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Over square

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sajdds(at)comcast.net
Guest

Posted: Tue Sep 13, 2011 7:39 am Post subject: Over square

p { margin: 0; }Please excuse my ignorance but I have never been able to understand the concept of over square. First, since over square is a relationship between two numbers it would seem the specific units of measurements would have to be specified. Our airplanes have a variety of different units so I do not see how the rule can be valid. Secondly, it seems to me that the concept of engine pushing the plane or plane pushing the engine is a function of attitude, speed, power setting, and prop setting. I can understand that for some engines using the prop to slow you down puts stress in areas that receive no direct oil and consequently can produce problems.  I just do not see the connection with under/over square. I can be either under or over square and still have the engine pulling the plane. Can anyone enlighten me? Also, I believe George Coy once told me that back loading was not really a problem for a Yak 52. Is that not the case?



Steve Johnson

Yak-52 N9900X

0B5

413 522-1130 Cell

From: cjpilot710(at)aol.com

To: yak-list(at)matronics.com

Sent: Tuesday, September 13, 2011 8:02:58 AM

Subject: Re: Yak 52 Questions



    I can only add one thing to Brain's post.  Keeping an engine 'over  square' can help with the longevity of the engine.  You want the engine to  be driving the prop - not the prop driving the engine.  This is known as  "back loading" for which the bearings and such in the engine are not really  design for.  This is particularly bad for bigger engines (W1820 &  PW1830).  The HS6a and M14p are somewhat "beef-e-er" for their size/hp so  they are not quite as critical for the occasional backloading.  A few  cycles in an 1830 and you'll be changing an engine way before that 1,000 hours  mark.

  

 As Brain pointed out the IAS at stall is what it is.  It is the  instrument you will be using for reference.  It can read in any units of  valve of what ever you want to call them.  And they are only good in your  airplane.  There are a great many errors from the real TAS at stall to that  of ASI simply because of system design.  You can even see this in a Cessna  or Piper.  Have you ever seen an ASI with one (1) kt/mph increments?   Also remember that a stall is really defined by the critical angle of attack of  that particular airfoil not a speed.

  

 Jim "Pappy" Goolsby

  

  In a message dated 9/13/2011 1:25:52 A.M. Eastern Daylight Time,  brian(at)lloyd.com writes:

 [quote]



   



[b]

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Jetj01(at)aol.com
Guest

Posted: Tue Sep 13, 2011 9:13 am Post subject: Over square

Nice work Brian Lloyd! 

Jj



Sent from my iPhone



On Sep 13, 2011, at 11:33 AM, Brian Lloyd <brian(at)lloyd.com (brian(at)lloyd.com)> wrote:



[quote]



On Tue, Sep 13, 2011 at 8:36 AM,  <[url=mailto:sajdds(at)comcast.net]sajdds(at)comcast.net (sajdds(at)comcast.net)[/url]> wrote:

 	  Quote: 	
		   Please excuse my ignorance but I have never been able to understand the concept of over square. First, since over square is a relationship between two numbers it would seem the specific units of measurements would have to be specified. Our airplanes have a variety of different units so I do not see how the rule can be valid. 

 



	




Bingo. It isn't valid. But it is an old wive's-tale that doesn't seem to die no matter how many stakes we drive through its heart.

 

 	  Quote: 	
		   Secondly, it seems to me that the concept of engine pushing the plane or plane pushing the engine is a function of attitude, speed, power setting, and prop setting. 

 



	




That is correct also. And it depends on engine design as well. 

 

 	  Quote: 	
		   I can understand that for some engines using the prop to slow you down puts stress in areas that receive no direct oil and consequently can produce problems.  I just do not see the connection with under/over square. 

 



	




There isn't one.

 

 	  Quote: 	
		   I can be either under or over square and still have the engine pulling the plane. Can anyone enlighten me? Also, I believe George Coy once told me that back loading was not really a problem for a Yak 52. Is that not the case?

 

	




I believe you and George to be correct. 

Here is the real scoop on "over square" operation. 

An engine is designed for some maximum loading on the pistons, connecting rods, and the crankshaft. These loads are relatively independent of RPM but ARE a direct function of combustion pressures. And combustion pressure is a function of manifold absolute pressure (MAP) because MAP tells you how much fuel/air charge is going to enter each cylinder on each cycle. When you light a fuel/air charge of a certain mass, you will get a fixed amount of heat released and a certain amount of expansion into the volume of the cylinder. That in turn produces a specific pressure on the top of the piston, which in turn produces a specific force on the con rod and the crank. OK? Good. 

 



Now there are some variations that occur as a result of RPM because the fuel burning process takes time. When the fuel/air charge in the cylinder is ignited by the ignition, it begins to burn, release heat, and expanding. At the same time the piston is coming to top-dead-center (minimum volume) and then moving downward. As the crank moves through its rotating the downward-moving piston increases the volume in the combustion chamber. The farther the piston has moved when the fuel/air charge has completed combustion, the larger the volume to hold the expanded gasses, the lower the peak pressure on the top of the piston and, hence, the lower the force on the con-rod and crank.

 



If you reduce RPM too far, the volume in the combustion chamber hasn't yet increased enough and you get higher peak pressures in the cylinder. If you run a high enough MAP (fuel/air charge entering the cylinder) and a low enough RPM (piston not increasing the cylinder volume fast enough) you CAN reach the critical pressures needed to cause piston, rod, and crank damage. This is the real, dreaded, "over-square" condition. But generally this requires really low RPMs and really high MAPs. You aren't going to achieve this with a normally-aspirated engine but you can get there with an engine that is running with a high level of boost. 

 



The big radial engines used on older warbirds actually came from the manufacturer with peak MAP numbers that could be used at a particular RPM. As RPM was increased, you were allowed to run a higher MAP without damage to the engine. In addition, those numbers also varied with the presence of things that slowed down combustion, e.g. higher octane fuel, water injection, alcohol injection, etc. Unfortunately this "knowledge" was held over by people who didn't understand even after we got engines for which this didn't apply. That is where this whole "don't run oversquare" stuff got started.

 



(And before someone jumps on me, I understand about detonation margins. They are also affected by the changing volume of the cylinder as the piston moves downward during the power stroke. The discussion is the same.)

 

-- 

Brian Lloyd, WB6RQN/J79BPL

3191 Western Dr.

Cameron Park, CA 95682

[url=mailto:brian(at)lloyd.com]brian(at)lloyd.com (brian(at)lloyd.com)[/url]

+1.767.617.1365 (Dominica)

+1.916.877.5067 (USA)

 

   	  Quote: 	
		  

	


[b]

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sajdds(at)comcast.net
Guest

Posted: Tue Sep 13, 2011 12:12 pm Post subject: Over square

Brian 



Thanks for taking the time to give such complete explanation. The explanations I have gotten in the past just never made sense to me.



Steve Johnson

Yak-52 N9900X

0B5

 

   

 

From: "Brian Lloyd" <brian(at)lloyd.com>

To: yak-list(at)matronics.com

Sent: Tuesday, September 13, 2011 11:33:20 AM

Subject: Re: Over square



 On Tue, Sep 13, 2011 at 8:36 AM, <sajdds(at)comcast.net (sajdds(at)comcast.net)> wrote:

  	  Quote: 	
		     Please excuse my ignorance but I have never been able to understand the concept of over square. First, since over square is a relationship between two numbers it would seem the specific units of measurements would have to be specified. Our airplanes have a variety of different units so I do not see how the rule can be valid. 

	
 



 Bingo. It isn't valid. But it is an old wive's-tale that doesn't seem to die no matter how many stakes we drive through its heart.

  

  	  Quote: 	
		     Secondly, it seems to me that the concept of engine pushing the plane or plane pushing the engine is a function of attitude, speed, power setting, and prop setting. 

	
 



 That is correct also. And it depends on engine design as well. 

  

  	  Quote: 	
		     I can understand that for some engines using the prop to slow you down puts stress in areas that receive no direct oil and consequently can produce problems.  I just do not see the connection with under/over square. 

	
 



 There isn't one.

  

  	  Quote: 	
		     I can be either under or over square and still have the engine pulling the plane. Can anyone enlighten me? Also, I believe George Coy once told me that back loading was not really a problem for a Yak 52. Is that not the case?



	
 



 I believe you and George to be correct. 

 



 Here is the real scoop on "over square" operation. 

 



 An engine is designed for some maximum loading on the pistons, connecting rods, and the crankshaft. These loads are relatively independent of RPM but ARE a direct function of combustion pressures. And combustion pressure is a function of manifold absolute pressure (MAP) because MAP tells you how much fuel/air charge is going to enter each cylinder on each cycle. When you light a fuel/air charge of a certain mass, you will get a fixed amount of heat released and a certain amount of expansion into the volume of the cylinder. That in turn produces a specific pressure on the top of the piston, which in turn produces a specific force on the con rod and the crank. OK? Good. 

 



 Now there are some variations that occur as a result of RPM because the fuel burning process takes time. When the fuel/air charge in the cylinder is ignited by the ignition, it begins to burn, release heat, and expanding. At the same time the piston is coming to top-dead-center (minimum volume) and then moving downward. As the crank moves through its rotating the downward-moving piston increases the volume in the combustion chamber. The farther the piston has moved when the fuel/air charge has completed combustion, the larger the volume to hold the expanded gasses, the lower the peak pressure on the top of the piston and, hence, the lower the force on the con-rod and crank.

 



 If you reduce RPM too far, the volume in the combustion chamber hasn't yet increased enough and you get higher peak pressures in the cylinder. If you run a high enough MAP (fuel/air charge entering the cylinder) and a low enough RPM (piston not increasing the cylinder volume fast enough) you CAN reach the critical pressures needed to cause piston, rod, and crank damage. This is the real, dreaded, "over-square" condition. But generally this requires really low RPMs and really high MAPs. You aren't going to achieve this with a normally-aspirated engine but you can get there with an engine that is running with a high level of boost. 

 



 The big radial engines used on older warbirds actually came from the manufacturer with peak MAP numbers that could be used at a particular RPM. As RPM was increased, you were allowed to run a higher MAP without damage to the engine. In addition, those numbers also varied with the presence of things that slowed down combustion, e.g. higher octane fuel, water injection, alcohol injection, etc. Unfortunately this "knowledge" was held over by people who didn't understand even after we got engines for which this didn't apply. That is where this whole "don't run oversquare" stuff got started.

 



 (And before someone jumps on me, I understand about detonation margins. They are also affected by the changing volume of the cylinder as the piston moves downward during the power stroke. The discussion is the same.)

 

-- 

Brian Lloyd, WB6RQN/J79BPL

3191 Western Dr.

Cameron Park, CA 95682

brian(at)lloyd.com (brian(at)lloyd.com)

+1.767.617.1365 (Dominica)

+1.916.877.5067 (USA)



[quote]

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brian(at)lloyd.com
Guest

Posted: Tue Sep 13, 2011 5:18 pm Post subject: Over square

On Tue, Sep 13, 2011 at 1:09 PM,  <sajdds(at)comcast.net (sajdds(at)comcast.net)> wrote:

 	  Quote: 	
		   

Brian 



Thanks for taking the time to give such complete explanation. The explanations I have gotten in the past just never made sense to me.



	




You are welcome Steve. This is why it is important to understand the physics of flight. Once you really understand how the things are working, you can make a much better guess at what is going to happen when you do something different. And sometimes you just have to get in the plane and go try it. (Engine excluded.)

 



The problem with engines is finding someone who truly has a clue. I have found that, 99% of the people who think they have a clue about engines, really don't. It can be very difficult to get good information. I have had good luck with persistent calling to Lycoming, Continental, and the overhaul shops who specialize in mods. On the M14P and Huosai .. not so much.  What you WILL find is people who will fight to the death about their opinions but then, it is just religion, not science.

 



So, we have to go back to the basic physics and make the best guesses we can.



-- 

Brian Lloyd, WB6RQN/J79BPL

3191 Western Dr.

Cameron Park, CA 95682

brian(at)lloyd.com (brian(at)lloyd.com)

 +1.767.617.1365 (Dominica)

+1.916.877.5067 (USA)



  [quote][b]

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dstroud(at)xplornet.com
Guest

Posted: Tue Sep 13, 2011 7:54 pm Post subject: Over square

So we shouldn't ask you about MMO then, Sparky ?    

  

  

 David Stroud

 Ottawa, Canada

Christavia C-FDWS

Fairchild 51  replica

under  construction C-FYXV   and survior of Sparky's  Waterfront Bar, St. Thomas, USVI

 [quote]   ---

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