Direct Drive

Discussion in '2-Stroke Aircaft Engines' started by mstull, Jan 17, 2006.

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  1. Jan 17, 2006 #1

    mstull

    mstull

    mstull

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    Does anyone have experience running a two stroke engine direct drive? I'd like to hear from anyone who's actually flown one... particularly a legal ultralight.

    Most 2 stroke engines don't have enough torque to turn a prop direct drive, nor enough power at direct drive RPMs to get decent thrust. I've been looking at trying it with a 605cc, MZ201 twin from Compact Radial Engines. That engine is specifically tuned for low RPM torque, and has enough power in the 3,000 to 4,000 RPM range for my plane.

    If this works, it could be a good alternative to the Rotax 447 with B box, for ultralights looking to shed a few pounds. I've been using single cylinder engines with belt re'drive. But I'm really tired of the short belt and pulley life. If it's not too loud, and doesn't vibrate too much, the engine should be very long lasting, cruising at around 3,000 RPM.

    Compact Radial Engines also offers 3 or 4 cylinder radials that they always run direct drive. So CRE is the only manufacturer I know of that deals in direct drive 2 strokes.

    For those unfamiliar with the concept... Reduction drives multiply engine torque, allowing a much longer prop, that generates much more thrust. The re'drive also divides the RPM, so a modern, high revving engine can turn the prop at a more reasonable prop RPM.

    Leon at CRE says he knows that people are using the MZ201 direct drive successfully. He even sells the prop hub for it. And Stuart at Powerfin says their E-2 blade prop should be about the right size, but no garantees. The engine would weigh about 65#, and put out about 35 Hp at 4,000 RPM.

    Mark E. Stull
     
  2. Jan 20, 2006 #2

    dustind

    dustind

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    I have never done that, but if the engine is running at it's design rpm and can handle the verious stresses of running direct drive then it should work.

    The powerband may be a bit too peaky to have any useful power at a lower than designed RPM. I am also not sure what the fuel consumption would be like. Would it just spit unburned fuel out the exhaust?

    I imagine two strokes without large expansion chambers would do better at low RPMs than ones that are highly tuned for one speed.
     
  3. May 11, 2006 #3

    Ted

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    Hello Mark

    Just noticed on the composite thread that you're planning to install an MZ201 in direct drive. I am considering this engine to replace the kfm 30hp in a Moni motorglider (kfm was a bit too light anyway). I got some interesting info off a French friend who ran an MZ201 in direct drive very successfully. Here's the content of his email to me:

    "I remember the MZ201 as an excellent engine. I was the first one to use it in a direct drive configuration (in France). I made a lot of testing before I was fully satisfied with the results. I used the Michel Colomban's bar method to explore the power curve, from 22 to 34 HP. The power was 26.5 at 3600. I used a 110/65 cm prop.

    I was so pleased with that engine that I drew the plans for another little plane which was intended to succeed to my Souricette (the proto is now being factory built).

    It is a sturdy motor, smooth and quiet. The fuel consumption was 8 l/h. If compared with the Hirth F23, it is far less. This last motor, on a souricette, burns 10 to 12 l/h. Here are the results when testing in the final configuration I chose: dual carburetors, selected jets, simple muffler, ...

    They are here compared with the data given in the F23 manual:

    rpm MZ F23
    3250 24.44 xxx
    3500 26.51 23.5
    3700 29.15 26
    4100 32.45 31
    4300 34.32 33

    Here are the true measurements I made on a friend's F23
    3190 20.1
    4000 30.13
    4140 30.6

    Both engines seem very close to each other. But the F23 I tested was equipped with an expensive dual tuned pipe muffler, while the MZ had only the simple exhaust that I took from the single cylinder engine. That gives a flat power
    curve and more torque at low rpm."


    Hope that helps and I am very interested to hear about your experience with the MZ201.

    Ted
     
  4. May 11, 2006 #4

    Ted

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    ah! - pic didn't go - trying again...
     

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  5. May 11, 2006 #5

    Ted

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    and from the other side...
     

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  6. Jun 22, 2006 #6

    mstull

    mstull

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    Thanks Ted,

    I notice the engine in the photos is a dual carb. The current MZ201 is single carb. Leon at CRE says the dual carb adds 10% more torque at the low, direct drive RPMs. Even so, thrust will be very, very low.

    As it turns out, the aluminum, belt, re'drive is only about 2 pounds heavier than the steel, direct drive hub. For those added 2 pounds, you get 2 to 4 times as much thrust... depending on prop, reduction ratio, and RPM.

    I loved the sound and simplicity of running direct drive. With the low RPMs, and prop in sync with the engine, it sounds very much like a 4 stroke aircraft engine... Totally cool.

    With a big Powerfin B prop, my plane now climbs at a rediculously steep angle. It has way more thrust than my plane needs. So I'm experimenting with different props to optimise fuel economy in cruise.

    The biggest problems I've had are vibration related... no surprise. I'm on my 4th muffler mounting system. The engine didn't come with any provision for muffler mounting.

    I wasn't satisfied with the stock, diaphragm carb. So I adapted a float/slide carb.

    Here's a fairly recent photo of my MZ201.
     

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  7. Jun 23, 2006 #7

    mstull

    mstull

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    Intake Manifold

    Here's a photo of my latest intake manifold. I modified the stock 1 into 2 one. The carb and rubber flange are from my old, Hirth F-33.

    Normally, two-stroke engines' props and reduction ratios are chosen so the engine can just reach peak horsepower RPM at full throttle. I ordered a much larger, 58" wide-blade (B), 2 blade prop to configure it for that. But since this engine has way more power than my plane needs, I can set it up for most efficient cruise. There's obviously other factors to consider, like noise, vibration, climb rate, etc. So I get to experiment.

    I stuck a 51", narrow (E), 2 blade prop, set for very high pitch, on my reduction drive. This prop is too small for this engine. Powerfin only rates it up to 30 Hp. So I can't use full throttle with it. My plane climbs excellent with this set-up, 5,200 engine RPM at 1/3 throttle. It cruises well too... 4,600 RPM at 1/4 throttle, at 45 mph. It burns 2.2 gph at that speed.

    We'll see what it does with the big prop. There are other possibilities. I could use a different reduction ratio.
     

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  8. Aug 26, 2006 #8

    mstull

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    MZ201

    Ted,

    The MZ201 is mostly reliable so far, at about 70 hours. I'm even still on the original re'drive belt. I kept upgrading my muffler mounting, until now it lasts fine. I've limited the throttle to 1/3... about 30 Hp. Above that, the engine would overheat without the optional fan cooling, on a pusher. So I'm operating it way below its limits.

    I'm using the 2.08 re'drive ratio, with a Powerfin BL2-58P prop. It could use an even bigger prop or taller re'drive. I had to set the pitch way high with the above set up. I'm burning just 1.6 gph, crusing nearly 50 mph. So it's still real efficient with my single float/slide carb.

    My only complaints: It vibrates a lot... but not really more than most engines. It is pretty loud... but really not more than most engines. The re'drive housing broke, but was replaced with a stronger one for free.

    There's a wide flat spot of reduced power between 35% and 90% throttle. I tried everything with the carb tuning to eliminate the flat spot. But it's not a fuel mixture problem. There's some kind of resonance in the one-into-two intake manifold. 35% throttle yeilds about 30 Hp.

    I'm fixing to stick a spark advancing CDI (from the Kaw 340 that didn't work out) on it, to replace the stock non-advancing ignition. That should smooth the idle a bunch. I'm still flying about 3 or 4 days a week... over 1,600 landings so far.
     
  9. Sep 8, 2006 #9

    mstull

    mstull

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    Spark Advance

    The spark advancing ignition from the Kawasaki engine didn't adapt.
     
  10. Nov 4, 2006 #10

    mstull

    mstull

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    Direct Drive Works !!

    I bought a wood prop, and started experimenting with direct drive on my MZ201 again. The 39" by 18" wood prop had too much drag to turn enough RPMs to climb well. It was a cheap prop with a thick airfoil and very blunt (3/16" thick) trailing edges.

    I reworked it, sharpening the trailing edges, decreasing the pitch, and making the tips more efficient. That helped only a little. Its airfoil was still too thick. That Arrowprop was made of a very light wood... poplar, I think. It was so light that the engine vibrated a lot... not enough gyroscopic effect. A heavier prop runs and idles much smoother, and is easier to start.

    I tried running a 38" Powerfin with E blades, and the pitch set very low (with the adjustment dowels all the way against the low pitch side of the windows). It worked great until the blades broke apart at around 4,600 RPM.

    I took a broken one apart to see what happened. There is a very small balance weight in each blade, about 14" out. It's just a blob of epoxy with a few little balls of lead shot. It had broken free, and flown out the end of the prop through the styrofoam core, and broke the trailing edge apart near the tip.

    I had some spare blades and decided to try to pin that weight in place with a 1/8" carbon rod/dowel. So I drilled a hole through the prop blades, near the outer edge of the weight, and epoxied the rod in the hole. Then I ground its ends off flush.

    I set the pitch flat, and ran it up well above my normal maximum RPM. It held. If anyone wants to know exactly how and where to put the carbon rod, let me know. I can even send you a couple inches of carbon rod, so you don't have to buy a whole, 100' roll.

    Performance and engine RPMs, with the modified, Powerfin prop, are almost identical to the big, reduction driven prop... amazing. The pulsating sound and vibration are gone, replaced by a clean, high RPM sound and vibration. Overall, the noise and vibration levels in the cockpit are about the same.

    The planform of the Powerfin isn't really right with the pitch set that low. I measured it at 16" near the tips. But in the middle of the blades, it measures 19".

    Performance is so good, I decided to stick with direct drive. I adjusted my CG for the lighter prop and lack of re'drive. My plane is about 5 pounds lighter... which is a lot on my true U/L.

    But don't assume direct drive will work on other engines (or planes). The MZ201 or MZ202 has 626cc, and very high torque at low RPMs. And my plane doesn't need much thrust to climb.

    But for someone who is tired of struggling with unreliable or gutless single cylinder engines, or looking to lighten your Rotax twin powered U/L down to a legal weight, this is the best choice I've found. You'll have to order an efficient 38" by 18" prop that has a thin airfoil and sharp trailing edges, or modify a Powerfin E2 prop.

    To save weight, Leon at CRE will sell you a stripped down MZ201, with no electric starter, no starter ring, direct drive hub instead of re'drive, and even single ignition if you want, at a discount. Actual flying weight comes out about 20 pounds less than a 447 with B box and big prop.

    I ordered a Prince, P-tip, composite covered, 38" by 18" prop. I'll let you know if it works any better than the modified Powerfin.
     

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  11. Feb 6, 2007 #11

    mstull

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    I tried my new, Prince, P-tip prop. It is dramatically more efficient than any other prop I've tried. It's tip losses are so low, it allows the engine to rev much higher, where the engine puts out much more power... 5,100 RPM vs. 4,000 RPM.

    When I push the throttle to the stop, it presses me back into the seat. (The throttle stop is set at 1/3 throttle, to limit engine power to prevent over-heating. Free-air cooled pushers don't get much breeze through the engine fins.) The take off roll is extremely short... about 1/3 of what it was. It only takes about 8 seconds to lift off. Then it climbs 2 or 3 times as steeply... amazing. So direct drive REALLY works now.

    Several prop manufacturers suggested that running direct drive on a two stroke engine is asking a prop to do what it's not good at doing. The thrust from outer few inches of each blade is lost, as the air circles back around the tips. But the inner 2/3 of each blade isn't going fast enough to generate much thrust. And much of that area is blocked by the engine, muffler, etc. On such a small, 38" diameter prop, that leaves almost nothing.

    So the key to making direct drive work is to use an extremely efficient prop. The curled over tips, of the Prince P-tip prop, eliminate much of the tip losses. So the outer part of each blade really generates thrust. My tip speeds are about 0.75 mach. The 14" pitch is ideal for climb. Climb performance is way better than I really need. This bodes well for my biplane project, which has a less efficient airfoil.
     
  12. Mar 14, 2007 #12

    mstull

    mstull

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    P-tip Prop

    Here's a photo of the carbon covered, Prince P-tip, direct drive prop. The thrust is amazing.

    I'm covering and painting my new U/L biplane now. I'm flying my U/L monoplane a lot lately, trying to reach 2,000 landings before I disassemble it.
     

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  13. Mar 14, 2007 #13

    orion

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    Well, there's efficiency and then there's efficiency. A few years back several organizations tested a variety of props on several different engines and aircraft models. As might be expected, the numbers were somewhat incongruent, with some props being better than others at some conditions while different units were better at others. There were however some pretty conclusive numbers for the P-tip configuration:

    The prop was shown to be quieter than all the others at just about any condition. It also tended to be smoother in operation, transmitting less vibration and/or noise to the cabin.

    The actual efficiency though was less than anticipated and in general, all tests resulted in lower numbers than that of the more conventional configurations. In one or two cases the differences were substantial.

    Even in the best of circumstances, the increase in tip efficiency would be relatively small. It is more likely that your dramatically increased climb performance is more due to a flatter pitch and better twist distribution than due to the bent tip geometry.
     
  14. Mar 14, 2007 #14

    mstull

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    P-tip Prop

    Thanks Orion,

    I'm sure the flatter pitch and accurate twist help significantly. I wonder if the comparison test you refer to, used the all wood or carbon covered Prince prop. The carbon covered one is advertised as having a thinner, more efficient airfoil, and sharper trailing edges. I think the advantages of an efficient prop multiply in a direct drive application... especially tip efficiency.

    According to Newton, thrust equals the mass of air that's accelerated, multiplied by how much that air is accelerated. You'd think the smaller direct drive prop diameter would accelerate much less air. And with just 14" of pitch, it can't be accelerating the air to any tremendous velocity.

    Here's the clincher: My cruise and top speeds are just as fast as with the reduction drive and big prop. And my fuel economy has improved slightly to 1.6 gph. I realize there's other factors involved, like fuel mixture and engine tuning. But in every way, my performance and efficiency is equal or better than with reduction drive. I'm amazed, and enjoying the heck out of direct drive... even though I can't figure out why it works so good with this prop. It sure worked poorly with the two other props I tried.

    I wonder how much power is converted to heat in a belt or gear drive.

    Here's a photo of my self-designed U/L biplane project. It will use the same engine and prop.
     

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  15. Mar 21, 2007 #15

    dustind

    dustind

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    Nice biplane. Are you going to completely take your current ultralight apart, or just remove the engine and prop?
     
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  16. Mar 21, 2007 #16

    mstull

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    Dustin,

    I plan to disassemble my monoplane pretty completely. But I'm saving all the parts so I can reassemble it if the biplane doesn't work out for some reason. In addition to the engine and prop, I'm also stealing the parachute, nose gear assembly, wheels, and a few cockpit and control system parts from the monoplane.

    I'm making fast progress on the biplane. It should be ready to fly next month. It just fits in my hangar with my monoplane. It's pretty small.
     

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  17. Mar 28, 2007 #17

    mstull

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    Re: Biplane

    I finished my self designed, scratch built biplane and flew it on Tuesday. It is very easy to fly and land. I did about 12 landings, and a few stalls. It weighs in at 206# dry, including the BRS parachute. That's 72# under the legal limit... amazing.

    The trim came out very close to perfect. I could fly it hands off on the first flight. The 4 ailerons are wonderfully powerful, with a spectacular/aerobatic roll rate. Adverse yaw is slight. The elevator and rudders work fine. There is not a hint of flutter, even when I try to wiggle the controls back and forth.

    It stalls at 30 mph indicated, with the flaps up. It doesn't drop a wing or the nose in a stall. It just shudders and sinks, and recovers instantly when I push the nose down. Bumps of wind feel gentler with the higher wing loading.

    This plane has a lot of drag, and the wing airfoil is less efficient than my monoplane. So it takes more power in climb and cruise. The glide slope is pretty steep too. It flies most efficiently between 40 and 45 mph. It has too much drag to fly much over 50 mph. The engine vibration is much less in the cockpit than my monoplane.

    I plan to install larger 5 by 5" aircraft tires so I can land on rough fields. There are only a few other minor adjustments needed. I'll probably upgrade some of the flying wires to the next size larger, since I have so much weight to play with.
     

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  18. Mar 28, 2007 #18

    wally

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    Congratulations!!

    Amazing job. Sounds like you have a nice UL there. It looks like you tried to keep everything simple as possible. You didn't have to bend very many big tubes did you.
    Wally
     
    Last edited: Mar 28, 2007
  19. Mar 28, 2007 #19

    mstull

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    Re: Biplane

    The ribs are all bent tubing. They work more like battens than real ribs. I can only think of one other bent tube on the plane. Here's the preliminary specs:

    Dry weight: 206# Including the BRS parachute
    Maximum gross weight: 450#
    Engine: MZ201, free air cooled, recoil start, single carb, direct drive
    Fuel consumption: 2.0 gph
    Fuel capacity: 4.9 gallons
    Endurance: 2.4 hours
    Prop: Prince P-tip 38" by 14", carbon covered, direct drive
    Cruise: 35 mph to 50 mph
    Vne: 60 mph
    Stall: 30 mph clean, 26 mph flaps down
    Maximum crosswind component: 15 mph
    Take off and landing roll: About 150'
    Wing span: 18' upper, 14' lower
    Wing area: 128 square feet, including the 4 flaperons
    Glide ratio: 4 to 1 estimated
    Controls: 3 Axis, with dual full flying rudders
    Design loads: +4 and -2 Gs minimum
    Rate of climb: 400 fpm
    Covering: Stits Poly Fiber
    Main gear: No suspension. No brakes.
    Nose gear: Steerable pneumatic tire
    Instruments: Altimeter, tachometer, air speed, visual attitude indicator
     

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  20. May 18, 2007 #20

    mstull

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    Just thought I'd keep y'all updated on my U/L biplane and direct drive experiments. The biplane has successfully completed its flight tests with no problems. The final testing included doing rolls.

    Vne is up to 70 mph. I upgraded the main gear to 5.00 by 5" aircraft tires for rough field landings. Empty weight is up to 228#, inluding the BRS parachute.

    The biggest problem I've had with direct drive was the noise. To get enough thrust with that little 38" by 14" prop, I was climbing at 5,200 RPM. My neighbors complained about the loud, scream, that sounded like a mad bumble bee.

    After studying my engine performance charts and prop data, I felt that my plane could climb if I could turn a 50" prop at 3,800 RPM. I figured I'd need to upgrade my engine with dual carbs to turn it. So I ordered them. But I stuck on my 50" Powerfin E-2 prop with extremely low pitch with the single carb, to see what would happen... and it turned it.

    My plane climbs just as well, and cruises a little slower, turning much lower RPM at much higher throttle settings. The sound is quieter, and has a much more acceptible, 4 stroke sounding growl. I'm climbing at 3,800 RPM at full throttle, and cruising at 3,500 RPM at 1/3 throttle. I'm averaging 2.0 gph.

    The Powerfin prop is set at such a low pitch, that is less than ideal for it's twist. The tips have almost no pitch at all, while the inner 1/2 of the blades have too much. Static thrust is excellent. But at cruise, the tips provide no thrust at all.

    When the dual carbs arrive, I should have enough torque to add some pitch to the prop to get the RPMs down a tad more, and get thrust from the tips to improve the cruise speed. That will make the twist a little more efficient too.

    One disadvantage of the lower RPMs is greater vibration. At the high throttle settings I'm using now, the power pulses create strong torsional vibration on this 626cc engine. It's still less than some of the single cylinder engines I've tried. And people smile when they see my plane, rather than hold their ears.
     

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