Wednesday, July 4th, 2012 08:40 am GMT -6 Wednesday, July 4th, 2012 08:40 am GMT -6Wednesday, July 4th, 2012 08:40 am GMT -6
Synergy aircraft

Synergy Aircraft has responded to our questions about their fascinating full-size airplane!




Introduction (by Carlos)

The first major section below, Synergy Model Questions, were the questions asked by the website members. There is clearly strong interest in building more RC model airplane versions of the Synergy. Unfortunately, Mr. McGinnis’ answers probably raised more questions than they answered.

The second major section, Synergy Full-Size Questions, were my questions. They were inspired by confidential information that was shared with me. If you read between the lines, my questions should provide a useful insight into the nature of the airplane.

I have a good enough understanding of aerodynamics to form a picture in my head of how Synergy is supposed to work. This picture is necessarily fuzzy due to the lack of public design details. The design ideas that I have seen are definitely interesting.

The challenges in creating a practical airplane out of these ideas should not be underestimated. Most likely, it will take many more years before a practical implementation of these concepts is realized.

Synergy Model Questions (from website members)

Could you make plans for the Synergy available?  I’d love to build a model.

Yes, we are working on several sizes of Synergy for the R/C market and we’re very interested to hear from prospective manufacturers. We won’t be able to offer plans quite yet, but that time may come as well. Qualified scratchbuild modelers are working on their own, of course, but some have expressed interest in collaborating.

The main thing for us is that we get the real airplane done. Having a bunch of models out there that probably aren’t properly designed for the unique challenges of the configuration could affect our development, since models are poorly understood by the public and we modelers crash them a lot.

Provide dimensioned three-views of the aircraft.

I will send a message to Carlos when they are available.

Provide nominal location and range of the CG.

This is a very important key, of course, and will be included in the three view. However, there are other considerations for airfoils and twist that work with the CG we use.

Provide the specific airfoil types for the upper and lower wings.

For a model you definitely do not want to use any of the airfoils we use in the full scale aircraft. They are high velocity, high Reynolds number airfoils. Like most high speed NLF (natural laminar flow) airfoils used in full scale, they have a ‘drag bubble’ at low speeds due to laminar separation. Their drag at model Reynolds numbers is many times higher than a normal R/C airfoil. This is one of the big changes that makes a small Synergy a bit more work than we’d like right now.

For the record, the wing airfoils used on the full scale Synergy v.31 aircraft were all designed by me and are as follows:

Inner Wing Root: JM48-312, an airfoil with its maximum thickness at 44% of chord; its maximum camber at 48% of chord; having a lift coefficient of 0.3 in the center of its minimum drag range; and a 12% thickness.

Outer Wing Root: JM48-314, an airfoil with its maximum thickness at 44% of chord; its maximum camber at 48% of chord; having a lift coefficient of 0.3 in the center of its minimum drag range; and a 14% thickness.

Wing tip: JM48-115, an airfoil with its maximum thickness at 44% of chord; its maximum camber at 72% of chord; having a lift coefficient of 0.13 in the center of its minimum drag range; and a 15% thickness.

Explain the interaction of the two surfaces that causes stability & control to be achievable in this tailless configuration.

The configuration is not tailless, it actually has two horizontal tails located above and behind each wing tip and four vertical tail surfaces. Because of wing sweep Synergy does not rely as much on its tail as some designs, but it really is a conventional tail-behind-wing aircraft in terms of its basic design and analysis.

There is a problem, though: when you put a tail close to a wing you have to consider many interactive effects. It also has to be large. These things have kept most designers from exploring the potential for a tail in this location. It is very easy to make big mistakes, and hard to see any benefits.

Regarding stability, the tails are not lifting, they are pushing down with more force than normal (about 8% of flight weight). This makes a high decalage effect, meaning that the aircraft is more stable against vertical gusts and statically stable in general. Synergy’s tails are very large for a swept wing aircraft and their sweep and control details provide dynamic stability. Yet because the tail is closer to the CG, it is not excessively stable from the pilot’s perspective. Not very much control travel is necessary to provide full control authority.

Also, the large area, high span, and high aspect ratio of the tail surfaces improve the tail authority greatly over that of most planes. Each tail can be understood as having its own winglets at each end, as well, since they are lifting downward, like a wing upside down. This greatly reduces the induced drag of the tail.

In operation, the tails act together to provide pitch control, and opposite to each other to provide roll control. This makes them have the function of both elevators and ailerons, which is why they are called elevons. Lots of planes have elevons, but they’re usually on the wing.

The elevons interact with the airflow over the wing and vice versa, by changing the lift as a system and by reducing the drag between them. (Normally biplane interference is a negative effect, but the counter-circulation of each airfoil in the staggered, vertically spaced condition creates a positive and beneficial biplane interaction in the Synergy design.

Finally, the effect of having a tail that is ‘pushing air the wrong way’ is good, when it is doing that in the right place. Synergy uses the fact that the wing throws air at the ground to create its stability and control. Let me explain: Airplanes throw air at the ground to fly, and because of this the atmosphere ‘tumbles into the hole behind the aircraft’ from the sides. The resulting swirling motion (called wing tip vortex) is started by the wings, and how strong it is depends on the aircraft weight and wingspan.

If we act against the speed and intensity of the wake vortex, with longer wings or by other means, we are acting against air already set in motion, and we are acting to reduce the lift-induced drag.

On Synergy, all of the air pushed ‘up and out’ by the tails (a small momentum) is meeting the air trying to crash into the “hole” created by the wings (because they already threw a lot of air at the ground.) This opposite momentum is like a headwind that slows down the speed of the resulting wake vortex, and increases its diameter. The result is less induced drag for the wingspan.

Another way of saying it is that it creates a high span efficiency. Although we’ve known about some of these things for many decades, there haven’t been designs able to use them to much advantage.

Provide total aircraft drag estimates as a function of airspeed with the proportions of induced, profile and parasitic drag.

Some limited information is found on our website in the FAQs, but most of this information is not public right now. Why not? Because it would only cause a fight, and it’s not yet demonstrated. We are roomy, quiet, and fuel efficient enough that we don’t need to talk about how fast we go; please just assume it’s over 200 MPH.

Howard Handelman has provided the proportions you describe on his website, for any Synergy-like aircraft capable of winning the Green Flight Challenge against the Pipistrel G4 winner. We have publicly said we could have done that (using our ‘passenger count and speed’ strategy), so it is a fair analysis. His numbers are not for Synergy and only use the public information, but they help to describe the space better.

Most experts all have the same tools for running the numbers at their disposal, and we trust them. Yet our basic calculation tools are built on a representative way of modeling lift and drag, not on the actual physics. Yes, let me say it again: using 1/2 rho v squared and a changing drag coefficient is not the same as using F=ma and fluid viscosity. They are different. So the physics we explore with Synergy* require MAJOR study and expensive research far beyond the level we can do on our own. (*powered drag reduction in combination with whole-airplane design)

Historical fact: none of that research is truly believable without a full scale flight test prototype, no matter who does it, so because I have so much background in this area I thought that I could skip the ‘debate in committee’ part of the process for now, and give the world something to analyze later. Many very important ideas have not been tested in good experiments because of the power we have given to our representative equations in early analysis. Who would fund a test of a design that looks mediocre in early analysis, in the gamble that it isn’t actually mediocre?

Another thing is that a synergistic system that works together can’t be defended part-by part. If I did it as a proper scientific experiment it looked like it would take another fifty years to test each ingredient in Synergy separately. I felt that we needed a new starting point.

Synergy Full-Size Questions (from Carlos)

Take off speed at full load?

Great question! Because of the gear arrangement we can fly at a lower speed than we can rotate at, unless we raise the nose. So until we do one of the cool things I have in mind to address this problem, we expect to take off at ~70 MPH.

Cruise speed?

Not saying, sorry! Fast enough for me. Howard’s spreadsheet shows what a normal airplane would do if it were capable of doing what we’ve said ours will do.

Is the design expected to be robust against surface imperfections?

Yes, we have great, long landing gear and a decent amount of prop protection provided by the split flap. I wanted to be able to fly into backcountry airstrips.

I just realized you are probably asking about the aircraft surface, and the answer is yes. It has to be able to tolerate bugs, dirt, and rain.

What are the numbers of the Synergy patents?

Our patent applications are pending but have not yet published. Stay tuned!

Conclusion (by John)

More questions are welcome, just send them as they come if you like.

Thanks Carlos.


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