Boom tail microlight/LSA

I was thinking which could be a suitable configuration if target would be to the microlight category (Finnish ultralight will be aligned with European microlight), or to EASA LSA category. The US-LSA category is stupid since it has some severe limitations which removes the reason to try to optimize anything – the speed limitation and also the stall speed limitation as clean -> with these limitations, it does not worth optimizing the aerodynamic performance or flap configuration, for US-LSA, the best solution probably would to design a plane, which is as lightweight as possible and which would not have any kind of flaps and which would achieve the stall speed only with the wing area (because that is what the limitation implies anyhow), so the utilized Clmax becomes close to 1.0, which is poor.

The main criteria in this more sane European category is the weight and the second main criteria is the stall speed. These are the most important features, other features are secondary. The performance can not be optimal, but it can be optimized to the constraints given by the weight and stall speed limitations. It would be also necessary to cut the part count to minimum, an inexpensive plane will be for sure more popular than the more expensive one, in the category where the buyers are not the richest people out there (who would anyhow order a Cirrus-Jet), but normal hobbyists who are not swimming in money.

So consider this:
– Plane structure would be based on carbon fiber rods (pultrusion rods).
– The fuselage would not be a structural member of the plane, but rather a baggage pod located below the spars. The twin booms would be a pultrusion rod each. The engine would be mounted to the wing spar rather than to the fuselage. These rods could have aerodynamic fairings on top of them (which also allow space for control cables etc.).
– high aspect ratio wing, which enables good climb rate with low power
– HKS700E engine in pusher configuration
– Fixed pitch pusher propeller behind the pod (but thrust line to the wing spar).
– inverted V-tail in the ends of the two booms, and the tail would connect the
two booms with the help of a pultrusion rod which functions as spar.
– Main landing gear connected to wing spar
– Nose gear located under the pod.
– Wing structure would be solid blue styrofoam, and in wing root there would be a large fairing which contains fuel (on both sides). The wing skin could be either carbon fiber or fiberglass (fiberglass to reduce cost obviously)

Think how many parts this requires compared to how many parts and layup schedules is usually needed. The pod type cockpit could be almost a complete monococue. There would be need for only instrument panel and some structure where one can assemble the pedals. Also the instrument panel, as we know it, does not need to be like it is, a panel. There are other ways arranging instruments in the plane than having a straight panel where everything is put with tiny screws. None of the modern cars use that old-fashioned way anymore. With modern avionics, you don’t need a big panel with lots of switches, knobs, circular gauges etc. You can have just two screens which display and control everything.

The only strength needed in the fuselage is for crashworthiness, it does not need to carry any loads, and it does not need to be shaped unoptimally to avoid flutter tendency for example, all structural members are straight lines and separate from the fuselage.

The idea comes from some NASA PAV concepts, but as modified. It also has some influences from the Sunseeker.

The concept could have idea of being as lightweight as possible (the lower power engine also supports this mission) and still being as highly performing as possible (that can be achieved with the light weight and aerodynamics, not so much trust is needed).

So the performance target setting for conceptual design could be:
– beat 100 hp Dynaero MCR-ULC in empty weight with large margin
– be on par with 100 hp Dynaero MCR-ULC in speed (with only 60% of the power available)
– and the rest comes from the category limitations
– be a lot cheaper than most other same category plane on the market
– climb rate 800 fpm (remember that because of the low climb speed, the climb gradient is high despite of the not so high number compared to high performance aircraft)

Compromise:
– beat 100 hp Dynaero MCR-ULC in empty weight
– cruise speed compromised to between 80 hp WT9 Dynamic and 80 hp MCR.
– climb rate 600 fpm

Failure:
– heavier than MCR-ULC
– slower than 100 kts in cruise
– climb rate less than 500 fpm

Anyone interested in a such thing or having ideas (for or against) for a such thing?

Here is an illustration about the idea (15 minutes of Rhino magic):

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    • dodlithr
    • January 20th, 2009

    Some great ideas there. One picture would tell more than 1000 words.

    • Karoliina Salminen
    • January 20th, 2009

    I can draw one in Rhino when I have time. Maybe weekend.

    • Matthew
    • January 21st, 2009

    I’ve been reading your blog for a few weeks now with great intrest.

    I find this concept very interesting. I am trying to design something similar,

    The design criteria for my project are;
    450kg mtow
    stall speed less thatn 65 kph (35 kts, 40mph)
    maximum speed greater than 160 kph (87kts 100mph)

    I am also looking at a twin boom pusher configuration for the benifits of forward visability and so that I can make use of reasearch done on pusher UAVs by the militery and hobbyists alike.

    • Karoliina Salminen
    • January 21st, 2009

    Your design criteria is quite modest, achieving more than 160 km/h at 450 kg doesn’t require very good aerodynamics.

    The 450 kg MTOW, if it is going to be the real MTOW (i.e. structural maximum 450 kg) rather than 600 kg like many of these ultralight/LSA -planes where the ultralight models are structurally LSA, that causes a hard requirement for the empty weight.

    If we count two not so heavy adults, 50 kg gasoline and 10 kg baggage, we need at least 220 kg useful load. This means that the empty weight of the plane must be no more than 230 kg including everything.

    To achieve this empty weight, Rotax is out of the question for sure, and so is Jabiru and all autoconversions. Only the HKS700E is pretty much left to the choices (because I think 2-stroke engines are not suitable for aircraft use, and they also have poor fuel economy in addition to being unreliable and unsafe).

    HKS web page promises 55 kg weight for the engine including exhaust system etc. peripherals.

    That leaves 230 kg – 55 kg = 175 kg for the airframe including all systems (batteries are heavy unless you use lithium polymer) and avionics.

    What I would do if I was doing this, I would design for the 600 kg LSA spec structurally and with the performance spec (i.e. have below 45 kts stall speed at 600 kg), and then try what is needed to achieve the required 65 km/h stall speed at 450 kg (or 472.5 kg if parachute is installed). This allows also other engine choices than the HKS700E, but the useful load is then pretty limited under ultralight/microlight spec.

    The 450 kg MTOW W/P becomes with HKS700E: 990 lbs / 60 hp = 16.5 lbs/hp. To achieve acceptable climb rate, high aspect ratio and low wing loading is needed. The takeoff performance is not very good with this configuration, but may be possible to make it acceptable compromise with sailplane -like aerodynamics.

    • Karoliina Salminen
    • January 21st, 2009

    I managed to draw this illustration last night, so I hope it describes something. It is not drawn to any scale and the shapes are not exact airfoils, wing area is based on nothing etc., but it is a concept drawing illustrating the configuration. Took about less than 15 minutes to do it, it was pretty easy:

    – I created a layer for helper lines
    – I set the layer to helper lines
    – I used the curve tool to draw half shape of the fuselage pod. I used osnap with end snap so it attaches to the grid
    – I approximated a laminar pod, not accurate but, just a look-a-like.
    – Then when the curve was ready, I used the Revolve tool.
    – Axis set to horizontal, 360 degrees revolve
    – I changed the pod to basic layer
    – Then I created wings -layer
    – I changed back to helper lines layer
    – I drew the airfoil shape (just an approximation of a look-a-like airfoil, not accurate anything)
    – I scaled two versions of it to do the trapezoid.
    – I placed larger airfoil shape curve to the center of the pod, and the smaller ones 14 major grid lengths away from the center each side
    – I lofted from the left tip through the center to the right tip. I used straight sections loft to get straight taper. Changed the layer of the wing to wings-layer
    – Then I created a circle to the middle of the wing, moved it to a place where the boom starts
    – Copied the circle and shift-moved it aft to a location which looked appropriate.
    – Then I lofted between the two circles, and changed it to the wings layer
    – Then I copy-pasted the airfoil shape to the tail
    – I rotated it some degrees, and copied and moved the other end to the middle line
    – I lofted between these two airfoil shapes
    – I used the mirror function to mirror the boom and one side of the tail
    – I changed the layer of these to the wings layer
    – And that was about it
    – Super-easy, only this easy in Rhino.

    • Karoliina Salminen
    • January 21st, 2009

    Forgot to add, that I hided the helper lines layer from the screenshot I took to this blog and created the canopy shape with a simple curve and projection function.

    • dodlithr
    • January 21st, 2009

    Basicly ok whatever.. if you need that kind of performance, then that it must be.

    The wing looks unpractically long. I would switch to box wing or biplane. So much wing makes it hard yo handle on the ground and take offs and landings. Why not use high lift devices to increase lift? Or the box wing?

    Where is the engine? Aft pusher? How do you avoid hitting the ground at take off with the propeller?

    The wing is very thin, where do you fold the landing gear? In the fuselage? Or they just hang out?
    No idea to minimize drag so much with the long wing and the loose it with the landing gear hangin out?

    etc. etc.

    But such long wings are very common today. I find them a bit stupid… (sorry to say). ūüôā

    • Karoliina Salminen
    • January 21st, 2009

    The wing profile is not accurate as I mentioned. And this is not a design, but concept sketch. People do these normally by hand. I use Rhino. Please don’t look at the details, the details are not the thing, it is just an illustration of a concept, not a detailed design phase drawing. Fitting things inside etc. are not yet considered at this preliminary design phase where initial configuration alternatives are though, it only comes in later.

    Why to use long wing and lose it with the landing gear? I was thinking about ultralight/LSA plane. A plane which is applicable to both european and US-LSA standards would need to be a fixed gear plane, RG is not allowed feature.

    Why not to use C-wing, box wing, etc.? Because it would make the structure significantly more complicated, it would require more parts to be fabricated, the control rods could have difficult issues with this type of config and the box wing or C-wing is not a silver bullet, a solution for everything that saves the World. Nothing in aircraft configurations is alone a silver bullet that saves the world. It is a compromise of everything.

    Why not use high lift devices to increase lift? Please notice this is not a detailed design picture, therefore high lift devices are not drawn in the pic.

    Why there are no ailerons visible? Please don’t ask, because this is not a good question for a concept sketch that does not have details on it.

    The config is a middle pusher. I haven’t drawn the prop on the pic because it is difficult and takes time, remember I only used 15 minutes to draw this thing.

    • Karoliina Salminen
    • January 21st, 2009

    And one more thing: why not biplane.
    Biplane has bad efficiency.

    And I find the contrary: I find the short wings pretty stupid. That is why long wings are used these days that they are more efficient, and here was a goal of getting good climb rate with very low engine power at low angle of attack. Very low aspect ratio wing leads to poor climb performance and poor high altitude performance and poor aerodynamic efficiency in cruise, and requires high amount of power, something to not consider for a HKS700E engine which is underpowered for most ultralights these days, for example put one of these to a TL-96 Star, load two people, baggage and gasoline on it, and it does not take off at all but ends up in trees and tears.

    And about fitting the landing gear: The landing gear can’t fit in the wing in many cases anyway. The only way to hide the landing gear is store it inside the fuselage, tail boom or engine nacelle (in case of twin) when the wing is small and has limited space inside.

    • lincoln
    • March 25th, 2009

    I’m really enjoying your blog, you’re willing to think about lots of unusual ideas, but you seem to do your homework and know something about what you’re talking about.

    However, I think if you are trying for low cost aircraft, you may be looking at too many bells and whistles. Perhaps I just haven’t completely absorbed the lessons of Graphlite, etc., but for LSA per US rules I think Barnaby Wainfan’s approach with the Facetmobile is more like it, except maybe for visibility.
    http://facetmobile.com/

    Under LSA rules, if you make the airplane really clean you will probably have to add drag or reduce power until you can’t climb well. (Ok, I might be wrong because I haven’t run the specific numbers, but am guessing based on my investigations of clean aircraft for part 103) So why not go for low expense, crumple zone for crashworthiness, really safe handling, low min. speed, etc? Recommend you read Barnaby’s report for NASA that you can find on the site I mention above.

    As I recall, Wainfan wrote that, based on scaling his flight experiments, you could make something with 80hp that carried 3 people with the same performance as a Cessna 150 with two people and 100hp. Plus having the other advantages mentioned above.

    Also, if you go for pod and boom, I think Strojnik’s configuration makes sense, although you have to put the engine high enough to let a decent sized propellor clear the boom. Might be fairly easy with PSRU to offset propshaft. Boom can be full length of craft and used for alighnment jig. Control runs for elevator and rudder are straight. If you made a low wing the runs for the ailerons could be quite simple as well, but that would probably cost a lot at low speeds.

    If you’re really into it, I recall that twitt.org sells a tape of a lecture from Wainfan on the aircraft. I recall that in the lecture if you were to make a Facetmobile big enough for 4 people, the interior volume would be so large that you could use it as an RV, i.e. all passengers could sleep in it. He also said that on the single place version he could extend his arms fully to both sides from the pilot’s seat. However, I think that his choice of facets is maybe not quite optimal, and a “smoothmobile” might be enough better to be worth a bit of extra effort. Another concept I’ve thought about is a smoothmobile ultralight glider. Maybe it would perform comparably to an ordinary hang glider, but be safer and more comfortable. (see Mike Sandlin’s BUG for another approach). Still, if I had to carry something up a cliff and jump off, I’d probably learn to use a hang glider.

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