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Tim. Before you proceed with the project, adding more power to the front of an Ercoupe, think of the consequences of such action. 1. The Ercoupe was designed to fly at a max. speed of around 100 miles with a 65 hp engine. Adding more horsepower showed that one does not gain a significant faster speed in cruise. It is that the airframe with it's beefy wings hits some invisible wall to fly faster. More horsepower does it make not much faster, a different airfoil certainly would. 2. The flying characteristics of an Ercoupe are a result of a two control system adjusted in flying tests around a 65 hp engine. With that it is stall proof, and coordinated in turns. Adding more horsepower and therefore more downwash on the tail might disturb this flying behavior. Using the split elevator opened the plane up for more horsepower. The 90 horsepower Continental seems to be the best fit for this airframe. The 100 hp engine in a 415-D requires a elevator restriction to keep it stall proof, mainly on climbout. With 25 horses more one would have to restrict the elevator even further or do some significant test flights with a split elevator in place. The control combination of ailerons, rudder and elevator is designed for 65 horses and a gross weight of around 1200 pounds. It has been already successfully been stretched to 1450lbs, but to go further, one has to do some testing. 3. Brings me to another important point. The dry weight of the Franklin is reported to be 206 lbs without accessories. One can assume that the flying weight of that engine will be at least 240 lbs with lightweight starters, magnetos, alternator and oil. 60 pound heavier than a C85/O-200 installation. The Franklin also has a fuel consumption of 6-7 Gallons in a cruise (75%power) setting, no data for full power is given , so I assume a 10 Gallons per hour in full power. The power has to come from somewhere. That means that you either can fly only for 2.5 h in the worst case or you have to get bigger tanks. Remember that you still can only fly at around a hundred miles, maybe 115 or even 120. But really not so much faster that the higher fuel consumption is compensated by higher speed. On a long Cross country, you'll lose more time by refueling than you would gain in traveling speed. Your average speed would be even slower compared to a trusty 75 hp engine installation. 4. Unless you get some bigger tanks. One could install the 30 Gallon conversion by sacrificing even more payload. Roughly 40 lbs. But now you'd have sacrificed a 100 lbs of payload already. Something you might not be able to afford, unless you always want to fly alone. Now, one could argue that a higher horsepower engine justifies a higher gross weight of the plane. True. And one did so with the 90hp engine installations in Forneys and Aircoupes. They got a mere 50lbs added to the maximal gross weight. To accommodate for the added 100 pound or more, I would suggest a max gross weight of 1500 if not 1600 pounds. The only problem I see with the higher gross weight is that the stall speed will now be higher too. The Forney with a 1450lb gross is reported to stall at 58 miles, a number I find a little low, but that's what the book says. An Ercoupe with a 1500 or 1600 pound gross would have a stall speed of over 60 Miles/hour, my guess is in the middle between 60 and 70. That is power off stall. Since you do not have positive control of the aircraft with your rudders in a stall ( pedals or not), a stall has to be avoided by design under all circumstances. That means you'll need to restrict the elevator so you never can fly that low, raising your speeds for climbout and approach to above 70 at all times. That in return will lengthen your take off and landing rolls, making your plane not safer, especially in high altitude airports. Airplane design is always dealing with tradeoffs. You can only have one thing. You can not have all the power, all the payload and all the altitude at the same time. One has to fine tune the desire with the targeted mission. Mission of an Ercoupe is to carry 2 lightweight persons and some baggage at speeds at around 100 MPH. You would have to change the airframe too, not the engine alone to change that mission. But there is one way to gain at least more speed on cross countries , where speed matters and that is to fly high. Fly at 10000ft if you can and you gain performance and save fuel, good winds provided. The other way would be to have a turbonormalized engine, which would assure 100% engine power up to 10000ft. That would be the real way to go. In the end, your 125hp engine develops only 65 horses at full throttle at 10000ft. A turbonormalized 75 hp engine could give you that too and maybe even more without all the penalties. Hartmut N3330H ----- Original Message ----- From: "Tim and Martha Briggs" <[EMAIL PROTECTED]> To: "Coupe-List" <[email protected]> Sent: Friday, June 25, 2004 9:15 AM Subject: [COUPERS-FLYIN] Franklin 125 hp engine in an Ercoupe?? > ----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]---- > > > Ercoupers: > > I was surfing around and found this website offering Franklin engine > conversions. The 4-cylinder version is a replacement for the O-200. Has > anyone tried this engine in an Ercoupe?? 125HP and a good prop would make > quite a difference!! > > http://www.franklinengines.com/4a.cfm > > Just curious > > Tim > > > ========================================================================== == == > To leave this forum go to: http://ercoupers.com/lists.htm > Search the archives on http://escribe.com/aviation/coupers/ > > > ========================================================================== ==== To leave this forum go to: http://ercoupers.com/lists.htm Search the archives on http://escribe.com/aviation/coupers/
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