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L/D redux

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wild.blue(at)verizon.net
Guest

Posted: Fri May 02, 2008 9:18 am Post subject: L/D redux

wild.blue(at)verizon.net
Guest

Posted: Fri May 02, 2008 9:34 am Post subject: L/D redux

Re-formatted just in case. Take two.

 

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wild.blue(at)verizon.net
Guest

Posted: Fri May 02, 2008 11:48 am Post subject: L/D redux

viperdoc(at)mindspring.co
Guest

Posted: Sat May 03, 2008 4:35 am Post subject: L/D redux

Jerry,

I thought that by pushing the pitch to full forward on the prop at cruise

manifold setting ( 600-750-800) and then pulling the MAP back to use the

prop as a speed brake would put you at risk for over speeding the engine?

Now you have set up a situation where the prop is driving the engine not the

engine driving the prop. Correct me if I'm wrong.

The translated 1990 RU manual that I have says for approaches set the pitch

at 80% and 400 mmHG on the advance (MAP lever)for the 52. I generally fly

70% and 400 mmHg or less (MAP) in the 50 as needed. I know at times being at

say 70-80% and 750-800 mmHG chasing down someone (lead or an adversary) then

pulling the MAP lever to idle to avoid overshoot is like having a big speed

brake out there! The but is in the huge change in pitch of the engine. I

always wondered if I was overstressing the reduction gears or the shaft

itself. Honestly I would rather go idle boards (deploy the speed brakes)

than use my engine as the speed brake.

I know a couple of old 17 drivers along with a 47 driver and T-28

driver/maintenance officer at the airport. I will ask then their take today

at our fly-in. If the weather permits!

Doc



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brian-1927(at)lloyd.com
Guest

Posted: Sat May 03, 2008 8:34 am Post subject: L/D redux

On May 3, 2008, at 5:32 AM, Roger Kemp M.D. wrote:



 	  Quote: 	
		   

 >



 Jerry,

 I thought that by pushing the pitch to full forward on the prop at  

 cruise

 manifold setting ( 600-750-800) and then pulling the MAP back to use  

 the

 prop as a speed brake would put you at risk for over speeding the  

 engine?


	


Not if your prop governor is working properly.



 	  Quote: 	
		   Now you have set up a situation where the prop is driving the engine  

 not the

 engine driving the prop. Correct me if I'm wrong.


	


Yes, that is the case. But remember, when the RPM increases above the  

set-point, the governor will drive the prop toward course pitch (lower  

RPM). This reduces the angle-of-attack on the prop blades and they  

will produce less lift (turning force). It is also why you get a  

longer glide with the prop set to low RPM -- the prop is not taking as  

much energy from the airframe to turn the engine.



 	  Quote: 	
		   The translated 1990 RU manual that I have says for approaches set  

 the pitch

 at 80% and 400 mmHG on the advance (MAP lever)for the 52. I  

 generally fly

 70% and 400 mmHg or less (MAP) in the 50 as needed. I know at times  

 being at

 say 70-80% and 750-800 mmHG chasing down someone (lead or an  

 adversary) then

 pulling the MAP lever to idle to avoid overshoot is like having a  

 big speed

 brake out there! The but is in the huge change in pitch of the  

 engine. I

 always wondered if I was overstressing the reduction gears or the  

 shaft

 itself. Honestly I would rather go idle boards (deploy the speed  

 brakes)

 than use my engine as the speed brake.


	


Well, think about it Roger. Do you think that the prop can generate  

more torque turning the engine or the engine generate more torque  

turning the prop? I think you will find that the torque is at its peak  

when the engine is producing maximum power. That means that the  

overall stresses on the gearbox will be less when the power is pulled  

back.



 	  Quote: 	
		   I know a couple of old 17 drivers along with a 47 driver and T-28

 driver/maintenance officer at the airport. I will ask then their  

 take today

 at our fly-in. If the weather permits!


	


Now here is something you need to be careful about. You cannot assume  

that the M14 or Huosai should be operated the same way that the big  

Pratts and Wrights should be operated. As I understand it, the crank  

on the Pratts and Wrights have just one oil journal and it feeds oil  

to the contact point for the master rod bearing when the rod is  

turning the crank. When the prop is turning the crank then the crank  

is turning the master rod which changes the contact point. This can  

lead to under-lubrication of the master rod bearing. That is why they  

admonish you to never let the prop drive the engine in the Wrights and  

Pratts.



So that brings up the question of how the M14 and Huosai engines get  

oil to the master rod bearing. If there are oil journals to ensure  

proper lubrication of the master rod bearing when the prop is turning  

the engine then there is no reason to worry about doing damage to the  

engine when you pull the throttle back to idle. Given that the M14 is  

intended for aerobatic use, I suspect it *does* have proper  

lubrication when the prop is driving the engine, hence the lack of an  

warnings about the prop driving the engine in the manuals.



But this is only supposition on my part. Only someone who really knows  

the engine can tell for sure.

--

Brian Lloyd                         3191 Western Drive

brian HYPHEN 1927 AT lloyd DOT com    Cameron Park, CA 95682

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry



PGP key ID:          12095C52A32A1B6C

PGP key fingerprint: 3B1D BA11 4913 3254 B6E0  CC09 1209 5C52 A32A 1B6C

viperdoc(at)mindspring.co
Guest

Posted: Sat May 03, 2008 1:45 pm Post subject: L/D redux

Brian,

Thanks for enlightening me. I was not thinking about the direct drive of the

P&Ws verses the geared drive of the M-14. Don't quote me but I believe the

M-14 master crank rod bearing has its own oil journal. I ran across that as

I was researching the oil supply of the prop governor for an engine

vibration occurring right at Vref. 

It can be found in the diagrams of the M-14 engine manual.

Doc



--

wlannon(at)persona.ca
Guest

Posted: Sat May 03, 2008 6:58 pm Post subject: L/D redux

You all are getting me confused!  High RPM, low MP, overspeed??, contact 

points for the master rod bearing??, direct drive, geared drive.  Where to 

begin---------



At some point on approach you will reduce the power and, if you follow 

standard practice, will move the prop control to High RPM. If you have 

reduced power enough you will not see any RPM increase because the governor 

has sensed an underspeed condition and has already moved the blades to full 

fine.

The only reason for moving the prop control is to set up for a possible 

emergency go-around.

Therefore the prop is always in the highest drag condition on a normal 

landing.



There can be no "overspeed" since with increasing power the governor will 

limit RPM to the max. setting.  However, if you run significantly reduced 

power at the max. RPM for any length of time there is the possibility of 

damage due to the increased centrifugal loading of the piston and pin. That 

is the primary reason for avoiding that scenario.



There is no such thing as a "contact point" on the master rod bearing, 

unless it is totally worn out and nearing  failure.  That is the function of 

oil. It's main purpose is to ensure there is NO contact.



The master rod bearing (and the inner bearing end of each link rod) is 

pressure lubricated from the crank journal cavity on ALL radial engines. 

Whether geared or direct drive makes zero difference in this area.



It is possible that the M14 is more tolerant of centrifugal piston loading 

than P&W or Curtis Wright engines simply due to the small piston size.



Cheers;

Walt

---

mark.bitterlich(at)navy.m
Guest

Posted: Mon May 05, 2008 1:23 pm Post subject: L/D redux

Walt would you please explain what you mean by "centrifugal piston loading" ??  I am not a radial engine expert, but I would think the term would be the same, and for the same reasons, as regular V-8 racing engines, which I do have a lot of expereince with.  In the world I come from, we call it "piston speed", and of course it is related to the stroke and the RPM of the engine.  If we are on the same page of music here, the worry with the race engine I work with comes from the exact failure areas you speak of, which is why the emphasis on light weight pistons, and is also why you can easily build a 327 Chevy to turn 9000 RPM, but why there is much more load on a 454 doing the same thing... Longer stroke (and thus higher piston speed) and heavier pistons in the 454.  That said, whether the engine is loaded or unloaded in my training does not make any difference.  When the piston reaches the end of it's travel and has to reverse direction, there is tremendous forces applied all over the place, but piston pin, piston, and rod itself are the biggest areas of concern. 



To repeat and simplify, the "big force" involved is simply when the piston reaches the top and reverses direction. The more it weighs and the faster it is going when this happens means "more force".  



What I fail to understand is why this force would be larger or smaller when the engine is producing power, or not producing power?  



Could you please explain what you mean by:  "if you run significantly reduced power at the max. RPM for any length of time there is the possibility of damage due to the increased centrifugal loading of the piston and pin." 



What increased loading are you talking about assuming the engine is still running under maximum RPM limits?  Why is there more loading with power off, than power on?  



Mark Bitterlich

--

wlannon(at)persona.ca
Guest

Posted: Mon May 05, 2008 8:21 pm Post subject: L/D redux

Mark; See comments below



 	  Quote: 	
		   

 MALS-14 64E" <mark.bitterlich(at)navy.mil>



 Walt would you please explain what you mean by "centrifugal piston 

 loading" ??  I am not a radial engine expert, but I would think the term 

 would be the same, and for the same reasons, as regular V-8 racing 

 engines, which I do have a lot of expereince with.  In the world I come 

 from, we call it "piston speed", and of course it is related to the stroke 

 and the RPM of the engine.  If we are on the same page of music here, the 

 worry with the race engine I work with comes from the exact failure areas 

 you speak of, which is why the emphasis on light weight pistons, and is 

 also why you can easily build a 327 Chevy to turn 9000 RPM, but why there 

 is much more load on a 454 doing the same thing... Longer stroke (and thus 

 higher piston speed) and heavier pistons in the 454.  That said, whether 

 the engine is loaded or unloaded in my training does not make any 

 difference.  When the piston reaches the end of it's travel and has to 

 reverse direction, there is tremendous forces applied al!

 l over the place, but piston pin, piston, and rod itself are the biggest 

 areas of concern.



 To repeat and simplify, the "big force" involved is simply when the piston 

 reaches the top and reverses direction. The more it weighs and the faster 

 it is going when this happens means "more force".


	


To be honest I was desperately scratching for a descriptive word here for 

the forces you have just described. Maybe sudden deceleration would have 

been better.



I learned about this teaching myself aerobatics in a PT26 with a Ranger 

engine and of course a fixed pitch prop. (In the early 1950's there were no 

aerobatic instructers around, in fact aerobatic seemed to be a dirty word).

To get enough speed for an immellman or hammerhead it was necessary to keep 

reducing power in the dive in order to stay just under red line RPM. ( this 

was not the worlds best aerobatic mount).



Started pulling cylinders due to low compression and found all six pistons 

cracked from the pin bore. Turned out to be an early model engine that did 

not incorporate piston pin plugs. The pins were being deformed by the force 

reversal causing the pistons to fail.

 	  Quote: 	
		   What I fail to understand is why this force would be larger or smaller 

 when the engine is producing power, or not producing power?


	


I think the reason is manifold pressure or maybe more accurately brake mean 

effective pressure (BMEP). The piston is "cushioned" on the 

compression/power cycle by combustion forces and on the exhaust/intake cycle 

by exhaust expulsion and, just before the piston reaches TDC, the input of 

pressurized air/fuel mixture from the manifold.



With the larger radial engines an excess of manifold pressure (over square) 

is necessary to provide this cushion. As an example the P&W R1340 numbers 

are - T.O. (600 HP) 2250RPM & 36"MP, 5 min limit -  Rated Power (550HP) 2200 

RPM & 32.5" MP, no limit - Typical cruise setting - (55%- 300HP)  1800 RPM & 

26"MP.



Jim Goolsby sent a very informative post on this subject with an insert from 

Mr. R Sohn describing the effect of these forces on the master rod bearing 

if the "cushion" is not maintained.

I believe he is absolutely correct.

 	  Quote: 	
		   Could you please explain what you mean by:  "if you run significantly 

 reduced power at the max. RPM for any length of time there is the 

 possibility of damage due to the increased centrifugal loading of the 

 piston and pin."



 What increased loading are you talking about assuming the engine is still 

 running under maximum RPM limits?  Why is there more loading with power 

 off, than power on?


	


Covered above, I think.

The M14P is impressive. Even with the very small piston the load reversal at 

2900 RPM must be awesome. That it would stand up to handling the throttle 

like an ON- Off switch is mind boggling.



Aside for Pappy from a P&W devotee;

The reason the R1830 has a smaller master rod bearing is because it has TWO 

of them and much smaller pistons flailing around.



Cheers,

Walt



[quote] Mark Bitterlich

 --

mark.bitterlich(at)navy.m
Guest

Posted: Tue May 06, 2008 1:27 pm Post subject: L/D redux

Walt... 



I would be glad to take this discussion off line if you would like, because I do not want to bore people who are not interested.  



Speaking to the "cushioning" effect.... 



There is a term in camshaft design called overlap.  In cams designed with overlap, both intake and exhaust valves can be open at the same time.  Engines with higher degrees of overlap normally do not perform well at lower RPM settings.  Engines designed with such cams usually have a sharply rising HP and Torque curve that maximizes at a designed RPM setting.  Past that setting, power will actually diminish.  Such engines normally have a very "lopey" idle, very much like aircraft engines, which by and large do the same thing.  



That said, during the cycle of exhaust followed by intake, there is very little if any "cushion" at the top of the piston's stroke.  If there was, exhaust gases would actually flow back out the intake valve!  So, one cycle out of the four at least does not exhibit the effect you speak of, if I understand what you are saying correctly?  Does this make sense to you?  



Further, in a car engine, every time you downshift and let off the gas, you are getting the exact same effect as what we are discussing with aircraft engines.  



I understand completely the issue of main bearing lubrication, which by the way the M-14 differs quite a bit in design from American made radials, yet that said, I do not think I am qualified enough to discuss those loading areas with confidence.  



So staying with the piston speed // compression // loading issues...  I think I see now what you are saying, but can you explain why a V-8 would not suffer a quick death from such reverse loading as experienced when downshifting, or ... Going down long mountain grades in a low gear... Etc., etc., yet an aircraft engine doing the exact same thing is "bad for it".  



Understand that I am not saying: YOU ARE WRONG.  I am struggling to understand in my mind how it can apply to a piston in one engine and not apply to a piston in another.  



Mark Bitterlich

--

wlannon(at)persona.ca
Guest

Posted: Wed May 07, 2008 7:14 pm Post subject: L/D redux

 	  Quote: 	
		   Walt...



 I would be glad to take this discussion off line if you would like, 

 because I do not want to bore people who are not interested.



 Speaking to the "cushioning" effect....



 There is a term in camshaft design called overlap.  In cams designed with 

 overlap, both intake and exhaust valves can be open at the same time. 

 Engines with higher degrees of overlap normally do not perform well at 

 lower RPM settings.  Engines designed with such cams usually have a 

 sharply rising HP and Torque curve that maximizes at a designed RPM 

 setting.  Past that setting, power will actually diminish.  Such engines 

 normally have a very "lopey" idle, very much like aircraft engines, which 

 by and large do the same thing.


	


The M14P has 45 degs.(crankshaft rotation) of valve overlap. Intake opens at 

20 Deg. BTDC and exhaust closes at 25 deg. ATDC.

The R1340 has a total overlap of 57 degs.

Both numbers are at the nominal valve clearance setting.



 	  Quote: 	
		   That said, during the cycle of exhaust followed by intake, there is very 

 little if any "cushion" at the top of the piston's stroke.  If there was, 

 exhaust gases would actually flow back out the intake valve!  So, one 

 cycle out of the four at least does not exhibit the effect you speak of, 

 if I understand what you are saying correctly?  Does this make sense to 

 you?


	


No. I have never heard of an aircraft engine without valve overlap. Even the 

old C series Continentals had some.  The flow is the other direction even 

without supercharging.   Overlap clears the combustion chamber of exhaust 

gases since the exhaust stroke is essentially finished.



 	  Quote: 	
		   Further, in a car engine, every time you downshift and let off the gas, 

 you are getting the exact same effect as what we are discussing with 

 aircraft engines.



 I understand completely the issue of main bearing lubrication, which by 

 the way the M-14 differs quite a bit in design from American made radials, 

 yet that said, I do not think I am qualified enough to discuss those 

 loading areas with confidence.


	


There is some difference but it may be more related to the crankshaft joint 

design. Hard to be sure without seeing the actual components.



 	  Quote: 	
		   So staying with the piston speed // compression // loading issues...  I 

 think I see now what you are saying, but can you explain why a V-8 would 

 not suffer a quick death from such reverse loading as experienced when 

 downshifting, or ... Going down long mountain grades in a low gear... 

 Etc., etc., yet an aircraft engine doing the exact same thing is "bad for 

 it".


	


No idea. Know dick about car engines.



[quote] Understand that I am not saying: YOU ARE WRONG.  I am struggling to 

 understand in my mind how it can apply to a piston in one engine and not 

 apply to a piston in another.



 Mark Bitterlich

 --

mark.bitterlich(at)navy.m
Guest

Posted: Thu May 08, 2008 1:47 pm Post subject: L/D redux

Ok, well we've beat this one to death.  



Mark Bitterlich

 



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