January 31, 2013 Leave a comment
A Murphy Rebel builder forwarded the comment below in brown from the Murphy builders list. He was interested to know how I would respond to the writers comments. Rather than send back a private email, I thought it was worth putting up here, as we have about 10 builders putting the Corvair on the front of a Rebel, and I wanted them to understand why we know the combination will work, and why the guy below is not correct in all the assumptions he wrote into a single paragraph…..
“I seriously looked at the Corvair engines – but decided that with the wide front end of a Rebel – that the faster turning and therefore smaller diameter prop’ ( which you HAVE to use – in order to let the engine get up IN to it’s power band RPM range ) would be “inefficient” on the nose of such a meaty plane ( big front end ) …….
So I was forced by common sense to revert to a Lycosaurus ( Lycoming – dinosaur ) engine – with it’s slower turning / bigger diameter / more efficient propeller !”
Where do I start? OK, I’m not fond of the term Lycosaurus, even when it is used by people planning on buying a Lycoming. Moving on to more technical points, the biggest single argument, and the easiest thing for new builders to understand, is that we have long been successfully flying a plane that is bigger than a Rebel, has more frontal area, more drag, and a greater payload. Our Wagabond, flying since 2005, works great and actually flew with more payload than it’s empty weight……On a 100HP Corvair. So maybe the comments that the writer made don’t count. Simply put, his evaluation was based on his eyeball look and a handful of old wives tales, on the other side we have my testing and a plane that has been flying for 8 years.
Above, a Murphy Rebel. The cabin on the plane is 44″ wide, and it has a comparatively blunt windshield. A guy commented that the recommended prop size is 74” by 56 or 58, but this is only the prop for a 160hp Lycoming. For our comparison, let’s have a reasonable comparison looking at a 3,000 cc Corvair vs an O-235 and a 2,700 cc Corvair vs a Rotax. Below is a Chart off the Murphy site. Like almost every other airframe factory chart on the planet, lets just call the numbers ‘optimistic.’ (We have an O-320 Rebel here at our airport and it doesn’t match the chart, but this is typical in our industry.)
|Engine||Lyc O-320||Lyc O-235||Rotax 912||Rotax 912||Rotax 912|
|Power Loading (lb./hp)||10.3||14.2||18.1||15.4||16.88|
|Gross Weight (lb.)||1650||1650||1450||1232||1320|
|Empty Weight (lb.)||950||900||700||625||700|
|Useful Load (lb.)||700||750||750||607||650|
|Wing Area (sq. ft)||150||150||150||150||150|
|Wing Loading (lb./sq. ft)||11.0||11.0||9.7||7.0||9|
|Rate of Climb @ Gross (ft/min)||1200||800||500||800||550|
|Climb Speed (mph)||65||65||60||60||60|
|Take Off Run (ft)||300||400||450||300||450|
|Landing Roll (ft)||400||400||300||200||300|
|50′ Obstacle Clearance (ft)||533||754||976||626||976|
|Stall (No Flap) Power Off (mph)||44||44||40||38||40|
|Stall (FULL FLAP) Power On (mph)||40||40||36||35||36|
|Cruise (65% POWER) (mph)||120||105||100||85||100|
|Top Speed (mph)||140||125||100||105||100|
|Fuel Burn (gal/hr)||7||6||4||4||4|
|Fuel Capacity (US gal)||44||44||44||22||44|
|Range (statute miles)||733||797||880||468||880|
|G Limit (Ultimate)||+5.7 -3.8||+5.7 -3.8||+5.7 -3.8||+5.7 -3.8||+5.7 -3.8|
O-235 vs 3,000 cc. The 235 listed above makes 116hp. If I asked the writer what exactly he ment by “with it’s slower turning / bigger diameter / more efficient propeller !” He probably wouldn’t have an exact number in mind for rpm. Does 2,800 rpm sound real slow? Well that’s the rpm required to get 116 hp. Think I have been an A&P for 20 years and don’t know what I am talking about? Read the last model on the Wikipedia page: http://en.wikipedia.org/wiki/Lycoming_O-235. Also note that just about every 235 has to hit 2,800 to make rated power. Second, a fully dressed 235 weighs 280 pounds, at least 40 pounds heavier than a Corvair. A 235 is wide, within 1/4″ of the width of a 320. You can’t put a sleek cowl over it.
Now lets look at prop size: Think more diameter is always better for low speed thrust? Think again. Last week I changed props on Grace’s 85 HP Taylorcraft. I am setting it up to tow our glider. It had a 72 x 48 wood prop on it, and I got a 74 x 46 metal to replace it. But before I mounted it, I took it to American Props and paid $865 to have it overhauled, the pitch reduced, and to have its diamerter reduced to 70″. I am not a fool, The diameter reduction allowed higher rpm, and improved the climb rate by 500’/min. Props with diameters of 74″ are only efficient on engines like the Continental 65 with a low red line of 2300 rpm. Low rpm isn’t efficient in itself. A 65 Continental becomes a 75 continental with respect to power output by just a jet change and an RPM increase to 2600. If turning the prop 300 rpm faster and using one with less diameter actually made less low speed thrust, than no one would have ever converted a 65 to a 75. Almost everything repeated in hangar flying stories or on discussion groups about rpm and efficiency is an old wives tale or pure BS that directly contradicts experience from certified engines and certified prop shops, but that never seems to stop people from repeating it as if it was told to them by Wilbur Wright and Kelly Johnson.
A 3,000 cc Corvair on a big plane can use either a 68 or 70″ in diameter prop. If the guy puts a 74″ prop on his 235, it is going to static near 2250 or 2300 rpm, 500 rpm below the engines rated power. He may tell people he has a 116 hp engine, but he isn’t going to get to use the last 12-16 horses unless he takes the diameter down to 70″ or so. Lets see…where is that big prop diameter difference the guy was speaking of? Yeah, it’s 2″, but don’t forget the Lycoming is 6″ wider, so which prop is operating with more blade area working in the clear?
We intentionally set up the Corvair to turn more rpm static, because more rpm is more power, and the Corvair builds hp much faster than prop efficiency decays, thus more rpm is a net increase in thrust. A flatter pitch prop on a 3,000cc Corvair will static near 2,800 rpm. The tips will be well below sonic, and the power output will be near 100 hp. The 235 with a high-pitched prop will not only make slightly less power at 2350 rpm, the critical difference is looking at the blade angle of attack: much of the high-pitched blade will be stalled, far more of the low-pitched Corvair prop will be working. The Corvair will accelerate much better. You may have to read that twice to follow it, but real learning and understanding takes a bit more time than memorizing and parroting BS phrases like “Keep your prop as long as possible as long as possible!”
Above, the wagabond outside our old hangar in Edgewater in 2006. It is built on a PA-22 airframe, which as a four place certified plane. It is bigger than a Rebel. It has taller gear, it has four lift struts, and at 147.5′ of wing area, it has just 2.5 feet less than a Rebel. In short, there is no rational reason to say that a Rebel would not fly as well or better on the same engine. In the photo, the plane is equipped with a basic 2,700 cc Corvair. Look at the prop, it is a 64″ diameter wood Sensenich. It worked great, it is using the same prop in the video link at the end of this story.