leading edge separation tendencies of the more classical laminar sections like the 63- to 66- series."

##### Well-Known Member
Quoting from Orion's paper on airfoil selection:

"Above 130 mph, I start looking at the laminar sections. My favorite has been the 747A315, which I have used with great success on several configurations. Although it does not have a high unflapped Cl, it does have a very low pitching moment, good stall characteristics, and some of the lowest drag numbers in the table. It also doesn't seem to have the leading edge separation tendencies of the more classical laminar sections like the 63- to 66- series."
Very intriguing airfoil. Did a quick XFLR5 (Xfoil) analysis on the airfoil and compared to others I was considering:

From the study, the NACA 747A315 has an impressively low pitching moment...even lower than that of the NACA 23015! However, at the Reynolds numbers I'm using at it seems like everything else under performs. Especially for the Clmax, which would limit this particular aircraft's ability to pull high Gs for aerobatic maneuvers at lower speeds and high AoA. It would however function superbly as an ultra low moment airfoil for something like a flying wing!

Right now I'm doing a detailed analysis for the NACA 1415. It is similar to the NACA 1412 used on the Decathlon, an airplane that I have quite a bit of experience doing post-stall maneuvers and aerobatics....and one that I never felt unsafe while doing so.

The NACA 1415 seems to have a slightly less steep stall drop off than the NACA 23015 and has almost as low of a pitching moment. It is also fairly symmetrical.

As mentioned, a large wing pitching moment results in unacceptable high horizontal tail forces when maneuvering. The NACA 1400 series has about 1/4 the pitching moment of the original Riblett airfoil, which would reduce the downward tail force according. For reference the NACA 23000 has about 1/10 of the Riblett Cm.

Now to plug in the new airfoil parameters into my stability spreadsheet...and modify my X-Plane test model!

PS: FYI, the NACA 23000 series is actually the most popular airfoil series ever, used by over 800 aircraft designs. The Clark Y seems to be second, followed by the NACA 00 symmetrical foils and the NACA 4400 series.

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Yeah, I am a bit concerned that the airfoil selection is more suited to a cruising platform than even a mild aerobatic machine...

I was wondering if you had an airfoil recommendation?

...

Primarily focused in the positive G regime.
I doubt the need for all that washout. Supposedly Harry's foils using the same foil root to tip do not need washout to have docile stalls.Three degrees is a bunch...

The negative g stall issues are there with the 23000 series too. Aerobatic ships that do both big positive and big negative g go with symmetric airfoils and zero washout. And aerobatic ships that are supposed to snap roll and spin sometimes go with thinner tip foils as well as symmetric foils and zero washout. Stadaucher/Panzl wings are done this way.

The positive g focus will help with limiting negative g. No inverted fuel or oil will help too. Design of the elevator control circuit could limit down elevator and make negative g stall tough to approach.

One other thing is that if you have a wing jig that can be adjusted, you can build your wings one way, and if you do not like the way it spins, stalls, or otherwise behaves, you can strip the skins, adjust the jig, and reskin with it the other way.

Billski

##### Well-Known Member
I doubt the need for all that washout. Supposedly Harry's foils using the same foil root to tip do not need washout to have docile stalls.Three degrees is a bunch...

The negative g stall issues are there with the 23000 series too. Aerobatic ships that do both big positive and big negative g go with symmetric airfoils and zero washout. And aerobatic ships that are supposed to snap roll and spin sometimes go with thinner tip foils as well as symmetric foils and zero washout. Stadaucher/Panzl wings are done this way.

The positive g focus will help with limiting negative g. No inverted fuel or oil will help too. Design of the elevator control circuit could limit down elevator and make negative g stall tough to approach.

One other thing is that if you have a wing jig that can be adjusted, you can build your wings one way, and if you do not like the way it spins, stalls, or otherwise behaves, you can strip the skins, adjust the jig, and reskin with it the other way.

Billski
Any experience with the nearly symmetrical NACA 1400 series (1412, 1415)? Apparently the only aircraft to use it were the Decathlon, Douglas Skyknight heavy fighter, and Convair Tradewind flying boat...

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Any experience with the nearly symmetrical NACA 1400 series (1412, 1415)? Apparently the only aircraft to use it were the Decathlon, Douglas Skyknight heavy fighter, and Convair Tradewind flying boat...
Understand that I just read the literature and can do the math. Lots of experience doing the synthesis so I can make the combination of theory work in my head pretty well. I can easily see that negative g stalls with a bunch of washout with a drooped leading edge wil tip stall, and just wanted you to be aware.

As to these foils, no EXPERIENCE with any of them. The knowledge is in what folks flying them have gotten. For instance, a Hershey bar wing with 23012 works fine for aerobatics as shown by the RV series, but expect a sharp stall in negative g. I suspect that down elevator is restricted in those ships too- I am going to check a buddy's plans on that this week... The Lancairs and Wittman designs all have limited down elevator, probably for the same reason.

I would not go crazy over my cautions, just understand that you may have to deal with the same thing some others have already dealt with.

Billski

#### TFF

##### Well-Known Member
Aerobatics is a broad term. Pulling a loop in a Cub is aerobatics just as much as a tumble in a Extra.

Quality of aerobatics is something else. As long as you can keep the plane from stalling when you don’t want to and stall it when you do want to, aerobatics will be, umm, aerobatics. If you wanted comp aerobatics, you would be picking a better airfoil. The Citabria which is a standard aerobatic airplane trainer is actually a pig of an aerobatic airplane. It makes you work for everything more complicated than a lazy loop or roll. On purpose. Pulling a loop anyone can do; pulling a loop that is judged perfectly round by a panel of judges takes built up skills.

Digging how deep you want to be in a subset of a category is as much a decision as being in the subset.

##### Well-Known Member
I would not go crazy over my cautions, just understand that you may have to deal with the same thing some others have already dealt with.

Billski
Yeah, I've been eyeing an airfoil replacement after going as far as calculating the MOI for the tail boom structure needed to handle the tail loads. Up until then, I thought dealing with the high wing pitching moment would just be a slight annoyance.

Anyhow, I was just wondering if there any weird quirks that wouldn't show up in a mathematical comparison, as I did in the picture above. For example, the NACA 23000 series would look almost perfect if I didn't know of it's faults from experience.

This is the level of acrobatic agility I'm looking for:

As to why I don't just buy a T-34, I wanted to justify paying for my Mech Eng degree since I don't use it anymore...that and I'm a millennial with a mortgage, so I can't afford it. Maybe after I get that raise...

But yeah,with the Riblett configuration handling-wise I was pretty satisfy with the results of the stability analysis. The X-Plane model also felt pretty crisp.

Of course I'll have to redo the stability analysis with the new airfoil and modify the X-plane model. With the amount of work I've put into automating my spreadsheets, though, it shouldn't take more than a couple hours.

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#### BJC

##### Well-Known Member
... For example, the NACA 23000 series would look almost perfect if I didn't know of it's faults from experience. .... This is the level of acrobatic agility I'm looking for: ...a T-34,
If you are looking for T-34 level of aerobatics, almost anything that will get off the ground will do.

There are many examples of successful aerobatic aircraft using the 230XX series, including aircraft that have won the World Aerobatic Championship.

Please describe the faults that you have experienced. (I have flown numerous aircraft using it, both tapered and rectangular platform, and I am curious as to what you experienced that I did not.)

(A guess as to the American Champion -8 series airfoil selection; spar placement fit well with the fuselage wing connection points.)

Thanks.

BJC