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  #31  
Old 02-26-2015, 08:27 AM
David Z David Z is offline
 
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Location: Thunder Bay Ontario
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The R-3350 from the piston airliner days has very little in common with a 4 cylinder Lycoming.

In any supercharged radial I've ever been around, full throttle is above the manifold pressure redline. Full throttle and full power are rarely the same. Back in the days of multiple grades of avgas, there would be different MP limits based primarily on fuel being used. Piston airliner reduced power take-offs will have more to do with turbo-compunder or turbocharger wear, plus detonation margins when using lower octane avgas.

Until we start installing turbo-compounders in RVs, I don't see any benefit to reduced power take-offs, and even then I don't see the safety trade-off as worth it. Does sound like a fun experiment though
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  #32  
Old 02-26-2015, 08:27 AM
pvalovich pvalovich is offline
 
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Location: Ridgecrest, CA
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Default Takeoff Procedures - KISS

Each to his own - but be sure to include all the parameters - especially operational (vice financial) in your reasoning.

RV-8A, IO-360, 180 HP. Operate out of Inyokern, CA, 2400' where temps are above 100 degrees F nearly every summer day. I do full power takeoffs, stick full aft, let it get airborne when it wants to. Lower the nose slightly to accelerate to 100 kts. and quickly climb at full power.

100 kts. gives me some airspeed margin - converted to time - for analysis during a potential "WTF was that?" low level event. 100 kts. and 800 ft AGL gives me the option to put it down without power, but with aggressive maneuvering with stall margin, on one of the three IYK runways - or do a controlled crash into the desert.

Note: this is not a reopening of the never-to-be-resolved turn back to the runway debate. The above has been practiced and works for me - your results may vary.
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  #33  
Old 02-26-2015, 08:31 AM
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Arie Arie is offline
 
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Location: Dubai
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Default Reduced power take off

Gents,
I do this every day for a living. There are actualy two ways of doing it. The one is a fixed de-rate on the engines ,and the other more popular way is to use an assumed temperature method. The crux of the matter with reduced thrust take off is to assure the minimum climb gradient incase you have an engine failure at V1 (which single engine aircraft dont have) and then to make the second third and fourth climb gradient on the remaining engines. This is ONLY for multi engine aircraft that can performance comply .IT DOES NOT applay to single engine aircraft AT ALL.
Use all the "power" you have on a single to get you in the air and as high as possible in the shortest time
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  #34  
Old 02-26-2015, 08:44 AM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by wirejock View Post
I'm no expert but I did drive race cars for a few years. Granted, they were very different from our Lycomings and driven very hard usually at redline as long as possible. Every failure I saw or experienced seemed happen coming off a high speed straight. Boom, oil all over the place. Never once saw a failure on hard a acceleration.
I distinctly remember engine parts flying across my hood just as the guy in front of me let off to get on the binders. Lucky for me, none of his parts came through the windshield!
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  #35  
Old 02-26-2015, 08:45 AM
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flightlogic flightlogic is offline
 
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Default save yourself

Bob Collins and Arie put it best, in simplest terms.
Comparing the original question about RV piston engines and airliners...is way off track.
Get to an altitude where that single engine four banger can glide someplace survivable. Period.
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  #36  
Old 02-26-2015, 08:59 AM
jimgreen jimgreen is offline
 
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Location: Vancouver island, BC Canada
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My local airport is close to town and we try hard to keep the noise level down.
For me this means 2600 rpm until airborne, then 2500 for initial climb.
With a c/s prop this is not much of a power reduction, but does seem to reduce the noise. At least it isn't so annoying.

I'm still way above the climb gradient of any non RVs on the field.

Are 150 hp RVs unsafe because they don't climb quickly? They still climb much quicker than a C 172.

Safety is a relative concept, and we all accept higher degrees of risk than necessary for expediency. Do we all climb to 15,000 to cross 20 miles of water? Do you fly direct routes over the rocks or follow roads?
Being safety conscious doesn't mean accepting no risk

Engine failure on take off is probably not the greatest risk we face. You stand a chance of walking away from it.
It's mid air collision that keeps me awake at night.
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  #37  
Old 02-26-2015, 09:09 AM
douglassmt douglassmt is offline
 
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Location: Missoula, MT
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Default Deakin/Braley

If I recall correctly from the GAMI seminar I attended last year, reducing MP lowers internal cylinder pressures (ICP) and correspondingly CHTs. However, contrary to my belief - and most others I suspect - reducing RPMs actually increases ICP, albeit only a small amount. The takeaway from the gurus was to climb at max power, max rpms, full rich with two exceptions: 1) if you want to lean in the climb (optional); or 2) if there are noise abatement considerations.

Plus, it's a lot more fun to let the horses run...
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  #38  
Old 02-26-2015, 09:14 AM
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Arie Arie is offline
 
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Jim, with all due respect, by reducing the rpm from 2600 to 2500 you are NOT doing a reduced thrust take off. By starting the take off run with less than all available power you are. You might not know ,but to do a reduced thrust take off in a jet liner and then comply with the climb gradient in case of an engine failure at V1 a C150 ,even on a hot day ,will out climb us, you will be lucky to see 2-300 feet per minute rate of climb. I will say it again, recuced thrust take offs are NOT for single engine aircraft. Save it for the airlines
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  #39  
Old 02-26-2015, 09:31 AM
engineerorange engineerorange is offline
 
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Location: Catawba, NC
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Quote:
Originally Posted by N941WR View Post
I distinctly remember engine parts flying across my hood just as the guy in front of me let off to get on the binders. Lucky for me, none of his parts came through the windshield!
That's usually because you are at max rpm (max piston speed/acc.) and when you let off the go pedal there is no boom to help turn the piston around and the rod and wrist pin have to do it all. You also get some shock cooling of the valves and every now and again a head pops off. Not to mention torsional reversals in the crank. Sometimes it gets ugly.
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  #40  
Old 02-26-2015, 10:12 AM
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bret bret is offline
 
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Also don't forget how the cylinder walls and cam get lube, from the oil being thrown off the rod big ends. Idle bad, rpm good!
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