Peter Sripol’s DIY Electric Ultralight MK4

Peter Sripol really likes building gravity defying death traps. He recently flew the fourth ultralight, which he designed and built himself. For a taste of what’s going on here, the wings have aluminum tube spars and are made of hot-wire-cut styrofoam sections.

To keep the plane simple, he got rid of ailerons entirely. For roll stabilization he angled up the wings noticeably, adding dihedral. This gives the aircraft passive stability, because as it rolls to a side, the upper wing’s lift decreases and the lower wing’s lift increases, forcing the plane to correct itself. Interestingly he kept the rudder controls on pedals instead of moving it to the stick, so the stick only controls the elevator.

It is powered by a single large brushless electric motor borrowed from the OpenPPG project. On the first test he used a two-bladed propeller, with a small pitch angle which required full throttle to keep flying. It can be compared to driving a car only in first gear. By moving to a three bladed propeller with a higher pitch angle, and increasing the length of the wings for more lift, [Peter] was able to cruise comfortably at about 30 MPH or 48 km/h.

Although this aircraft definitely performed better than [Peter]’s previous ultralight builds, piloting something like this isn’t for the faint of heart. Although he does extensive weight-loading and thrust testing before taking to the air, adding tail weight to piloted aircraft by simply taping a water bottle to the tail just felt wrong. But we aren’t aviation experts, so we won’t pass final judgement.

70 thoughts on “Peter Sripol’s DIY Electric Ultralight MK4

  1. I really like Peter’s model builds and prototypes, but draw the line at his ultralights. Some of his wing and structural assemblies look rushed and flimsy and the taped water bottle ballast seems bordering on being a little careless.
    Come on Peter, it is not worth risking your life or serious injury for the sake of some fame and a few thousand ‘likes’.

    1. mehh, It’s not like the weight is crucial to make it flyable. If it’s for just a short test, it’s not that big a problem. And if it falls off, he just needs a bit more nose down elevator. Would it have been better to have a dedicated weight box designed in? Sure. But this works for a quick test.

      1. Actually, the weight IS crucial, both in terms of lift/weight and weight and balance. I know of people who died because of that attitude so that attitude is dangerous. BTW, I’m a pilot of 43 years, aircraft technician of 35, and with an education in engineering.

        1. So, you’re a pilot and technician with an education in engineering, and you’re telling us that one pound of weight on the tail is crucial. Got it. Just like to point out, if the bottle falls off, the plane will nose down ever so slightly. Not going to kill anybody.

          1. >BrightBlueJim
            It’s called Moment. Please don’t ever fly. Where it’s not adequately designed, or not adequately built, or a pilot hasn’t paid adequate attention to how he’d loaded his plane, and the balance-weight for single/double ‘occupancy’ hasn’t been adequately ‘retained’, this has killed people.

            If he’s got the weight balance design done well and if properly designed & built stabilizers & controls with adequate control authority, AND it’s loaded within design limits… You can get away with a lot of crap design & crap builds at RC scale due to Reynolds number (Re), loads & material proprieties, but that same leniency or lack of understanding does NOT scale well to the loads seen in human flight.

            And the above (and other design or build deficiencies seen in Peter’s meant-for-human ‘aircraft’) are things that have caused or contributed to flutter ‘incidents’ in aircraft. Some resulting in controls or entire aircraft coming apart in the air (with corresponding fatalities), some getting back to the ground with damaged parts or with a rebuild-able airframe, and others where the plane appears good but is now only safe for suspending inside the hanger as a decoration.

          2. I fully understand moment. YOU clearly don’t understand real-world magnitudes. We’re talking about maybe ten foot-pounds here. But this doesn’t even require an understanding of moment. It’s ONE POUND. If your elevator doesn’t have 1 pound of authority to spare, you’re dead anyway.

  2. At least it’s got significantly less flex & flutter than his earlier builds.
    Still, watch the one wing lift as it gets loaded on takeoff.
    Still wouldn’t do a circuit for $100 in cool still air.

    Hilarious watching something with that much dihedral; doesn’t need that much for two-axis controls.

    1. This shows a great deal of improvement over the previous two models – less useless weight and simpler. The two-axis controls save a lot of complexity and weight, and the whole build is simpler.

    2. I flew a Teratorn T/A ultralight in the early-1980s. The T/A stood for tri-axis as the transition from weight-shift was nascent. The roll axis was supposedly controlled by spoilerons, boards about 6″x18″ on the top of each wing that flipped up to spoil lift on a small portion of the wing, much like asymmetric air-brakes. They were grossly ineffective, to put it kindly.

      In practice, we never used left-right stick, instead steering exclusively via the rudder pedals. Due to the wing’s high dihedral, uncoordinated flight increases AOA on the forward wing and decreases it on the trailing wing, resulting in the desired roll. Slipping wasn’t even possible, as the spoilerons had so little authority.

      1. This was originally going to be powered by the paramotor motor, because his earlier two planes, both electric, were way underpowered. Don’t know where he got the current electric motor, but he mentions somewhere that the vibration with the 2-stroke was bad, and he worried about whether the airframe could handle it.

    1. This is part of the false safety of staying low. Without flying high enough to safely establish the stall speed, he won’t know until he’s in a situation where he’s too low to recover.

    2. The policy of “low and slow” is based on the facts that low potential energy (“low”) and low kinetic energy (“slow”) leads to low total energy if and when you crash. Unfortunately, the combination of low and slow means that when you stall, you will be too low, and you WILL crash.

      1. At the speeds involved its hopefully more a forced landing than a real crash… Depends on the stall characteristics, but with so much dihedral and that layout Its not likely to really corkscrew into the ground easily is it? I’d think the stall of that sort of plane would be relatively safe..

  3. His builds are sometimes interesting, but so much of his “engineering” is guess and check style. Pretty terrifying when it starts to involve human lives. This reminds me of his general nonchalant attitude about building a hydrogen blimp too which he set on fire and then had to stamp out because he didn’t appear to have a fire extinguisher…

      1. But yeah. This is an RC airplane scaled up. Which points out the difficulties in scaling up.

        On smaller scale, you can afford carbon-fiber spars, and don’t need to hollow out the foam either. Document laminating film adds a lot more strength at that scale than the vinyl film, too.

        And then there’s the whole not-having-to-carry-a-payload thing, which really frees up the design a lot. He spends a lot of weight in the frame b/c he needs to be carried.

        But it’s cool that he gets it this far. I’m surprised and impressed.

        Just imagine what he could do with a real budget…

  4. If anybody wants to the other end of building an air plane ( as in over the top professional), check out Mike Patey’s youtube channel (, and his series on building Scrappy (and earlier Draco). That man knows what he’s doing. The subject, building a carbon Cub with an 8 cylinder 300+ HP Lycoming engine, using an airboat prop to be able to sink all that power and a landing gear with several feet of compression travel.

    1. True, Mike Patey is in a league of his own when it comes to building airplanes, but his budget is also in a league of its own. I doubt Peter has millions of dollars to put into his projects, which is required to build on Mike’s level. Mike once talked about the cost difference between building materials, and I was flabbergasted at the huge costs involved with carbon fiber anything.

      Personally I like what Peter is doing, but it seems a bit reckless at times. There are better and safer ways to home/garage build ultralight aircraft.

  5. I find Peters builds to be very inspirational 👍🏼 – and a bit scary at the same time. But I believe we’re living in the second golden age of personal Aviation these days. I never thought I’d live long enough to see a man fly with jet engines on his back or on his feet or a man in a lawn chair with 12 electric motors or etc etc…
    and here is Peter building multiple different flying aircraft, in a similar way that early pioneers of Air flight would have done in 1912. I literally cried when I watched him take off in that first bi plane and cruise around. To me it encapsulated the best of humanity, amazing creativity and boldness of spirit.

    What an inspiration!

    Yes he’ll probably die sometime because of his crude RC engineering adapted to full size aircraft… just the same as many people died in the early 1900s to further the cause of Aviation. Is it a waste of life? I don’t know, who’s to say…what do you think?

    but we all know about the various sayings, ” better to risk it doing something great, than to stay home, scared & afraid”. Insert your own risk affirming saying in the comments below 😄

      1. Aviation in itself is not inherently dangerous. But to an even greater degree than the sea, it is terribly unforgiving of any carelessness, incapacity or neglect.
        – – Captain Alfred Gilmer Lamplugh, CBE, FRAeS, MIAeS, MCAI, FRGS

  6. Does it not have ailerons or am i missing something? No ailerons in a model is fine. No ailerons in something i’m flying in – oh hell no! I’ll take all the control I can get!

      1. Bloop 1-4, Weedhopper, Quicksilver, Easy Riser, Wing Ding 1 & 2, plus more gliders than I care to list all lack ailerons.

        In an ultralight aircraft ailerons only add weight, and complexity unnecessarily. Going that slow with such light wing loading you don’t need to bank any more than the dihedral with the rudder permits.

        1. People say that having no ailerons makes it impossible to take off in a crosswind. One designer/builder of a 2-axis ultralight (Sky Pup?) counters, “what crosswind? I can take off in the width of a standard runway. I can always take off into the wind!”

  7. All engineering eventually gets to the check our assumptions phase. Nothing he is doing is all that unusual, sure with bigger budget you’d do flowsims and structural stress modelling in the computer for months before building anything and then probably stress test a model or two. But you still get to the we guessed this will work, does it? phase. Take the Millennium Bridge in London, modern science and design goes into it, and it wasn’t fit for use when it was initially finished..

    All it takes is one small oversight in the model or modelling systems to have a huge failure that is probably worse than anything Peter will experience as its his life if something does seem good enough it will be changed – needed or not- all that theoretical work gives too much confidence the design has no weakness, so even if you do think ‘hmm that seems a bit ‘ while building you assume it will come good once the assembly is complete, because the maths says so. Maths is wonderful, perfect stuff, but actually applying it correctly to every atom of your design never happens…

    Also why waste a fire extinguisher when you can use other free methods of putting it out… May not be that expensive, but they do cost and if you are using them every time you might possibly need too that’s going to add up fast. If they didn’t have one at all its poor planning – on the off chance you need it you should have it..

    1. I think Peter does a great job at proving he can actually fly on the planes he builds however at this point I guess he should start using remote control to avoid any terrible accidents. Stick a GoPro on the airplane and add the equivalent to his weight and there you go.

  8. Everybody getting their panties in a twist because someone build a flying minibike and puttered around in it himself. Yet I bet every single last of you would get behind the wheel of a koeningsegg or lotus 7 if offered, knowing damn well that handling a car with that power/weight ratio and no drivers aides takes no prisoners.

    look, I get it, I work in aviation and I see it daily. somehow dying from flying is worse and more scary than being killed safely on the ground doing something we all think is normal. And I’m no less guilty of it than anyone else. I design aircraft parts. important ones. I know how the industry works. And I’m scared shitless in the air, yet think nothing of driving and I drove some sketchy contraptions way to fast. People brains are not wired for accurate risk assessment of flight, but we are wired for fall risk. 2 legs and a mushy brainbox on top kinda demand that.

    But although we are all conditioned to feel safe under the pressure of His Noodley Appendages, approach the risk honestly: the speed isn’t that high, he isn’t flying that fast, the plane has been reasonably tested (not to aerospace standards but we are scaredy-cats by profession), it’s fine.

    Most importantly: He build it, he’s flying it over empty ground, no-one but him is at risk! it’s his life, his risk, and no-one elses’. he isn’t even doing anything illegal!

    1. He is also careful not to give too much detail about his planes. In response to people asking for plans, he has been adamant that he will NOT publish plans, because while he has weighed the risks for himself, he wants to be sure that any error in his design doesn’t kill anyone else.

  9. If I wanted my wife to collect the most possible life insurance, you can’t beat a home-built aircraft.

    Actually, it an interesting build, I’ll give he credit for both that and his courage. I considered building an ultralight at one point, as a coworker had built two himself. But I also remembered that I’m afraid of heights.

      1. Very, very true Ctiatura. I helped build over a half dozen Van’s RVs and even owned and flew my own RV-6 for half a decade before hanging up my wings to pursue other life adventures. The collective homebuilt fleet of Van’s various models flying worldwide has an identical, if not slightly better accumulated total safety record than the certificated fleet of piston single engine factory built planes from Cessna, Piper, Beech, Mooney, Stinson, Aeronca, Bellanca, Luscomb, et, al….

        Besides, virtually all life insurance policies exclude dying while acting as a pilot or flight crew member of a privately owned, non-commercially flown aircraft anyway, lol! So the previous poster seems to just making a wisecrack to disparage homebuilt airplanes in general.

        I’m also deathly afraid of heights, yet had no problems flying my RV-6 solo across the Rocky Mountains four times, getting it up to as high as 16,500′ MSL. Sitting on top of a slab of aluminum wing and looking out a bubble canopy at the beautiful mountain scenery is very comforting….. especially with the control stick in my hand, lol!

  10. For the people overly concerned with the safety aspect of this all, perhaps to get some perspective try and find the Scapheap / junkyard wars special “Flight of the century” and see that the British team pulled off in that show..

    In comparison, this video looks like a documentary of NASA testing protocols for human rated flight.

  11. What’s with all the fearmongering and naysayers?

    There are thousands and thousands and thousands or home build air craft flying legally and safely all around the world. Some of them have even literally been flow around the world, east to west, west to east, and over the poles too.

    Wittman Regional Airport in Oshkosh, Wisconsin literally becomes the busiest airport in the world for that week.

    1. Armchair wannabe pilots who’ve never flown an actual airplane maybe? Or have seen the long lists of ADs and SBs on factory built, certificated small piston singles (which still pretty much get assembled the exact same ways, by hand, as someone building a Vans RV, a Glasair or a Cub clone rag and tube plane.

      I had a friend who was a lot like Peter Sripol, in that he loved to build and fly unique experimental planes. He built a Smith biplane out of wood and fabric, owned an RV-6 for a while, and was in with a group of WW-I replica builders and fliers when he died. Aviating didn’t kill him, a drunk driver crossing a highway centerline and hitting his car head-on at high speed killed David and his wife in January 2016.

      Oh how I miss my friend and I miss Oshkosh/AirVenture, I flew my own plane to there 10 years in a row, the last time was 2016.

  12. Everyone is concerned about Peter’s lack engineering and how it might kill him. Boeing recently put two in and have made major mistakes in the past, even with the best engineering. At least he is not put passengers in danger.

  13. Peter is an inspiration to me. I am still designing my ultralight going on over 8 months now…(family obligations and work come first).
    Always remember, there is no reward without risk. We would not be flying at all if it wasn’t for people like Peter in the early 1900’s.
    Also, how many test pilots have died flying experimental aircraft that were developed by teams of aeronautical/aerospace engineers?
    If you don’t like what he builds and flies, don’t watch the videos.

    1. Very true. I really like his videos and perseverance. Building an airplane isn’t easy, plus like you said there are many other crazy people that make crazy contraptions that are way worse than Peter.

  14. Peter gets things done and has great enthusiasm. You are lucky to have less regulation than here in the UK. Here, sub 70kg foot launched powered planes do not require third party insurance by law, but if you fit wheels (and still stay under 70kg), you have to have insurance. I asked the UK CAA exactly what additional hazard was caused by having wheels – they have not been able to reply. I noted that Peter did not use any drag bracing in his wings, so is relying on shear stress in the foam/covering and some assistance from the struts to resist drag loads.

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