|
-
POSTING RULES

-
Donate yearly (please).
-
Advertise in here!
-
Today's Posts
|
Insert Pics
|

08-09-2011, 04:09 PM
|
 |
|
|
Join Date: Oct 2005
Location: 08A
Posts: 9,500
|
|
Quote:
Originally Posted by Moura
What is best in that case (flying low), high MAP and low RPM, low MAP and high RPM or somewhere in between such as 24/24?
|
In the context of detonation, any of the above is fine. Only the high MAP, low RPM case has limits.
I do think you can detonate a 8.5 -8.7 CR normally aspirated Lycoming. I just don't think the "red box" for those engines is anywhere near as large as the oft-repeated Deakin graphic would suggest. The graphic is a great illustration of general principles, but the numbers are not gospel. For our little engines I don't think they are even close.
Lycoming offers guidance (or did) on operation limits due to detonation. It's right there on the old power charts; look for the line titled "Limiting Manifold Pressure For Continuous Operation".
Here's a sample chart for an O-320 (an illustration; find and use the chart for your engine). The assumption is best power mixture, meaning the limit line is a worst case. Any combination of RPM and manifold pressure to the left of the line is acceptable at any mixture, since richer or leaner than best power is less likely to detonate, not more. You could probably move to the right of the line if you canceled detonation with a very lean or very rich mixture.
And the first question I proposed back in post #6, about engines on the dyno? Look at the chart....given CHT within limits, you can't detonate it at 2700 RPM. Without a turbo there isn't enough manifold pressure.
So, the second question. If detonation risk is low, then what is the failure that brings on early cylinder replacement?
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 06-08-2017 at 12:00 PM.
|

08-09-2011, 04:44 PM
|
 |
|
|
Join Date: Jul 2009
Location: Mojave
Posts: 4,652
|
|
Good info Dan,
The thing I find interesting is the fact that despite the warnings of this power chart, many pilots are in the habit of pulling the prop back right after takeoff for a variety of reasons (noise, "easier" on the engine, etc), yet still maintaining WOT and best power mixture. There are plenty of times that an engine is operated at high enough MP and low enough RPM to be to the right of the limit line of this chart... Heck, there are plenty of fixed pitch applications that run in this zone for a large percentage of each flight. For all the people that do this, where are the failures?
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
|

08-09-2011, 05:38 PM
|
|
|
|
Join Date: Jul 2005
Location: Henderson, NV (KBVU)
Posts: 129
|
|
Detonation Experience - what it looks like
I had a problem with an electronic ignition several years ago that resulted in detonation. It was certainly moderate detonation, if not heavy. I did a short flight and shut down, then restarted again and took off. CHTs were probably just under 400 degrees at the start of the takeoff roll. By a couple hundred feet in the air, the CHT alarm was going off and the #4 cylinder CHT was climbing rapidly through 430 degrees. I don't remember the exact temps or rate temp change, but it was readily obvious something was wrong. I pulled the power way back and came back to land. Temps came back down with the power reduction.
I checked the cylinder. It was very clean, but the plug looked undamaged. The inside of the cylinder and piston looked okay. I've got another 400 hours on the engine now.
To summarize, detonation and its attendant CHT increase will not go unnoticed with a full engine monitor and good alarms.
Seb Trost
RV-7A
Boulder City, NV
|

08-09-2011, 05:48 PM
|
 |
|
|
Join Date: Nov 2006
Location: Oklahoma
Posts: 672
|
|
Quote:
Originally Posted by DanH
So, the second question. If detonation risk is low, then what is the failure that brings on early cylinder replacement?
|
Does it have to do with lack of carbon build-up on the pistons, which acts as an insulator, thereby protecting them from intense heat when ham fisted pilots don't stay away from the "red box"? It seems I read something along those lines in the Sacramento SkyRanch Manual a few years ago.
__________________
Joe Schneider
RV-7, IO-360, BA Hartzell, N847CR
Flying since 2008
|

08-09-2011, 06:47 PM
|
|
|
|
Join Date: Apr 2006
Location: Santa Monica, CA
Posts: 325
|
|
Detonation?
Quote:
Originally Posted by SebsRV7A
By a couple hundred feet in the air, the CHT alarm was going off and the #4 cylinder CHT was climbing rapidly through 430 degrees.
|
Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?
__________________
-John
Flying a Glasair I FT
Planning to build an RV...someday
|

08-09-2011, 07:25 PM
|
|
|
|
Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
|
|
With regard to the LIMITING MAN PRESS FOR CONTINUOUS OPERATIONS note on the performance charts, it appears the limitation applies only to engines with a CS prop.
Engines certified with a fixed pitch prop do not have the limitation. The only time they are right of the limitation line is for take off. My engine isn't even on the chart at 28" MP and 2180 RPM, but as soon as RPM is up to 2300, it doesn't matter, it would be right of the line if there were one.
So, I guess the bottom line is Lycoming does not recommend continuous ops at say 27" and 2000 RPM (which of course is impossible with a FP prop.) That may explain an old procedure to reduce MP to 25" after take off. It put the operation left of the line no matter what the RPM.
(Except of course with the H6 Subby. With it, it was WOT and manage power with RPM. The ECU responded appropriately, you could cruise around at 1700 RPM, WOT and be burning 5-6 gph. There are good things to be said about modern engine ECU timing and fuel control not possible with a Lycoming.)
__________________
RV-12 Build Helper
RV-7A...Sold #70374
The RV-8...Sold #83261
I'm in, dues paid 2019 This place is worth it!
|

08-09-2011, 07:41 PM
|
|
|
|
Join Date: Sep 2010
Location: Charlotte NC
Posts: 1,166
|
|
Quote:
Originally Posted by jtrusso
Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?
|
I think in this situation what he means is that the CHT rising rapidly above the norm was caused by detonation. The detonation could have started with the CHT's much lower but drove the temps up rapidly. The detonation itself was caused by the ignition issue.
George
|

08-09-2011, 07:54 PM
|
|
|
|
Join Date: Feb 2005
Location: Chesterfield, Missouri
Posts: 4,514
|
|
Quote:
Originally Posted by jtrusso
Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?
|
Nope. Max continuous for high performance cruise is 435, for economy cruise it is 400 and the never exceed limit is 500.
Lycoming says 400-435 is OK for cruise, Mike Busch says for long engine life 400 should be the red line and the norm for cruise 360. I buy into that. At 400 the cylinders are at half strength.
Without a knock sensor, I don't think it is possible to detect detonation, these things are so noisy. If you hear the familiar pinging like in an old stick shift auto, it is detonation. Even with a knock sensor, it might not work because there is a lot of normal knocking going on all the time. How would it know what is normal and what is detonation?
There are a couple simple techniques to set the engine at less than 75% power. With a fixed pitch prop, it is 23" or less MP. With a CS prop the 23" will work but RPM enters the equation also, if it is set 2300-2400, you are safe. At less than 75% power, the engine can not be harmed by leaning, it might quit if you're too aggressive but you can not harm it.
Seriously, if the engine does quit due to over leaning, push the mixture forward and it will spring back to life immediately. Some times when flying into cold air on a cross country, it will sound like it is ready quit, push the mixture in a bit and problem over.
The premise of detonation occurring is a stretch with these engines unless one drags it around on the right side of max MP line. I don't know if Lycoming meant that line to be a detonation line or not, probably not since its limitation is not for continuous ops. If it were a detonation line it would say never exceed.
__________________
RV-12 Build Helper
RV-7A...Sold #70374
The RV-8...Sold #83261
I'm in, dues paid 2019 This place is worth it!
Last edited by David-aviator : 08-09-2011 at 07:59 PM.
|

08-09-2011, 08:40 PM
|
 |
|
|
Join Date: Jan 2006
Location: Green Bay, WI (GRB)
Posts: 479
|
|
"So, the second question. If detonation risk is low, then what is the failure that brings on early cylinder replacement?"
I'll take a shot at that question:
Valve failure caused by poor valve geometry, angles not matching with the seat, valve not concentric with the seat leading to the valve not fully transferring it's heat to the head.
__________________
Don Pansier
Green Bay, WI (GRB)
RV-7 N450DP
W9LYX
Antennas for Experimental Aircraft
www.deltapopaviation.com
|

08-09-2011, 08:55 PM
|
|
|
|
Join Date: Jul 2005
Location: Henderson, NV (KBVU)
Posts: 129
|
|
Rate of increase was the issue here
Quote:
Originally Posted by jtrusso
Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?
|
The temperature was increasing more than 1 degree a second (give or take, no stopwatch going at the time). The rapid increase was the point, not a particular value. It may have come apart well before getting to 500 degrees.
Seb Trost
RV-7A
Boulder City
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
All times are GMT -6. The time now is 12:42 AM.
|