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Thread: Homebuilt, low wing, 5+2 place, mazda powered twin

  1. #1

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    Homebuilt, low wing, 5+2 place, mazda powered twin

    Hi all,
    I am currently 15 and working on getting my private pilot license. My dad has a Grumman AA1B, but we have always wanted something a little bit bigger (we are a big family of 7). I am very good with mechanical stuff and problem solving. I recently decided that i would like to build my own plane. I know it seems like a very ambitious project, but i am not expecting to finish it any time soon. The plane is going to be a 5 + 2 place (there will be 5 main seats and 2 removable jump seats for small people in the cargo area in the back of the cabin) low wing RG twin. My plane is going to be very heavily based on the Diamond DA-62 (it will have a straight tail). It will have a wingspan of about 33 feet, with an LOA of 26 feet. it will weigh about 3500-4000lbs and be powered by 2 (heavily) modified 13B Wankel engines from the Mazda RX-7; each engine will hopefully put out about 700bhp. The cabin and interior is heavily inspired by the Diamond DA-62, with a 3 place bench seat in the middle, and a large cargo area behind the bench seat. due to the size of the cargo area, there will be 2 removable "jump seats" that can seat people up to roughly 100lbs. I will be developing all of the systems that do not pertain to avionics myself (engine instruments, safety systems, etc.) with the help of my good friend (for all the computer related stuff). The plane has a very heavy focus on safety, with 2 fully redundant electrical systems and separate flight computers. There is a built in fire suppression system that has access to both engines, cabin and all cargo holds. The cabin is a completely sealed (not pressurized sealed, just sealed from all liquids) and there are no fuel lines that run through the cabin. The cabin is also completely encased in an aluminum structure that adds rigidity to prevent deformation in case of a crash. The 2 feet of the wing closest to the root is reinforced to prevent bending and jamming doors closed. the doors are a traditionally opened (like a Cherokee) and the frame around the door is reinforced with steel to prevent deformation. The plane will also be equipped with a trailing link, hydraulically retractable gear. the reason that I choose a trailing ink setup is because it allows the aircraft to land on rough strips without the concern on collapsing the nose gear. The flight controls are all linked to the control stick (no yoke) using control rods instead of cables; additionally, the aircraft will be fitted with a 2 stage flap system (will explain later, its complicated) that will allow the flaps to come down in more gradual notches from barely down to being a giant barn door. The plane will also be equipped with an electronically controlled spoiler system and constant speed, fully automated (no prop lever), reversing propellers. There are many other aspects that I would be happy to discuss, just let me know. I realize that it may seem like the plane is already built but it is still in the "drawing in a notebook" stage, (with a lot of dreaming).

    I mentioned powering it with 2 wankel engines from the Mazda RX-7. I am wondering just how common this is (i have seen a few youtube videos on it, but only 2 or 3) and how hard it would be to do. i have browsed the rx7 forums, and given that you can get 750 bhp from a 2 rotor 13b if you push it, i do not think that 700 would be that bad. if there are other engine ideas, then please let me know. i would like to keep it above 400 hp per side and keep the cost under 15k per engine (a 13b is about 8k for a remanned one) and associated equipment. i would also like the engine to be liquid cooled and burn about 12 gph at max (preferably mogas).

    if you have any pointers for me, then please let me know, because despite how much help i already have (several mechanical engineer friends are willing to help me), i am searching for any tips i can possibly find. if you have another idea, then let me know, and if there is anyone who has done anything remotely similar to what i am trying to do, then please let me know...

    thanks,
    Christian L.

  2. #2
    cwilliamrose's Avatar
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    Pulling that much HP out of an automotive engine is going to be a problem. The duty cycle in cars is completely different to an aircraft -- where the car may pull high HP for a short time an aircraft needs that power continuously. Also, think about how many family members will be wanting to go on trips in the airplane by the time it's finished and developed enough to carry passengers. That number could change a lot in 10-15 years.

  3. #3
    cluttonfred's Avatar
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    Christian,

    Welcome to the EAA Forums. If you haven't already, you might also think about joining the HomebuiltAirplanes.com forum, which is not affiliated with any group. Both are great resources. You project sounds very ambitious--very large, very complex, and hugely boosted engines. Personally, I would suggest something more modest as a first step--say building an existing, proven, simple, single-engine, one- or two-seat homebuilt. As you look around online and in the magazines, you'll find that twin-engine hombuilts are very uncommon, certainly less than 1% and probably less than 0.1%. Ditto anything with more than 300 hp or so, period. I don't want to discourage your enthusiasm, but what you've described is like saying, "I want to build my own home and I want it to be a 40-story skyscraper" instead of starting with, say, a small cabin as a first step.
    *******
    Matthew Long, Editor
    cluttonfred.info
    A site for builders, owners and fans of Eric Clutton's FRED
    and other safe, simple, affordable homebuilt aircraft

  4. #4
    Sam Buchanan's Avatar
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    Christian, I applaud your endeavor to obtain your ticket! My suggestion is to continue your training to include instrument and multi-engine ratings as you consider your aircraft design. You will need those ratings to utilize a twin-engine aircraft.

    Mazda-based rotaries have been used on a few aircraft but they require a tremendous amount of engineering along with complex gearboxes. They are usually limited to ~200hp in order to obtain some degree of reliability.

    Best wishes for an enjoyable pursuit of your ratings!
    Sam Buchanan
    The RV Journal RV-6 build log
    Fokker D.VII semi-replica build log
    YouTube Channel

  5. #5

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    Quote Originally Posted by cluttonfred View Post
    As you look around online and in the magazines, you'll find that twin-engine hombuilts are very uncommon, certainly less than 1% and probably less than 0.1%.
    Exactly! There is a surplus of small single-engine homebuilt designs that can't carry much very far. Which is fine if that's what you want. In constrast, there are woefully few homebuilt designs to choose from that can carry 7 people. As pilots decline as a percentage of the population, the fleet needs more aircraft that can carry more passengers per crew member.

  6. #6

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    It's neat you are in an aviation family.
    My family was 8. I found that most in my family don't want to fly much. Rarely more than one at a time.

  7. #7

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    Hi all,
    Thank you very much for your support. I really appreciate all the encouragement and advice. i mentioned getting a lot of hp out of the Mazda engines, and the only reason that i would think to do that would be because they are the Wankel rotary engines, not a traditional recropraciting engine. I may be wrong, but some of the research that i have done says that because the Wankels are so simple (3 critical moving parts in a 13b), that it is not excessively hard on the engines to run them at high power settings for extended periods of time. another piece of information that makes me think this is because drag racers who use the wankels run them at extreme levels of power (750-800bhp for a 13B) in a much harsher way, then especially because an aircraft engine is mostly running at a constant RPM, it would not be the end of the world. if I am wrong, then please correct me, and if you have any other engine suggestions, then please let me know. i have also considered several marine engines, as boats generally (similar to aircraft) cruise at higher power settings (greater than 60% throttle). this seems to me as if it is the perfect solution (marine engines are extremely durable and long-lasting), but there are no PSRU's available for any marine engines that i can find. I considered rebuilding an old aircraft engine, but it would limit my options to liquid cooled engines (i really want water cooled, i know that it is more complicated, but it gives you far more control over the plane) which there are not that many of, and then i would still not get the power that i want. as i mentioned before, please give any recommendations and advice that you have.

    thanks,
    christian L.

  8. #8
    Just a few things:

    1) I would be very interested to see the design because it sounds interesting.
    2) If you are looking to save money, it would be difficult to build a twin for cheaper than you can buy one. I saw a flying Travelair on Barnstormers the other day for less than $35k, and Cessna 310s regularly go for under $70K.
    3) Unlike a one-off design, a certificated airplane can be financed, which may make it more accessible than building.

    Assuming you are not looking to save money:

    The Mazda rotary has had limited success in aviation. I think it is related to the fact that they can be turned up to such high power levels. (I am not an expert but) based on what I have seen in RX7 and RX8s, it is really difficult to manufacture Apex seals that can stand up to constant heat of high horsepower applications. As a result high horsepower builds often work in drag cars or street cars that only use that horsepower for short amounts of time, but they are not used in circle track applications that run high, sustained load. Airplanes are more like circle track applications.

    If you want to use the rotaries, I would suggest dialing back your HP goals to the 250-275 range. But rotary engines are still appealing for weight advantages.

    If you can afford slightly more weight, a few Vans RV builders have had good luck with Chevy LS V8s. 400 HP, fuel injected and lighter than many aircraft engines with 1/2 the HP. Plus they are cheap, around $10K-15K for a new engine, converted for aircraft use.

    If you are looking for some less conventional inspiration, take a look at some Burt Rutan designs including the Defiant and the Starship.

  9. #9
    rwanttaja's Avatar
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    Quote Originally Posted by cluttonfred View Post
    Christian,

    Welcome to the EAA Forums. If you haven't already, you might also think about joining the HomebuiltAirplanes.com forum, which is not affiliated with any group. Both are great resources. You project sounds very ambitious--very large, very complex, and hugely boosted engines. Personally, I would suggest something more modest as a first step--say building an existing, proven, simple, single-engine, one- or two-seat homebuilt. As you look around online and in the magazines, you'll find that twin-engine hombuilts are very uncommon, certainly less than 1% and probably less than 0.1%. Ditto anything with more than 300 hp or so, period. I don't want to discourage your enthusiasm, but what you've described is like saying, "I want to build my own home and I want it to be a 40-story skyscraper" instead of starting with, say, a small cabin as a first step.
    I'd like to echo Matt here. This *is* a highly ambitious project, and you probably would be better off to scale things down a bit.

    Developing and testing a new auto-engine conversion will take the same amount of time...or more...than the building of the airplane. Reliable auto-engine conversions have been rare; the accident rate for auto-powered homebuilts is significantly higher than for traditional engines.

    In the archives of the EAA Forums you'll find at least one thread dedicated to the Twin Jag aircraft, an RV-6 with two Corvair engines. The first posting was over five years ago; the airplane just recently flew for the first time.

    Consider: The builder took a flying RV-6, and installed two Corvair engines (which are commonly used on homebuilts) and it STILL took him over five years.

    If I were you, I'd bag the Wankels. There are a lot of older twin-engine aircraft that can be bought for a song; use one to supply engines for your project. Considering the large size of the aircraft, you MIGHT consider turboprops.

    Like Matt, I recommend you build something NOW; something you can reasonably complete in a reasonable amount of time. Something like an RV would be perfect, considering you're planning to build your dream bird out of aluminum. Work on the new design while building the RV. You'll undoubtedly learn a lot in the process that could be used to make your design better. Once the RV is completed, you can start building the new design. The RV would keep your skills sharp while the new plane goes together.

    Ron Wanttaja

  10. #10
    Dana's Avatar
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    Welcome Christian,

    As others have said, you're proposing a really ambitions project... one that even most experienced aircraft designers would shy away from. Designing even a simple aircraft is not a simple thing, and the complexity of a fast twin with all its systems makes it several orders of magnitude more difficult. Without going into the big picture, let's focus on a few aspects of your proposal:

    First, the engines. Yes, Mazda rotaries can make absurd amounts of power... for a very short time. So can big block V-8's and other automotive engines. The problem, in an aircraft application, is usually cooling those engines. A drag racer runs for a few seconds and shuts down, the engine has time to cool afterwards... and even so, the engines only last for a few races. Another issue is engineering the reduction drive... you can't efficiently turn a propeller at the rpm a modern auto engine is turning, so you need a redrive. Reduction drives look simple, but the engineering to make one reliable is anything but. Torsional resonance is just one of many issues that have ended many otherwise promising designs.

    Marine engines are heavy and have an unlimited amount of cool water available for cooling.

    You're talking a lot about minor details like fire suppression systems, landing gear geometry, and control linkages. It's tempting to get interested in those details because they're easily comprehensible, and many inexperienced designers make the same mistake of fixating on the little details before the basic configuration is worked out, I've been guilty of it myself. But it's way too soon to worry about those kinds of details.

    Now, let's compare your concept with the Diamond DA-62 you mention. You're assuming a similar weight, but a much smaller aircraft 33 vs. 48 foot wingspan), with a lot more power (never mind that the power isn't realistic). Have you considered what that means in terms of stall speed and runway requirements (not to mention pilot skill)?

    Don't stop dreaming, but at this stage of your aviation experience, focus on learning to fly and learning more about what's out there, both production and experimental airplanes. See what ideas work and what ones haven't been successful, and why. You can learn a lot about designing and building with R/C models and ultralights. In a few years you'll be much better able to judge what's realistic (or useful!) and what's not.

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