Question for Paul McBride, our engines expert: Paul, I came across your address when Google answered my question regarding EGT. I have an O-235, vintage 1948 in a Bakeng Duce. Takeoff and climb show about 2,400 rpm. Out of respect for the old girl, I try to keep down to 2,200.
I just installed these baffles. Figured as long as I can get 2,200 rpm all should be good. Your input would be appreciated.
Ed Kusy, Schulenburg, Texas
Answer: Ed, you’ve asked a question that I’m not certain I can give you an honest answer for. Since Lycoming engines horsepower is calculated running the engine with straight exhaust stacks, I’m not sure what loss you might expect from these baffles.
Each airframe manufacturer has the responsibility of designing and installing the exhaust system on all normally aspirated engines. It’s different when the engine is turbocharged, however.
Anyway, when the airframe manufacturer installs the exhaust system, we know that this will cause a slight loss of horsepower. It’s commonly known as an installation loss. However, as minimum as it may be depending on the specific design, there is a slight loss. This installation loss is also caused by installing other engine accessories, such as an alternator, etc.
Since you have already installed these new baffles, my only suggestion is to fly the aircraft and see if you notice any difference in performance. I appreciate your point that you want to treat the old girl with respect by using 2,200 rpm, but most of the O-235 series engines are rated for 2,800 rpm.
My concern here is — as I’ve mentioned many times before — the accuracy of the tachometer. If it hasn’t been calibrated recently, you may be operating the engine at a much higher RPM than what you’re seeing on the tach.
I would think a static RPM on the ground would be around 2,400 rpm for your engine.
There are several things to take into consideration, and you are more the expert here when it comes to the operational aspects of the aircraft.
I think it would be ideal if you could contact someone who has an aircraft/engine configuration like yours and compare operational details. As an example, if he has an O-235 series engine, what static RPM does he see? Of course, there are other factors like prop, etc. I’m sure there must be a Bakeng Duce flyers group and that may be a really good source for all kinds of information for you.
Not knowing your full circumstances Ed, I’d really like to see you operate the engine at a higher RPM for better performance.
I was taught to run the engine at full T.O RPM to make sure you can develop full power. Pull the RPM back after gaining enough altitude to circle back if needed. Reduce power once you have established an acceptable climb rate to altitude and cruise.
Do not lug the engine rpm around thinking it will save the engine. You’re not going anywhere either. The lower combustion pressures and temperatures cause issues internal to the engine as well. Lead will accumulate on the plugs, lead will contaminate the oil. The better lubrication oils we have now are far better than those of 60 years ago.
I tried to provide URLs to the O-235 engine manual that states certain versions allow 2200 rpm, but the site blocked me. Is that a feature, or a bug?
The pdf is available at the Lycoming site.
O-235 and O-290 Series – Lycoming Engines
https://www.lycoming.com
The engine should be run at at least 60% power to keep the ring pressure up, reducing oil use and keep from sludging up the compression rings.
The restart C-182 uses a Lycoming engine rated 260HP at 2700 rpm. Cessna derated it to 230 hp to meet the old type certificate by limiting it to 2400 rpm. The engine runs cool, strong, and economical (admittedly, some thought went into engine cooling, a novelty rarely enjoyed on legacy aircraft). If Cessna can do it, why not Ed’s O-235? What does the engine manual say?
Lycoming publish power charts for their engines. Unless there is a compelling reason to deviate, stick to theses settings. Decide what power you want to use for cruise and adjust RPM with altitude to maintain your engines output. Remember, for best performance, lean appropriately at all cruising altitudes and for take off/climb at higher elevations.
Yes, the O-235 engine operating manual is available as a pdf online.
It lists all of the models and the performance specs.
Also, there are a number of suppliers of exhaust systems for this engine.
Some are stainless steel, and others are Inconel, as supplied o the C152.
There is a great iPhone app called Engine RPM that provides an acoustic tachometer. It is the same technology that I used to use to hunt submarines. It is probably accurate to 10 RPM or better.
I have found most GA Aircraft to read about 50-100 RPM Low
I used to baby my 320 and I ended up with washboarded cylinders. After replacing the jugs I’ve been running it close to full rpm. 2550 to 2600 rpm at cruise. I use wide open for take off and climb to a best tail wind if possible altitude. Mixture as appropriate during climb to alt. I end up with about 8 gal per hour. Gets average cruise 90 to 95 knots. An old AI/AP Recommend this after discovered low compression and jug removal showed the damage. I’m also a licensed A-P mechanic. Seemed logical to me. No low compression lately.
What’s a washboarded cylinder? What causes it?
A safety recommendation regarding engine performance:
Engines statistically fail most frequently after a change of state. That means that you are more likely to have a problem when you change an engine setting, RPM, MP, or mixture. It follows that on take off it is safest to not change anything until you have enough altitude to turn back to the runway or at least have another viable landing area available.
Please share the data you used to form your ‘failure with changed state’ hypothesis.
Cheapest, easiest, fastest way to check calibration of your tachometer:
Wait till dark, park under the FBO ramp lights, rev the engine to 1,800 rpm. You should be able to “stop” the shadow of the prop at exactly 1,800 rpm indicated. If it is off, you’ll know by how much (so get it fixed). Different number for three blade prop and in Canada (50cps electricity, not 60 cps).
Power company regulates the frequency very carefully, so this is dependable.
Best of all, this test is FREE!
I don’t believe that you would find 50 hz power anywhere in Canada. Perhaps the odd pocket 70 plus years ago, but not today.
Perhaps a more visual result was damaged couplers on my backhoes. They connect the engine crankshaft pulley to a hydraulic pump via rubber insulator bushings. Problem was an operator running low RPM while digging. The pump equates to a prop as a load. The lower the RPM, the more spread between power pulses, the more load on the engine being transmitted through the coupling trying to turn the pump/prop. Had trouble explaining to him that rated horsepower is not constant across the entire RPM range.
0-360 I use 2700 for takeoff, 2600 after breaking ground, 2450 cruise.
I agree with Dan. One should use all the power that the manufacturer of the engine intended during takeoff and initial climb. It won’t hurt a thing to do so, and not doing so could do a lot of harm one day.
I don’t think running your lycoming at 2200rpm’s is doing anything good for it, not to mention the performance you are leaving behind during a time when you may need it most, at take off or a critical go around. If you want to baby your engine, not that it needs it, throttle back to 2200 after take off when you have sufficient altitude to make a safe landing should you have an engine failure. Some engine manufacturers recommend full throttle for 5 minutes after take off then to throttle back.
To run an engine that’s rated for 2800rpm’s at 2200rpm’s just because “it’s an old girl” may not be the right thing to do, in my opinion.
Another thing to consider is how is the extra back pressure that your baffles created going to affect your engine. Maybe make your cylinder heads run hotter? Maybe burn exhaust valves? Ruin exhaust guides ?
I think the best thing you can do is run your engine the way the manufacture intended it to run. Trying to reinvent the wheel here might create problems that you wouldn’t normally have.
I have tried a verity of mufflers on my O320 and I was able to gauge the power loss pretty well by comparing take off rpm’s. I got about a 200 rpm decrease with most of the mufflers I tried. I was using motorcycle and race car mufflers and while they did help a lot with cockpit noise, I was surprised to find how hard the exhaust pressure was on the mufflers. They started coming apart in just a few hours.
I do agree that it’s is a good idea to check the accuracy of your tachometer. I used a True Tach that just sat on the glare shield