Operating Oversquare
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- Опубликовано: 17 ноя 2018
- Many pilots of constant speed prop airplanes have been cautioned never to operate oversquare, with manifold pressure (in inches) above rpm (divided by 100), to avoid damaging the engine. This is dead wrong according to engine guru Mike Busch.. SavvyAviation offers Professional Maintenance Services to owners of General Aviation aircraft, such as: Savvy Mx (Professional Maintenance Management), Savvy QA (Expert Consulting), Savvy Prebuy, SavvyAnalysis (Engine Data Analysis) and Breakdown Assistance. Savvy also publishes a monthly newsletter with lots of interesting information for the general aviation enthusiast; subscribe to it at www.savvyaviation.com/home/ge.... For more information, visit us at savvyaviation.com. This webinar was hosted by the Experimental Aircraft Association (EAA).
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It's rare to hear someone with such knowledge at the tip of his tongue. And it's interesting to hear an authority who provides information generally hidden from the aviation public. Terrific stuff!
Mike Busch explains all these very clearly and modestly. He is undoubtedly the expert on engine topics.
After a military and airline maintenance career, I’m enjoying these lecture/talks since I’ve transitioned into a general aviation environment. Mike is covering topics I haven’t thought or dealt with since A&P school. Thank you EAA for sponsoring these RUclips posts.
Just found the answer to oneof my questions. I'm also a Canadian AME also started the same year as you 1965. Apprenticed on Bell 47s. I read about Lindgergs technics on P38s
Low RPM and higher throttle settings= Less noise and increased efficiency... Gee is that why they started using overdrive transmissions in cars???
I am burning 9 GPH in cruise with my Cessna 182 with an O-470-R using these techniques: slightly lop with carb heat, throttle almost full open and 2100 rpm. I had to burn some fuel off before maintenance so I ran ROP at 2400 rpm at the throttle almost wide open and burned 16 GPH. 9 GPH at 130 mph or 16 GPH at 150 mph. It’s amazing how much you can extend range with these techniques.
The fuel savings over 400NM flight that will last only 24min longer are just ming boggling - 14gal!
My Rv8 has a O-320 carburatted Engine , with fix pitch, but it’s quite interesting to learn about low RPM operations.
If we had begun measuring manifold pressure in, say, hecto-pascals instead of inches if mercury, the oversquare argument would never have originated.
You are so correct!
I have noticed some FIs take the square numbers as a gospel still. Basically “With the american units it so happens that the low RPMs of an aircraft engine coincide with the manifold pressures and the engine is designed around that.” One such FI has checked me out in a PA28-180 with an electric MT prop. It limits the continuous RPM to 2500 as that is the maximum position of the regulator. We took off and I was told to reduce the MP to 25” for the climb. The aircraft climbed like a 150 on a hot day. My investigation into the POH supplement revealed one should keep full throttle and just reduce the RPM to 2400, and you basically have the same engine regime as with a fixed pitch prop during Vy climb. That however breaks their logic :)
Great news MIKE , thanks CW
If you look at the old 1960s Cessna owners manuals for cruise performance at various altitudes, you will see recommendations that are very over square. This info was right before our eyes back in those days. The book showed that it was ok, but nobody ever talked about it. The "over square" warning was always taught to us until years later.
Mike, any recommendations on running a PPonk O-470-50?
Awesome information..!
I once flew a return flight in my 185 in Africa at 8000 ft , 1700 rpm and LOP , full throttle and a fuel flow of 7 gph ,egt lowered by 250-300 degrees. My indicated air speed was 90-100 mph but I had a 50 mph tailwind so thats why I opted to experiment with ultra low power settings…!!!
We have found, using the JPI EDM-700 that with the throttle at full throttle, with either a carburetor or fuel injection, you will have more even EGTs and CHTs than with the throttle less than wide open. With a Fixed Pitch propeller, you are always "over square" on take-off at sea level: 29" MP and 2300+/- rpm when the throttle is pushed to full throttle at the beginning of the take-off roll.
Less throttle, more cooling, maybe?
The test at about 40 min. Was there a difference in speed from over to under square?
IIRC it was Lindy's techniques that made the interception of Yamamoto's bomber possible.
Lower RPM equals higher propeller advance ratio (J), which corresponds to higher propeller efficiency. More brake horsepower converted to useful thrust horsepower.
Lower RPM reduces friction horsepower, less power lost due to the rubbing together of all the moving engine parts. More fuel energy converted to mechanical torque vs lost as heat.
Lower RPM leads to increased volumetric efficiency. The air being sucked through the induction system by the racing pistons must travel through a series of tubes and pathways which are imperfect and introduce some level of friction. The faster the pistons attempt to suck, the less oxygen is delivered to be used in the power stroke. A lower RPM allows each intake stroke to gather a more substantial fuel air charge.
"Reducing the propeller and don't touch the prop" - I think Mike meant "and don't touch the throttle."
Not to quibble, but you may want to correct the notion that Col. Lindbergh introduced the idea of oversquare operation to the P38 pilots. The Allison V-1710 is a supercharged engine, and is routinely operated oversquare, as are most high-performance fighter, bomber and large transport aircraft engines of the piston age: the R-1820, R-1830, R-2600, R-2800, R-3350, R-4360, to name a few, are probably never operated undersquare either.
On the contrary, many pilots of that era were advised never to let the engine run undersquare, to avoid having the prop drive the engine.
Col. Lindbergh's contribution involved a mix of reduced RPM with increased manifold pressure, true, but his chosen power settings reduced cruise airspeed to 185. . . bringing the airplane closer to its maximum range airspeed. And it's probable that these same settings put the engine in a state where the "red box" didn't exist, making it safe to operate the engine at best power mixture continuously. It was the combination of best-power mixture and reduced airspeed that provided the extended range.
@Johnathan Celso Note, please, that none of the engines I referenced are horizontally opposed. I read the article you mentioned around the time it was published, and in it Mr. Deakin confirms that one of the high-performance radial engines that was most common in that era ( R-2800,) was indeed subject to failures caused by 'engine driving prop,' and he explains that in detail. . . The V-1710 is not radial, but it is still true that "pilots of the era" were told not to run undersquare. Mr. Deakin didn't prove anything that the engine and airframe manufacturers didn't already know. To quote from the C-47 series Air Force T.O.:
". . . it is important to cushion the high inertia loads on the master rod bearings which occur at conditions of high RPM and low manifold pressure. As a rule of thumb, it is well to remember that each 100 RPM requires at least 1 inch Hg manifold pressure; for example, 23 inches Hg at 2300 RPM. Operation at high RPM and low manifold pressure should be kept to a minimum."
So I don't see what your skepticism is all about.
Those airplanes were also not expected to survive until normal TBO. B17s were lucky to hit 500 hours
As for the prop driving the engine that is only slightly related to being undersquare, it is mainly caused by rapid power reductions at high airspeeds (or if a making a dive), but power can be reduced faster with a low rpm setting.
I have been playing with running my o-360 at 1800-1900 rpm when solo or lightly loaded. Any slower and maintaining above best L/D speed and altitude becomes difficult at or below 19" MP. Operating much above that MP seems to cause detectable vibration "(lugging? as mentioned below)". Have yet to get serious about calculating fuel savings because most of my flying in this condition is in ground observation ops (low speed for more time to look at the ground/ take pics), but noise is reduced and i cant help but believe that less rpm at a relatively low cylinder pressure must equal less wear, piston inertia engine friction, and fuel burn. I wish i could operate in the region my Hartzell prop/ Lyc powerplant combo is Placarded against (2000-2250 rpm). When trying to make a moderate speed (traveling) i almost always run 2350 rpm, as low as i feel gives me a safe margin from the restricted rpm region).
software being the pilot then?
Fantastic information. I love getting rid of old wives tales with data!
Tell Mike if he pulls the mixture to cut out on downwind he can even land without touching his throttle too!
If one buys an aircraft with 50% of TBO which has been operated under square and the new owner begins operating over square, I wonder if there are any negative ramifications from changing operating styles.
Mike: why don't you make the point that slower RPM means greater angle for the propeller, ie. greater air movement (greater work for glven power). Much like using a canoe paddle 90 degrees to water as apposed to less paddle work. This is higher gear relationship to eg. and overdrive in a car.
Tnks for the great content! I have 2 questions:
1. I fly nowadays a PA34 seneca V, (220 Hp Turbo). The POH power table says that 30in/2300rpm is acceptable, (normal cruise) so I follow that, but I fell that it loose some GS using that instead 28/2400rpm. Am I Wrong?
2. About the LOP for this specific airplane/engine, (Seneca V), do u have some tip how to do that correctly? I don’t ever did LOP at a turbo aircraft, and I can’t find anything in its POH.
Tnk u again.
Are you at the same fuel flows for 30/2300 and 28/2400?
@@davidspeyers5740 yes!
@@thiago.marcato Then I would say that 30-2300 is farther LOP than 28/2400 even though the fuel flow is the same. How much speed loss are you seeing? 1-2 knots or more?
should post airspeed differential with chart at 40-41minutes
All that over square Business is born of shear lack of understand the combustion engine physics and science
And the fact the inches of mercury being the standard of measurement only just so happens to be less than 30 at sea level as well as your maximum engine RPM being always under 3,000.
If you use PSI for manifold pressure, than its 14.7psi and what, 1,470rpm? What if we use ATM? 1.0 is the max, we see 0.7atm at altitude so we can only use 0.7 RPM? Its nonsense.
A fixed-pitch propeller static run-up might only be 2,150rpm. Thats at MAXIMUM throttle and Max manifold pressure as well. You lean to achieve maximum static rpm, richen very slightly, then release the brakes. For a max performance takeoff. Nobody has a problem with this.
I saw a ABS presentation that said operating my A36 below 2300RPM , ever, especially for extended periods of time, is harmful due to engine harmonic wear/damage.....any opinion on this?
I never heard of this on a Bonanza. However, on my buddy's Mooney M20C, I vaguely recall there is an RPM range somewhere around 2,000 RPMs where the book says not to fly continuously at that setting due to "vibration". But there is no marking on the tach to warn of this. It may have something to do with propeller vibrations as suggested in the note below by another pilot. I guess "harmonics" and "vibrations" are in the same family because they both are "waves". Hopefully somebody else can chime in on this. What does your engine manufacturer say? I'd call the tech department if it were my airplane and was that concerned. Hope that helps.
I have always flown my Mooney with the IO360 at 24 squared, careful not to go over square. Also, on the recommendation of another Mooney owner on the field and my instructor I've been using the 50 ROP method by hitting peak EGTs with JPI700. It always felt wrong to me. I just ordered Manifesto and Mike on Engines. Looking forward to seeing what I can get out of the plane with these new procedures!
Curious to know if you changed your techniques and what differences you've experienced if so.
@@justcommenting4981 Absolutely! I've learned so much since then. Thanks for bringing this back around. I now avoid ROP ops for cruise. I'm WOT from take off until landing prep. I run LOP but still usually at 2400 rpm. Gets me 150mph true at 9gph or less depending on altitude. Just installed a Surefly EIS so I'll need to monitor CHTs and fuel burn for changes.
Any recommendations on a 0-470? Where could I fine Continental's information for running oversquare for any 0-470? Best Regards.
My question too. My POH has me higher RPM over MP on almost every altitude and setting. Just trying to see if I can run 23” and 1900rpm.
@@bmshope You should search for the engine specific manual. Even the O200 has one, you should easily find one out there.
@@bmshope Just follow what your POH says . . .
I'm getting confused. If I set 65% power at 4000', according to the aircraft manual, I should set 2200 RPM and 22.4" MAP. If I now open the throttle all the way and get an increase in MAP, am I not also increasing the power setting beyond the desired 65%? This is a Lycoming IO-540, normally aspirated engine.
You have to figure out what settings get you the power/speed you desire.
For most GA planes: Fly at 8,000' wide open throttle for 75% manifold pressure. And fly at 12,000' wide open for 65% manifold pressure.
(Assuming naturally aspirated and not turbo).
Set RPM as desired. Lean to observe engine roughness and richen slightly. This should have you just slightly lean of peak. And well lean of maximum-power mixture.
Excessively rich mixtures are one cause of rapid wear in the Exhaust Valve Guides. Vary the throttle position to find the most even EGTs. The optimum throttle position can change with altitude, but we find that wide open is usually the best position, especially at higher altitudes, like 7 to 12 thousand feet. Reducing power by leaving the throttle at full throttle until usually at about 3,000 feet or so and leaving the mixture where it was set at 7.5 to 10 thousand feet and slowly reducing the RPM from cruise, such as cruise at 2350 rpm, to about 2,000 RPM is an efficient method to maintain cylinder temperatures and low fuel flow. It is almost impossible to have detonation, using the correct fuel, at below 65% power. For "over square", some manuals state not to exceed 2" to 3" above the RPM for cruise. On a 300 HP Continental or Lycoming, with a maximum RPM of 2700 RPM, you are 2" over square on take-off at sea level. Manufacturers de-rate engine power by lowering the RPM, but seldom reduce the throttle manifold pressure. O-540 Lycoming engines that produce 235 HP on take off run at 2575 RPM with full throttle, which is 29" of manifold pressure. The Continental TSIO-360-__ engines run at 2575 RPM and 40" of manifold pressure, as in a Seneca II. I do not think that low of an RPM, 2575 @ 40", is a good idea, but that is what it was certified for.
I am a bit confused. Why is the fuel flow the same above and under square? What is the point of this then? Sorry if i missed it…
Less RPM:
-> less wear and tear,
-> cooler temps -> less wear and tear
-> allows you to run LOP better, -> fuel savings, lower temps -> less wear and tear
Imagine what would be better for your car if you could get the same performance and mileage driving at 2500RPM or 2000RPM.
Is there a document that has suggested settings for various engines?
Bert Penney yes, its called a pilots operating manual. Each aircraft has one for it specific engine and performance specs
With regards to full power takeoffs, should a reduced RPM takeoff be done at extremely low density altitudes (i.e. below -1000ft) to keep the engine within its rated power range?
Nahhhh! Take off like a rocket. It’ll be fun😅.
its only like 2% and a minute of use, besides that usually low DA is correlated with low air temps
I agree that running "square" is a totally arbitrary rule of thumb and is foolish to teach as universal to all engines.
However I must point out some questionable statements in the video.
1. While WW2 is a rich source of R&D, combat aircraft were on average expected to last a few hundred hours before being destroyed, and even when they weren't lost a short TBO was an acceptable cost if it won an extra battle.
2. The statement about props liking low rpm is less than a half truth, the full story must include prop diameter, torque, tip speed, and true airspeed (true blade angle of attack) so unless you are changing the prop, specifying the target air density, and adjusting the TAS "slower is better" is a statement just as hollow as "always fly square"
3. It is barimetric efficiency not volumetric efficiency. The volume is always 100% filled.
4. With the exception of 5 second bursts, street cars normally operate at very low manifold pressure and don't stress valves. They actually can't operate substantially lean of peak with modern multistage catalytic converters which are required to do both oxidation of carbon compounds AND reduction of nitric oxides.
I can’t understand SR22 story operating at 2700 rpm , and Cirrus factory lack of diagnosis ...
They were probably operating on the cost-saving idea of fixing the cheapest thing(s) first. Why they thought their specs were required for that specific engine is hard to say. But, he suggested a small operational change and the problem disappeared, so Cirrus now has to know they have some variability in their engines and the problem isn't always another piece of hardware or the fuel system.
My instructor is on the 'square' side of the equation on a DA40-180. The question is is a flight school operated aircraft similar to the air operation mentioned where this type of service is not permitted?
I would think the over square issue was to assure prevention of "lugging " the engine, as it is known in the automotive arena. That condition, along with detonation, isn't very detectable in a/c due to typical noise environment, power plant roughness and sometimes turbulence. It's created by low rpm, high power demand and possibly inconsistent fuel quality. Lack of variable timing is part of the problem and I can only imagine what fuel inconsistency was " back in the day ". The "over square " was probably created as a convenient catch phrase like gump and all the others. On the practical side probably weren't a lot of pilots flying around cross country at max weight, wide open throttle and 1200 rpm in the first place. As to Lindbergh, as we see more and more, celebrity status doesn't assure thorough understanding of a particular topic.
I think oversquare danger zone comes from boosted radials...?
They are ALWAYS over square, due to the fact they are 1: Boosted well above 29.92 inHg and 2: Cannot rev higher than 2,500rpm. Its all ancient pilot numerology hokus pokus.
Manifold Pressure could be 29.92 inches of mercury or it could be 407.17 inches of Water. Or maybe 14.696 PSI or it could be 101.32 Kilopascals.
All of these ARE THE SAME PRESSURE.
There IS NOTHING MAGIC about "Square" power settings.
In your automobile, pressing the accelerator pedal to the floor results in 100% of available manifold pressure due to the throttle plate going wide-open. SAME AS the airplanes Throttle being fire-walled.
A Car can be shifted into 2nd gear at full throttle, and "pull" through the Rev-range at Maximum manifold pressure, yet low sweeping to high RPM. And again and again for hundreds of thousands of miles for each and every gear shift.
Over Square/Under Square is literally Numerology hokus-pokus with no basis in engineering principals.
I think the basic premise that RPM must be increased before throttle is certainly valid, and was taught for all highly supercharged piston engine aircraft operations around the second world war. But those are very special cases. And they CERTAINLY operated "Over Square" as the P-51 Merlin ran at 61 inhg, and a maximum RPM of 3,000... But, you don't want to shoft the supercharger into high gear, whack open then throttle, with the propeller rpm selected for a low cruise rpm. Because this manifold pressure at low piston speed results in extreme cylinder pressure, and large pressure spikes due to flame front speed burning all of the fuel/air mixture long before the piston reaches top dead center, which is going to cause detonation or even catastrophic pre-ignition.
Downside to very low RPM: Oil pump speed is also lower and therefore there is lower oil volume thru the bearings and potentially lower oil-film thickness.
And regarding Volumetric Efficiency: With a tuned length intake, there is a specific RPM that gins maximum benefit. Also, the swirl/tumble is improved which aids mixing. So VE has an efficiency "island" of operating parameters, going to far above or below will cause efficiency to suffer.
Turbochargers, superchargers, engines, intakes, exhausts, even propellers, aircraft themselves, all have efficiency maps. Operating in the area of peak efficiency is usually optimal. And each item should be carefully selected by the engineers to happen simultaneously where its most often used. (Although most aircraft fly waaaay faster and at a lower lift coefficient than optimal. Sacrificing substantial lift/drag for speed).
Exhaust valve failure is caused by a lean mixture at a higher rpm. By reducing rpm the cylinder has more time for the flame to finish burning before the exhaust valve starts to open.
It also holds the exhaust valve against the valve seat longer allowing the valve to transfer heat to the cyl head for a longer amount of time.
And the cylinder cools it with airflow both inside and out. That valve is opening over 1200 times per minute
Your premise is wrong. Rich mixtures cause carbon bromide to form like a shellac on the valves and then they get sticky and brake. See the July issue of AOPA for an excellent article on why lean mixtures are better for exhaust valves and why rich mixtures are bad.
@@daffidavit there is no bromine in the system
@@dragthatsht the percentage of time in contact with the seat is exactly the same at all rpm
Why have an intro each time introducing the presenter when it’s the same guy presenting. Pretty sure he can do his own intro at this point.
Some very interesting charts but some flaws and your confusing explanations make it a struggle when it needn’t be. First using a chart with mislabeled lines is inexcusable and lazy. Second in chart with labeled N42MM …. Near the beginning you say the over and under square examples have the same fuel flow and therefore the SAME HORSEPOWER so that you can go on to to prove they have DIFFERENT HORSEPOWER 🫤. I guess your first horsepower is as measured at the piston crown? You might say constant fuel flow means same work done but you know that’s NOT true either. The point your trying to make is that with constant fuel consumption the case with more
waste heat going out the exhaust and convecting from the cylinder heads, should have less useable output horsepower. If you just had dynamometer data and clearly defined terms, this would be so much more clear.
As an automotive machinist of 13 years specializing is high horsepower race engines. I must say there are many half truths here, Be careful of being blinded by graphs and words that do not tell the whole story.
Please enlighten us “LordJiggle.” Can you present specific concerns? Better yet, can you bring forward your operational research/expertise in a well-thought presentation? I’m betting you can’t. In the meantime...
@@paulwikstrom Well said!
OMG, you talk so slow and thanks to RUclips speed control, playing the video at 1.75 is just about the right speed.
ADHD . . .
Chen. Content over style is more important than style over content.
1.25 works great..
I prefer 1.25
1.10 would be better.