I think all need to be aware of the possibility of tach errors, when adjusting my prop on my new 701, I realized when it took 12.5 -13 degrees to get the engine max RPM below 6,000 that something was wrong, and my mechanical master tach confirmed it . it was almost 1,000 RPM's fast , the 912s motor will not make 100 HP unless it reaches 5,800, now I have to calibrate the tach. and flatten the prop , usually 9.75-10.5 works best for short take off since it reaches 5,800 on lift off    BEWARE...

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Bob,I have mine set so at WOT I get around 5725 rpm in straight and level flight. I don't get anywhere near 5800 on take off closer to 5450 rpm. If yours produces 5800 rpm on takeoff what will it go to at WOT in straight and level flight, way over 5800? I'm worried about a throttle cable breaking and going to full throttle which in my case would be 5725 or so. What would yours go to? Do you have the throttle springs reversed? I'm considering this as my throttle will want to creep forward on approach and landing if you are not holding it back. I can cruise at 100 mph at 5500 rpm, but my takeoffs are closer at gross to 250-300 feet, at 3000 ft density altitude, probably still not in a high enough angle of attack and I have the slats removed and VG's. I'm probably a little gun shy still from my takeoff accident! My prop is a 72 inch Sensenich set at the zero setting which is the flatest pitch with the pins they give you. Very curious if flattening out the pitch would give me much better takeoff without overspending the motor in case the throttle cable breaks. When I first flew the 750 at Carson City the 0-200 would go over redline at cruise very easily but was MAYBE 85 mph cruise. I think my plane would beat it in takeoff now as well. Thanks for the post. Chris

chris, you have several confusing issues here, every thing in aviation has a trade off , the 912 makes 100 hp only with the tuned intake and exh. you can use the 100 hp where ever you want ie. take off , cruise ,climb..it doesnt make any difference if you break a throttle cable , once you drop the nose its going to go over red line, unless you turn off the ignition ,it sounds like you are more concerned about speed then take off performance ,otherwise you would not have the slats removed , if its speed you want then you should build a 601 , , 5725 is over the Rotax cruise , 5600 is max , and the 912 is too small to make a good airplane out of a 750, as far as throttle creep you need to tighten the tensioner, the take off performance is totally dependent on take off HP which is dependent on the RPM, which is set with prop pitch , so is climb or cruise, if you are ok with the poor take off , and ok with the cruise then that is as good as it gets 5600 is the max rpm you can pull out as a continuos setting  

Yikes! Here's Chris' departing a dirt mountain strip at 4,500 ft, not too shabby with a 200 ft roll, DA was higher, This is real life, not some sea level high wind deal, and I've watched him come and go with passengers on this 800 ft dirt strip with obstacles. If your 750 is not an excellent performer with the 912 there's other issues, it's not the Rotax. There's too many of us wringing this 750 out with the 912 to not know it's performance capabilities. 


750 STOL country, resting after giving rides…...

Chris, whatever you do do NOT reverse or remove those throttle springs! If the throttle linkage were to break, it's far better for the engine to go to WOT than to idle. That's why they are there. :)


I finally got my throttles silky smooth and now the springs really pull, SOOO I'll just tighten the cable tensioner a little tighter on approach and all good like from the factory meant it to be. Rotax recommends that if the engine over revs above 5,800rpm, but less than 6,500rpm for more than a minute, that the crank should be checked and the push rods should be checked for straightness. With that in mind, I don't want to be right at 5800rpm on take off then level out and over speed the engine. The article also said to be approved for far 33 Rotax has to run the engine at redline for a 100 hours tear it down and must be within limits. I'm sure there is wiggle room here, but I want my engine to go to TBO without any trouble. We always want more but I'm very happy with my set up.

About tach errors -- that is a common problem with aircraft tachometers, both mechanical and electronic -- they frequently are not accurate. There is an optical tach available for aircraft that actually counts the prop blades going past and uses that to generate a very accurate RPM number. They are expensive (almost two hundred bucks) but they work very well in pretty much all conditions. Some light aircraft pilots have one velcroed to the top of the glare shield and use it full time as an accurate tach. Many owners and mechanics have one and use it to check the aircraft's tach now and then but don't have it mounted in the plane. It is a fairly expensive solution to the problem but one that is designed to work with piloted airplanes. http://www.aircraftspruce.com/catalog/topages/trutach.php?clickkey=...

A less expensive optical propeller tach is available through the model airplane market. These units are intended to be held within a few inches of a model airplane prop and give an accurate RPM reading to check the tune of the little engine. Because of that design intention they are not intended to reliably read a prop blade that is several feet away and they are also oriented to have the optical reader pointed ahead of your hand while the readout can be read from behind the device while you have it pointed at the model plane's prop.

However, these model plane optical tachs WILL successfully read a piloted aircraft's propeller through the windshield and you can use them to check the aircraft's installed tach for accuracy. They are twenty or thirty dollars compared to the big plane version which costs almost two hundred. Because they were intended to be pointed at the little model plane prop and be read from behind, they have to be held a bit awkwardly to point it at the prop of your airplane from the pilot seat, but it can be done easily enough and you can read the RPM readout. Sometimes they will not get a reading and that just means there is not enough contrast between the passing prop blades and the sky so the little tach cannot "see" the prop blades. Turn 30 or 40 degrees to get a different sun angle on the blades and that little tach will indeed start counting blades and give you an RPM readout.

It's cheap and accurate and lets you check the accuracy of your cockpit mounted tach easily. I keep it in my plane and check the tach every week or two to make sure it has not drifted. My model plane tach is very old and reads in 30 RPM increments. That is plenty accurate to check your plane's tach with -- if the readout is steady you know the "real" RPM is within plus or minus 15 RPM of what the little tach is displaying and it's hard to read most panel mount tachs within 15 RPM anyhow. If it is skipping back and forth between two readings 30 RPM apart, you know the actual RPM is somewhere between those two numbers which again is close enough to check tach accuracy.

I suspect newer model plane optical tachs are even more "tight" than my old dinosaur, but even if they aren't that 30 RPM step is plenty good enough for our purposes. These things read up to astonishing RPM numbers because those little model airplane engines turning short little model props can turn up into the tens of thousands of RPM's but they are accurate down in the RPM range we need as well, which is all that matters to us. Here is one I found with just a second of Google work to show you what I am talking about, I am sure there are dozens to pick from. http://www.horizonhobby.com/product/airplanes/airplane-field-equipm...

from Walt Snyder

>>>. If your 750 is not an excellent performer with the 912 there's other issues, it's not the Rotax. There's too many of us wringing this 750 out with the 912 to not know it's performance capabilities.

I don't have a 750 but am basically familiar with it. Like Walt, who does have experience, I would think the 912 ULS would be the better performer by far over the direct drives that some use. It's the lightest out there that I know of and has the redrive which allows us to run the all important big prop. In comparison. the O-200 has to be a dog...50 or so more pounds and no redrive. Maybe there are other motors out there that are comparable to the Rotax that I don't know about. That said; I don't even like my 912; didn't wanta buy it when I did; and don't really like operating it and especially working on it. But it had the best power/weight out there and for maximum STOL I bought it. It WILL perform!

Thanks for the tach reminder Bob. I have been aware that tachs may have errors for decades and have checked several with the handheld checker borrowed from a friend. I mysteriously lost some performance on my 701 some time back and went to great lengths to find it. The search became an obsession but I never did find it. Checked everything imaginable over and over. Except the tach. Getting old I guess. I'll check it.


Off thread, but there are those that make the 0-200 work for them. If you couple that with a heavily built 750, the performance would be less than stellar, but a lightly built 750 with the 0-200 in mind would work satisfactorily. Maybe not a STOL contest winner, but the familiarity of a good ol' aircraft engine, plus the cost, trumps the STOL that's lost. 

Plus everyone looks up when the familiar 0-200 flies overhead :) 

Well said.

Define your mission, build it, fly it and have fun.


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