The "Beast One": What doesn't kill you makes you stronger

[Scheny]

Eh? A light wing loading doesn't affect cruise...? That's a new one! You should ring up all the glider manufacturers and tell them not to bother with ballast tanks and stuff. They'll be pleased to hear this news!

You misunderstood. If course it effects in general, but we calculated 3 different configurations and it turned out, that the two lighter configs only differ at landing speed. At cruise speed there was almost no visible difference. The config with the highest wing load was dismissed, as it failed the requirement for stall speed.

 

[Scheny]

Short status update:
We wanted to start building this summer, but our lead customer has lost 90% of his revenues due to the Corona outbreak (he owns an aviation business). So, the build is unfortunately on hold.

As a mitigation, we are setting up a small workshop for carbon composites, where we focus on small series productions. We will start working on the Beast in parallel, as fast as money will be flowing in.

 

[Topaz]

Very sorry to hear that, Scheny. Best of luck to you and your team.

 

[Scheny]

I just got asked how our wing got developed. As I am much for playing with open cards (as far as not giving away IP), I decided to disclose some of the info here.

The foils are very close to the NACA 64215 (1% camber, max thickness at 40%, 15% thickness), but optimized for low speeds and Reynolds numbers, as they are the main problem for the behaviour of the BD-5. While performing similar at cruise speed, we see two digit improvements at stall speed. This is why we were able to use 1% camber only, which has a benefit in cruise over the wing which Sonex is using for the Subsonex.

The planform is slightly resembling an ellipse (for lower drag at low speeds where the turbine is less efficient), which normally would cause serious issues with stall propagation at the ailerons. To counter this, the wing is slightly swept forward and tapers into another profile during the section of the wing where the aileron is located. This section reduces washout by only 1.25° (to match the working points of the profiles) and moves the point of max. thickness to the front. These measures additionally add to reducing stall speed at super low Reynolds speeds.

Outside of the ailerons, we have some "winglets", which are mostly cosmetical. There was not much impact on performance, so I designed them to have a pleasant look, both from being looked at from the ground and from the cockpit (as the owner is the most important person from our selling perspective).

I will add some performance comparison charts to the NACA 64215 when I have some less work to do.

BR, Andreas

 

[Lendo]

Aerowerx, I got the scares and broken bones to prove it. I will never be as strong again, neither physically or mentally, mind you age does't help either.
George

 

[Scheny]

Aerowerx, I got the scares and broken bones to prove it. I will never be as strong again, neither physically or mentally, mind you age does't help either.
George

OK, I get it, I shouldn't have called the thread this way .

I for myself am an overly cautious pilot, but my experience shows that there is a linear proportional correlation between money in your pocket and being ruthless. Unfortunately this seems to be more my clients than the average homebuilder

 

[Scheny]

Here the comparison of the airfoils that I promised:
The left side shows the wingtip profile and the right side shows the foil for the inner part of the wing (until beginning of ailerons). Both are compared to a NACA 64-415.

The left charts are showing what the ailerons are subjected to in a normal landing situation (Reynolds appr. 500k). While having the same drag, the stalling behavior is as smooth as it can be (while the NACA gets very abrupt) and gives 30% more lift before fully stalled. The NACA is already a very benign profile, as the lift at least does not sink, while drag increases. You can also see that pitching moment is reduced in half for all normal flight conditions.

On the right side, you see the profile that is used for most of the wing in a standard cruise situation (Reynolds about 10M). Not only does it have 20% less drag in the economy cruise condition, it also has 27% higher lift before stall. And, to add up, pitching moment is again highly reduced, unloading the tail.

Another point for selecting this configuration was, that due to the lower camber (1% and 1.5% at the tip), the negative lift is superior to other aircraft, helping in inverted flight at lower speeds.

 

[Aerowerx]

Here the comparison of the airfoils that I promised:
The left side shows the wingtip profile ....

Do you mean "polar" instead of profile? Those looks like airfoil polar plots from xfoil.

 

[Aerowerx]

Aerowerx, I got the scares and broken bones to prove it. I will never be as strong again, neither physically or mentally, mind you age does't help either.
George

"Scares"or "scars"? Or both, maybe?

 

[Lendo]

Typo, but Both will do!

 

[Scheny]

Do you mean "polar" instead of profile? Those looks like airfoil polar plots from xfoil.

You are right, that these are polar plots. But I meant what I said. The right shows the polar plots for a different profile (in- and outboard are different).

 

[David Lewis]

This (low wing loading) leads to problems in case we have a tiny girl flying with empty tanks through moderate gusts.

Besides an uncomfortable, bumpy ride, what are the problems when wing loading gets too low?

 

[Scheny]

Besides an uncomfortable, bumpy ride, what are the problems when wing loading gets too low?

The speed Va gets quite low (~120kt). As you normally cruise between 150kt and 220kt, it was important to have Va near to 150kt (in actual config, it's 145kt).
We will investigate if we can shrink the wing a little bit further without having too much penalties at low speed.

 

[Jay Kempf]

Shrink the wing further? ? ? Said no one ever building an airplane of this type. No airplane design gets lighter as you start trying to build it. This size airplane grams will matter.

 

[Aerowerx]

You are right, that these are polar plots. But I meant what I said. The right shows the polar plots for a different profile (in- and outboard are different).

Remember that these are polars of a 2-D airfoil. The results with a 3-D wing will be different. Smoother with not as many sudden jumps in the curve. And also remember it is a simulation, not reality.

 

[Scheny]

Shrink the wing further? ? ? Said no one ever building an airplane of this type. No airplane design gets lighter as you start trying to build it. This size airplane grams will matter.

"Said no one ever building..." I love that . You are 100% right, that the challenge for a plane that size is not only weight, but in addition, where it is. Getting the CoG right proved almost impossible.

Unlike the Raptor, I am keeping track of every single item in the aircraft. I based the weight values on similar but larger aircraft (for instance the Silence Twister which is of similar construction), so it already contains some margins. Then I substituted these values by calculated ones, as soon as I had/have them available.

From my composite building experience I know that by now I manage to get very accurate, but I add 20% to the calculated value to be on the safe side. For the boat "Girl #4" (an ultralight 14 footer) that I am building for myself at the moment, I calculated the last plate I laminated at 2380g (excl. margin) and happened to arrive at 2430g. All parts have been within 5%, although hand laminated, so "that's fine for me" .

The wing is really tiny, still it has a loading of only 75kg/m² (1780lb/ft²). This is a good value for something cruising at 120kt, but for a jet you would normally rather go for 100-120kg/m². My aerodynamicist/testpilot keeps me at staying with the large wing, rest assured, and I don't question him. Only checking every angle.

Remember that these are polars of a 2-D airfoil. The results with a 3-D wing will be different. Smoother with not as many sudden jumps in the curve. And also remember it is a simulation, not reality.

The wing layout was started with support of the aerodynamicist who invented the profiles. He has proven the polars in a windtunnel (together with Mr. Eppler, if I remember correctly?), so these are quite trustworthy as I know the correction values to be applied. He also gave me the advice for the geometric and angular washout in the tip.

Then I optimized the wing with vortex lattice method computation (I know it's not perfect) and prooved the results with CFD. This was done with the oversight of the second aerodynamicist, who is the one participating part time in the project. He has worked on a short haul airliner, so I hope he knows what he is doing . He also happens to be the test pilot, with experience in fighter jets and B777 / B767. Meaning he has some good arguments for prooving it with a sceptical eye.

I already learned my lessons the hard way with my second boat, where all CFD proved to be wrong and the drivetrain went broken within single hours. Since then, I learned a lot on how to get real life values and not trust what CFD tells you. Still, CFD is very good for visualization of potential problems. Interestingly, the drivetrain suffered from almost the same problems as the Raptor . The second generation worked perfect ever since.

 

[wktaylor]

quotes that may have value in this conversation...

" ‘What doesn’t kill You makes You stronger’... is a fairy tale... ‘What doesn’t kill You changes You, for better-or-worse’.” –paraphrased, unknown

"The J3 Cub is the safest airplane in the world; it can just barely kill you." ~Max Stanley, Northrop test pilot

… on the other hand...

"Happy are those who dream dreams and are ready to pay the price to make them come true.” --Cardinal Leon J Suenens

 

[Lendo]

Scheny,
Overall your approach seems to be very thorough, using everything it seems at your disposal, including professional advice.
However from my knowledge a wing tapered from the Root to the Tip ( say from 15% to 12 %, as in your case), causes the Tip to stall first. To counter this the wing needs washout, but with washout there is reduced Lift at each AOA, with less lift there needs to be more wing, so YES! I would be listening to your Aerodynamicist in this regard.
Advice to me is to keep Wing thickness the same, to avoid this situation and maybe have less Drag.
Everything is a trade-off.
George

 

[Lendo]

Scheny,
Yep Getting the CG accurate is problematic in preliminary estimates - but the Neutral Point is yet another matter.
George

 

[jedi]

Scheny,
..........
However from my knowledge a wing tapered from the Root to the Tip ( say from 15% to 12 %, as in your case), causes the Tip to stall first. To counter this the wing needs washout, but with washout there is reduced Lift at each AOA, with less lift there needs to be more wing, so YES! I would be listening to your Aerodynamicist in this regard.
..............
George

A proper raked wingtip with BSLD and reflex can help in this regard.

Reference: Sonex Wingtip Extentions