Oowweee! I gotta headache from all this. I'd have to have been there and seen it with my own two eyes to understand and believe this. But, since you were there, and you saw, I accept it.
Larry. John Cooper wrote: > In a message dated 00-01-01 13:49:51 EST, [EMAIL PROTECTED] writes: > > > The C-75 carb is designed to supply all the fuel > > the engine can use at 2275 RPM. thus producing 75 HP. Increasing the > amount of > > fuel supply, without increasing the RPM, could result in a rich condition > that would > > actually decrease HP at the same 2275 RPM. If you cannot take advantage of > the extra > > fuel, you've gained nothing, and possibly lost some. As someone else out > there > > mentioned, ya gotta do the whole job to get the added power. As there are > no other > > changes than carb, i.e.. no cams, displacement, higher comp., the only > thing > > that gives the extra power is RPM. > > > Well, turns out I didn't like my own response to this. Seems I'm having a > tough time saying what I want to say in a clear fashion. I'm gonna take one > more swipe at it then I'll let it go. > > First, to clarify the question I'm trying to answer: Given that the only > difference in the engine between a C75 and a C85 engine is the carburator, if > someone changes just the carb on an otherwise stock C75 installation to the > C85 carb, is there any increase in HP? > > To try and present an understandable explanation of why I say the answer is > yes, let me start with a slightly different but hopefully more easily > understood example. I'm going to make up some simple numbers to illustrate > the example. The exact numbers are illustrative only and not intended to > represent any real airplane. (See attached spread sheet) No lab animals > were injured in the preparation of this e-mail. > > So, imagine we have a Bugmasher 100. It's certified with the C75 and the C85 > is optional. Operating limits with the C75 are 2275 redline, static minimum > 1850, maximum 2000. Operating limits with the C85 are 2575 redline and > static minimum 2150 and maximum 2350. > > Lets start with an airplane with a bone stock C85 installation, standard > fixed pitch prop. Our sample airplane gets 2250 RPM static and at full > throttle level cruise gets 2575 RPM, indicates 100 mph and burns 7.4 gallons > per hour. In a 70 MPH indicated climb at full throttle it gets 500 feet per > minute and is turning 2350 RPM. 75% cruise is 2350 RPM and we indicate 90 > MPH and burn 5.3 gallons/hour. > > Now, Imagine that we adjust the full throttle stop on this plane so that the > throttle opens such that the carb intake area is 90% of what it was in the > previous example. Then we go and do some flight testing. Conditions are > exactly the same as before. What will we see? Well, it'll be something like > this: Static RPM will be lower since we're no longer at full throttle, so > say 2100 RPM (note: this is outside the allowable range and thus the AC is > not airworthy in this condition). Full throttle cruise is really only 90% > throttle cruise, so we'll see maybe 2450 RPM and 95 MPH at 6.2 gallons per > hour. Climb performance at 70 mph is going to be something like 400 FPM and > 2250 RPM. We can still cruise at 2350 RPM, indicate 90 MPH and burn 5.3 > gallons/hour. > > Next, let's take our crippled carb off and install the one for the C75 with > the 1 5/16 inch venturi, which is 10% smaller than the one we took off. This > time we'll leave the throttle screw set for full throttle. > > Go out and do the flight tests again and we'd expect to see no change since > our new carb has the same flow rate as our crippled C85 carb had. > > Next, let's remove the prop and install the standard prop for the C75, since > that's what we've now got (remembering that the only difference between the > engines is the carb.) Do all the tests again. What will we see? Something > like this: Static RPM 1950. Full throttle cruise 2275 RPM 95 MPH and 6.8 > gallons per hour. 70 MPH climb is at 2100 RPM and 350 FPM. (Remember, we > didn't change anything except for the prop, and we went in the "cruise" > direction, even though what we've now got is "standard" for the C75, note red > line at full throttle in level cruise.) Using the performance information > for the C75, we cruise at 75% at 2060 RPM, indicate 85 mph and burn 5.0 > Gallons per hour. > > Now, finally for the one we've all been waiting for: Go back to the C85 > carb, but keep the C75 prop. What will we expect to see? Something like > this: Static RPM is going to be a little higher, since full throttle is more > fuller than before, say 2050 RPM. (Note that this is outside the allowable > range for both the C75 and the C85, so again this airplane is technically not > airworthy). Full throttle cruise will be a little faster for the same > reason. Say 2325 RPM and 97 MPH. (but we're over the redline for the C75 > and burning 7.0 GPM). Climb will be a little better, too, say 400 FPM at > 2150 RPM. 75% cruise using the the C75 numbers is unchanged, 2060 RPM, 85 > MPH and 5.0 GPH. For the C85 we can't hit the specified RPM, but note that > at 2325 RPM we're going faster and buning more fuel than the bone stock C85 > setup does at 75%. This is because we have this extreme cruise prop out > front and we're actually making more than 75% power, but were outside the > certification envelope. > > So, you see that there actually is an RPM increase, in climb and at full > throttle level cruise. Those are the only places you'll actually get a power > increase. With the same prop turning the same RPM there cannot be any change > in power in level flight. There can in climb, though, because you can > increase the angle of attack which increases the prop load. Allowing it to > "absorb" more power at the same rpm, much like the reason the static RPM is > lower than the full throttle cruise RPM, even though both are full throttle. > > Out. > > ------------------------------------------------------------------------ > Name: PERF.XLS > PERF.XLS Type: Microsoft Excel Worksheet (application/vnd.ms-excel) > Encoding: base64 __________________________________________________________________________ ______ To unsubscribe from this list please send mail to [EMAIL PROTECTED] _____________________________________________________________ Get your favorite topic delivered daily. http://www.topica.com/t/11
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