Oversquare Confusion

[201er]

I saw a post somewhere recently about oversquare operations which reminded me of my own confusion on the matter. I don't understand why at sea level, the POH allows a maximum of 27" MP at 2200RPM but at higher altitudes the max oversquare condition shrinks. For example at 4,000ft 24.4" MP is the max at 2200RPM even though higher MP may be available (26.2" at 2400RPM). If the plane can be flown at 26.2" to produce less than 75% power, why can't it be flown at 2200RPM instead at the same WOT to produce something between 62% and 74% power which are given by the POH at 2200 and 2400 RPM respectively?

I don't understand why a greater condition of being oversquared is permitted at sea level and higher power but not at higher altitude and lower power? There have been many times in cruise where I wanted to begin a descent leaving throttle full and simply pulling back the prop to reduce drag, but decided to bring the power back to the maximum POH applicable MP for that lower RPM setting. Is this really necessary?

How substantial is this concept of oversquare? I understand that even the POH allows it in certain configurations but are there examples where you really can be too far oversquare? Not that I would, but is 29" and 2000RPM bad for the engine? I have not yet found a useful power setting that exceeds the prop governor capability. Where would that be?

[jlunseth]

The concept of "oversquare" is not very relevant to engine management.  It is a handy but outmoded rubric for training. 

At higher altitudes, props generally become less efficient at lower rpm's.  What you are most likely seeing in your POH are higher RPM settings at higher altitudes, required in order to produce a higher BHP.  And that's because the prop is more efficient at the higher RPM at that altitude. 

In my 231, according to the manual, the engine won't produce 75% HP above 22,000 no matter what MP I use, unless the prop operates at full rpm's for the engine, which is 2700.

I use just two prop speeds.  I use 100%, which in my engine is 2700, for takeoff and climb, and any other operations requiring 100% power.  And I use 2450 for cruise, which is a speed at which my engine feels nice and smooth.  I will then vary the HP to get the % HP I want.  I will bump the cruise rpm's up for cruise if I am in Flight Levels.  Otherwise I just use 2450, the engine seems to like it. 

Some people use a third prop speed, say 2200, for descents.

[jlunseth]

Ah, here we go.  A pretty good read on "oversquare."

http://www.avweb.com/news/airman/184483-1.html

[201er]

I read that before I posted. Really didn't answer my question about just how much over square it's ok to go and what happens if you go TOO much over square. Basically it said to consult the POH and the POH seems to be more restrictive for oversquare operations at higher than lower altitudes which seems odd.

[danb35]

Quote: 201er

I don't understand why at sea level, the POH allows a maximum of 27" MP at 2200RPM but at higher altitudes the max oversquare condition shrinks.

[Cruiser]

what Dan said ........

The POH settings are what you can use and expect based on testing. Other combinations are possible and acceptable unless specifically prohibited such as the MP/RPM range limitations

[201er]

Alright, well in that case, why do you think they didn't include higher MP at lower RPM at higher altitudes but did at lower? According to the oversquare article linked above, your best bet may be to stay at full throttle and reduce RPM to set power.

 

Also, still wondering what the practical limit of oversquaring is. What is wrong with cruising at WOT and 2000RPM at low altitude rather than higher RPM and bringing the throttle back? What is behind the whole don't go oversquare rule of thumb?

[Jeff_S]

I've seen the answer to this, which was not addressed in the Mike Busch article but if you search around you will find it. As I recall, in the older radial engines, especially the warbird trainers of WWII and before, there was a problem with flying oversquare that shortened engine life and caused some other problems. Much of general flight instruction was formulated during this period, so many of these practices are still taught today even though they are outdated.

[Hank]

Quote: 201er

Alright, well in that case, why do you think they didn't include higher MP at lower RPM at higher altitudes but did at lower? According to the oversquare article linked above, your best bet may be to stay at full throttle and reduce RPM to set power.

 

Also, still wondering what the practical limit of oversquaring is. What is wrong with cruising at WOT and 2000RPM at low altitude rather than higher RPM and bringing the throttle back? What is behind the whole don't go oversquare rule of thumb?

[MARZ]

Quote: 201er

Also, still wondering what the practical limit of oversquaring is. What is wrong with cruising at WOT and 2000RPM at low altitude rather than higher RPM and bringing the throttle back? What is behind the whole don't go oversquare rule of thumb?

[Shadrach]

Quote: jlunseth

At higher altitudes, props generally become less efficient at lower rpm's.  What you are most likely seeing in your POH are higher RPM settings at higher altitudes, required in order to produce a higher BHP.  And that's because the prop is more efficient at the higher RPM at that altitude. 

[Shadrach]

Quote: 201er

I don't understand why a greater condition of being oversquared is permitted at sea level and higher power but not at higher altitude and lower power? There have been many times in cruise where I wanted to begin a descent leaving throttle full and simply pulling back the prop to reduce drag, but decided to bring the power back to the maximum POH applicable MP for that lower RPM setting. Is this really necessary?

[Hank]

It's quite possible that I blew the analogy. But I don't fly WOT/low RPM anyway. And I try not to drive the Interstate in 3rd or around town in 5th, too.

[Shadrach]

Quote: Hank

-For your last question, it seems that down low, WOT/2000 is rather similar to cruising on the interstate at 75 mph in 3rd gear instead of in 5th . . .

[stevesm20b]

I don't think your going to hurt anything or reduce engine life on a O-360 or the IO-360 by running low RPM (2200) and max M/P, even at low alt in a normally aspirated Mooney. 

[jlunseth]

Quote: Shadrach

Hmmm, I'd like to see where you got this info, or the efficiency calculations involved. I've never done the calculations so I can't say what exactly what RPM is ideal. I know temperature, air density and prop diameter are the main players all other things being equal. IIRC prop tip speeds of .88 to .92 mach is ideal. If you have a linky, please post. Norman, can you ring in?

[Shadrach]

Quote: jlunseth

Now I'll have to go look it up.  It seems to me it was "Flying High Performance Singles and Twins" by John Eckalbar. 

[jlunseth]

Didn't find the material in Eckalbar's book, but did find some other stuff, and am going to have to leave this for the weekend, but here is what I have.

On the general proposition that props become less efficient at higher altitudes, here are a couple of general articles.  http://www.experimentalaircraft.info/flight-planning/aircraft-performance-3.php

http://avstop.com/ac/4-10.html

This proposition is fairly simple.  A prop generates lift, and as density altitude increases and there are fewer molecules, lift decreases.  This is true for wings also. 

To some extent, the constant speed prop is capable of compensating by going to a coarser pitch for a given density altitude, thus taking a bigger "bite" out of the air and, if you will, catching more molecules.  But props, whether constant speed or fixed, are all designed to be optimal for a particular part of the aircraft's performance envelope.  You can make a "climb" prop, or a "cruise" prop, or a "high altitude" prop, but you can't change out the prop at 20,000 feet to go from one phase of flight to another.  High altitude props generally have bigger blades, both length and width, for better performance at a high altitude, the better to catch more molecules if you will.  The props on Mooneys, I believe, are generally designed as cruise props.  The implication of this, is that at some point as the aircraft goes higher, the prop's ability to develope thrust is going to begin to deteriorate.

Another piece of the puzzle is that for higher KTAS, a higher RPM is required for maximum prop efficiency.  Here is a pretty good article on that:

http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf

And what happens for a given power setting as altitude increases?  KTAS increases. 

So for a given prop design, as altitude increases, lower RPM's and coarser pitch are going to be more efficient up to a certain point, and after that, as density altitude increases, RPM's are going to have to increase to maintain prop efficiency.  And if you extrapolate that out far enough (fly to a high enough altitude), the prop is no longer going to be able to develop enough thrust even at max power (assuming the engine can still develop max power), for the aircraft to climb higher.

Am not an engineer and do not know alot about the particular design of the Mooney props.  But going back to the OP's question about why less "oversquare" is apparent in the POH tables for higher altitudes, it has little to do with being oversquare, but has to do with the fact that both the engine (especially if normally aspirated) and the prop are running out of efficiency, so both need to be set higher.  That leads the the appearance that there is less "oversquare" tolerance at higher altitude, when that in fact is not what is going on at all.  What is going on, is that both prop and engine need to be set to higher numbers in order to generate sufficient thrust. 

Hope that is somewhat helpful.

[201er]

Very interesting response. I'm going to have to read those articles and give this some more thought. Does this mean that running 2700RPM at higher altitude is less bad than cruising 2700RPM down low (noise, engine wear, oil burn, etc)?

However, I think you missed part of the oversquare point for high altitude and lower MP recommendations by POH. It would have been my guess that if 27"/2200RPM is allowed at sea level, that you would be good to use WOT above any altitude yielding 27" MP or less. However, if you look at the 4,000' performance table I posted, you'll see that although 26.2" MP are available  (2400RPM column), a maximum of 24.4" is recommended for 2200RPM. This yields 62% power. There is clearly some kind of purposeful intent her rather than providing a 65% setting and whatever MP between 24.4" and 26.2" that would provide it. Also, they did not provide a 26.2" MP power setting for 2200RPM even though it probably would be available as well.

Based on the table it doesn't seem that 2200RPM is an inefficient prop setting at 4000ft. In fact the opposite. 2400RPM and 23.3" yield 65% power, 9.2gph, and 152ktas. 2200RPM and 24.4" yield 62% power, 8.5gph, and 148ktas. My guess is that at 2200RPM and 25" you could get about 8.7gph and 150ktas. For these reasons, except if oversquare is the culprit, I cannot understand why the POH does not provide a 65% or full throttle power setting at 2200RPM at 4000ft as just one example.

 

[Shadrach]

Quote: 201er

Very interesting response. I'm going to have to read those articles and give this some more thought. Does this mean that running 2700RPM at higher altitude is less bad than cruising 2700RPM down low (noise, engine wear, oil burn, etc)?

However, I think you missed part of the oversquare point for high altitude and lower MP recommendations by POH. It would have been my guess that if 27"/2200RPM is allowed at sea level, that you would be good to use WOT above any altitude yielding 27" MP or less. However, if you look at the 4,000' performance table I posted, you'll see that although 26.2" MP are available  (2400RPM column), a maximum of 24.4" is recommended for 2200RPM. This yields 62% power. There is clearly some kind of purposeful intent her rather than providing a 65% setting and whatever MP between 24.4" and 26.2" that would provide it. Also, they did not provide a 26.2" MP power setting for 2200RPM even though it probably would be available as well.

Based on the table it doesn't seem that 2200RPM is an inefficient prop setting at 4000ft. In fact the opposite. 2400RPM and 23.3" yield 65% power, 9.2gph, and 152ktas. 2200RPM and 24.4" yield 62% power, 8.5gph, and 148ktas. My guess is that at 2200RPM and 25" you could get about 8.7gph and 150ktas. For these reasons, except if oversquare is the culprit, I cannot understand why the POH does not provide a 65% or full throttle power setting at 2200RPM at 4000ft as just one example.

 

[carusoam]

Why would anyone want to operate intentionally oversquare by lowering RPM?

I understand oversquare and highest rpm available (2,500 on the O1) = best power on T/O and climb out for a relatively short period of the life of the engine.

But selecting lower RPMs with a high MP sounds like unusually high and unmeasurable ICPs (internal cylinder pressure).  Even when running 250 deg ROP.

You can drive an LT-1 Camaro in 6th gear at 1000 rpm around the neighborhood.  Just because it can be done does not mean it should be done (picture a bucking bronco when the process gets out of balance).  I expect high ICPs are not good for mechanical connections or piston rings.

I think the data that we are missing would be related to the prop.  What rpm is most efficient/powerful for the density altitude we are flying at.

Best regards,

-a-

[Shadrach]

Quote: carusoam

Why would anyone want to operate intentionally oversquare by lowering RPM?

I understand oversquare and highest rpm available (2,500 on the O1) = best power on T/O and climb out for a relatively short period of the life of the engine.

But selecting lower RPMs with a high MP sounds like unusually high and unmeasurable ICPs (internal cylinder pressure).  Even when running 250 deg ROP.

You can drive an LT-1 Camaro in 6th gear at 1000 rpm around the neighborhood.  Just because it can be done does not mean it should be done (picture a bucking bronco when the process gets out of balance).  I expect high ICPs are not good for mechanical connections or piston rings.

I think the data that we are missing would be related to the prop.  What rpm is most efficient/powerful for the density altitude we are flying at.

Best regards,

-a-

[carusoam]

Ross,

I agree with your insight above.  It seemed that somebody was interested in running WOT at 2,200 rpm at low altitude (high MP) for an extended time.

I just was wondering if there is a point to running under those conditions.  The Camaro would achieve it's highest mpg.  The Mooney, probably not. 

I just got the picture in my head of Carson's speed for Camaros....It may be efficient, but so slow, most owners don't want to go there.....

Best regards,

-a-

[201er]

Well let's say you're just sightseeing or local flying with no need for speed. Naturally you want to pick whichever power setting that will cost you the least gas for the same distance. The option is to stay at a faster RPM and throttle back or to run at a higher MP and reduce RPM. Let's not bring mixture into this because it should apply equally regardless of MP/RPM producing the same power.

Looking at the 20J POH, 2700RPM/18" MP can produce 55% 138ktas at 8.6gph. 2200RPM/22.2" can produce the same 55% and 138ktas 7.8gph. Clearly reducing RPM is more effective (assuming POH is right). My take on complex airplane operations (and I could be completely wrong, still learning) is to pick a power setting and then find the lowest RPM that can produce that much power for maximum fuel efficiency and equal speed.

I still don't understand the reason why the POH recommends diminishing max power/MP for low RPMs as altitude increases. Why is max recommended power at SL for 2200RPM, 27" and 68% but at 4000ft, 2200RPM, 24.4" 62%. Once again, let me remind you this is not for lack of greater available MP because higher power settings are recomended with higher MP.

[carusoam]

201er.

Things to consider (thinking out loud)....

[1] The why anyone would want to fly oversquare....GPH.  I answered my own question....

[2] It seems your lowest FF is at 2200rpm and 22.2"  - admittedly, not very much over square.

[3] 2200rpm is more difficult for the engine to push, Is it possible that the MP reading may not max out the same as when at 2,700rpm?

[4] Small experiment:  Check actual FF at three points.  2200 / 22.2,  2700 / 18, and midway  2450 / 20.1  - assuming no yellow arc for this one.  (who can read to 0.1" MP anyway???)

[5] Watch cyl temps don't fall out the bottom of the green arc.

[6] Do this while LOP if possible. (re-lean between operating points)

[7] Take note of how quiet or low vibrations are.  How well does the engine run at each point?

[8] Combine the idea of Carson's speed.  This will get you best mpg to go along with best gph.

[9] Best overall fuel use would be something like this: Carson's speed, low rpm, LOP adjust MP to maintain speed and altitude.

I think you may find that the actual FF savings between each of the three operating points above does warrent operating outside of the known envelope.

Best regards,

-a-