Hi all, I am trying to work out the intricacies of flying with a variable pitch propeller, and found this youtube video very helpful. There might be fellow simmers who are, just like me, unsure of how to properly operate the infamous blue prop lever, and how an airplane with a constant speed propeller works. Try this link, I hope you find it as useful as I did. 

 

Edited November 15, 20205 yr by charliearon
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Thanks!   I always thought it was the cigarette lighter! 😄

Seriously though, good video. 👍

"Square rule" aka rpm is never higher than manifold pressure is up for debate. While we are all taught to do it, it appeared a lot of aircraft manufacturers never cared to put this rule in POH/AFM. Although  I believe "square rule" is valid for some older type of airplane we never used it in Bonanza BE35B  for over 7 years and still doing fine 

41 minutes ago, sd_flyer said:

"Square rule" aka rpm is never higher than manifold pressure is up for debate. While we are all taught to do it, it appeared a lot of aircraft manufacturers never cared to put this rule in POH/AFM. Although  I believe "square rule" is valid for some older type of airplane we never used it in Bonanza BE35B  for over 7 years and still doing fine 

It is a rule of thumb, probably based around Lycomings in GA planes.. The fact it worked at all is coincidental.

If we had measured manifold pressure in PSI or barr or hPa/kPa or even mm of mercury instead of inches of mercury it would never have worked at all.  Similarly if traditionally engine speeds were quoted in revolutions per second it would not have worked either.

It never worked with supercharged radials anyway. Typically you are talking 30 odd inches of manifold pressure at maybe 2200 rpm on one of those old radials. If you check this old DC3 handbook below the Pratt and Whiney recommended cruise was 25.25" at 2050 rpm ... takeoff was 48" at 2700 rpm !!

http://pcmuseum.tripod.com/dc3/dc3operatinglimitations.pdf

Edited November 14, 20205 yr by Glenn Fitzpatrick

13 minutes ago, sd_flyer said:

"Square rule" aka rpm is never higher than manifold pressure is up for debate. While we are all taught to do it, it appeared a lot of aircraft manufacturers never cared to put this rule in POH/AFM. Although  I believe "square rule" is valid for some older type of airplane we never used it in Bonanza BE35B  for over 7 years and still doing fine 

He discusses this briefly in the video, worth checking it out. ( Btw if you mentioned things like manifold pressure to me yesterday i would have gone all slack-jawed and “durrr” 😀)

1 hour ago, Glenn Fitzpatrick said:

It is a rule of thumb, probably based around Lycomings in GA planes.. The fact it worked at all is coincidental.

If we had measured manifold pressure in PSI or barr or hPa/kPa or even mm of mercury instead of inches of mercury it would never have worked at all.  Similarly if traditionally engine speeds were quoted in revolutions per second it would not have worked either.

It never worked with supercharged radials anyway. Typically you are talking 30 odd inches of manifold pressure at maybe 2200 rpm on one of those old radials. If you check this old DC3 handbook below the Pratt and Whiney recommended cruise was 25.25" at 2050 rpm ... takeoff was 48" at 2700 rpm !!

http://pcmuseum.tripod.com/dc3/dc3operatinglimitations.pdf

I asked this question to FAA examiner about “square rule” he quoted older supercharged engines where MP exceeds certain RPM may blow cylinder and throw the rod. But all modern normally aspirated engines  don’t suffer from it. However, you see many CFI teach this  as they been taught,  and  so tradition  continue . I’ve been taught this too by the way 🙂

6 hours ago, sd_flyer said:

I asked this question to FAA examiner about “square rule” he quoted older supercharged engines where MP exceeds certain RPM may blow cylinder and throw the rod. But all modern normally aspirated engines  don’t suffer from it. However, you see many CFI teach this  as they been taught,  and  so tradition  continue . I’ve been taught this too by the way 🙂

Yeah, it was just a rule of thumb for your typical aviation engine of the time, that somehow became taught as a fixed limit.

Not all engines are "typical" even back then. Hence the stock supercharged V12s used in the  P51D ran a max of around 60" manifold at 3000 rpm according to documentation from the day and they were not blowing apart after a few minutes.

http://www.wwiiaircraftperformance.org/mustang/mustangtest.html

Now yes, a Reno racer might blow apart if ran too long at max boost. The P51D Voodoo in it's record breaking 531 mph run in 2017 suffered engine problems on the fourth run of the day. But  Voodoo was up to 120 inches manifold at 3400 during the record attempt. That is not even close to "square" but we are talking a world record holder that pushes the limits of what can be done here.

Worth a read:

https://vintageaviationecho.com/voodoo-speed-record/

 

Edited November 15, 20205 yr by Glenn Fitzpatrick

35 minutes ago, Glenn Fitzpatrick said:

Yeah, it was just a rule of thumb for your typical aviation engine of the time, that somehow became taught as a fixed limit.

Not all engines are "typical" even back then. Hence the stock supercharged V12s used in the  P51D ran a max of around 60" manifold at 3000 rpm according to documentation from the day and they were not blowing apart after a few minutes.

http://www.wwiiaircraftperformance.org/mustang/mustangtest.html

Now yes, a Reno racer might blow apart if ran too long at max boost. The P51D Voodoo in it's record breaking 531 mph run in 2017 suffered engine problems on the fourth run of the day. But  Voodoo was up to 120 inches manifold at 3400 during the record attempt. That is not even close to "square" but we are talking a world record holder that pushes the limits of what can be done here.

Worth a read:

https://vintageaviationecho.com/voodoo-speed-record/

 

I can’t find my p-51 manual at the moment  but here is P-47D. The only thing you care “running away “ super charger . Everything full max and full on

 

2 hours ago, sd_flyer said:

I can’t find my p-51 manual at the moment  but here is P-47D. The only thing you care “running away “ super charger . Everything full max and full on

 

 

It actually says manifold pressure should not exceed 52 inches Hg in section 3(d) at the bottom of your page. Note that 52 inches at 2700 is definitely not square  😄

 

Seems like the P47D was tested at up to 70  inches Hg at 2700 rpm with water injection though they recommended 65 for the R-2800-63 with 70 requiring more testing . Temps were an issue in climb as you would expect.

The P47D in level flight was 32 MPH faster at 70 inches versus 52 inches. It also climbed at 1000' per minute faster at 65 inches (climbs at 70 were not tested)

 

http://www.wwiiaircraftperformance.org/p-47/p47-26167.html#:~:text=VII Conclusions-,A.,temperatures and carburetor air temperatures.

Quote

Preliminary tests were run to clear the airplane for performance with higher powers with and without water injection. Detonation equipment was installed to determine if any flight condition became marginal as to detonation, cooling or improper operation of auxiliary parts. No detonation was observed in level flight up to 65.0" Hg. without water and 70.0" with water. No detonation was observed in climb up to 65" Hg. without water. Detonation occurred at 65.0" with water in climb but was remedied by using a No. 18 water jet. Cylinder head and carburetor air temperatures remained below the limits in level flight. Excessive cylinder head and carburetor air temperatures were encountered in climbs, limiting the duration of any climb to a point where limits are reached.

       The airplane and engine handled well at all altitudes at the higher powers. At 70.0" Hg., water injection, a maximum speed of 444 MPH was obtained at 23,200 feet. At 65.0" Hg., with water a high speed of 439 MPH at 25,200 feet and a maximum rate of climb of 3260 ft/min. at 10,000 feet were obtained. At 65.0" Hg., without water a high speed of 430 MPH at 25,400 feet and a maximum rate of climb of 2850 ft/min. at 12,000 feet were obtained. At 56.0" Hg. without water a high speed of 418 MPH at 29,600 feet and a maximum rate of climb of 2330 ft/min. at 12,000 feet were obtained. At 52.0" Hg. without water a high speed of 412 MPH at 31,400 feet and a maximum rate of climb of 2030 ft/min. at 12,000 feet were obtained.

 

Quote

VII   Conclusions        A.  The R-2800-63 can be operated at 65.0" Hg., 2700 RPM, in level flight and climb without water injection when using 44-1 fuel. It can be operated at 70.0" Hg., 2700 RPM with water injection with 44-1 fuel. Climbs at high power must be limited because of high cylinder head temperatures and carburetor air temperatures. Short climbs can be made without difficulty.        B.  A gain of 19 MPH can be realized by using 65.0" Hg., 2700 RPM over 56" Hg., 2700 RPM. 8 MPH can be gained at 65.0" Hg. by using water injection. With water injection at 70.0" Hg., 2700 RPM, 7 MPH can be gained over 65.0" Hg., 2700, water injection.        C.  In climb operation a gain of 510 ft/min. by using 65.0" Hg., 2700 RPM over 56.0" Hg., 2700 RPM can be realized. 410 ft/min can be gained at 65.0" Hg., 2700 RPM using wate injection. No 70.0" Hg. climbs were made. VIII  Recommendations        1.  It is recommended the Pratt & Whitney R-2800-63 engines be rated at 65.0" Hg. with and without water injection when using 44-1 fuel or its equivalent.        2.  It is recommended that the use of 70.0" Hg. be further investigated.        3.   It is recommended that pilots using these higher powers be cautioned concerning the high cylinder head temperatures and carburetor air temperatures which may be encountered in extended climbs or level flight.

 

Edited November 15, 20205 yr by Glenn Fitzpatrick

1 hour ago, Glenn Fitzpatrick said:

 

It actually says manifold pressure should not exceed 52 inches Hg in section 3(d) at the bottom of your page. Note that 52 inches at 2700 is definitely not square  😄

 

Seems like the P47D was tested at up to 70  inches Hg at 2700 rpm with water injection though they recommended 65 for the R-2800-63 with 70 requiring more testing . Temps were an issue in climb as you would expect.

The P47D in level flight was 32 MPH faster at 70 inches versus 52 inches. It also climbed at 1000' per minute faster at 65 inches (climbs at 70 were not tested)

 

http://www.wwiiaircraftperformance.org/p-47/p47-26167.html#:~:text=VII Conclusions-,A.,temperatures and carburetor air temperatures.

 

 

That is military power you are referring and war emergency power which had nothing to do with “square rule” in my book 

Also notice reasonable  logic all controls forward to the stop beyond which is military power which is time limited. Military or war emergency power is outside “square rule” because it’s not continuous maximum power 

24 minutes ago, sd_flyer said:

That is military power you are referring and war emergency power which had nothing to do with “square rule” in my book 

So Thunderbolt pilots back in the day applied the square rule unless in actual combat?  Possible I suppose, though I find it unlikely the P47 never flew with more than 27" of manifold pressure unless being shot at - which is what the square rule would specify.

Not to matter it is beside the point, even the DC3 continuous figures above exceed the square rule and note it is a civilian DC3 not a Dakota so no WEP here. I was just trying to show that even back then the square rule did not apply to every engine. The square rule seems to be a general rule of thumb used on a batch of old (probably normally aspirated) engines and some how that "rule of thumb"  has become received wisdom that people try to apply to everything with a piston.

Come forward 50 years and even a modern supercharged Lycoming does not fit the square rule:

https://www.lycoming.com/sites/default/files/Horsepower%2C Manifold Pressure%2C and RPM Values.pdf

Potentially you could use the square rule with something like a normally aspirated 912 Rotax as if I recall correctly the numbers are in the right ballpark, but you are still better off sticking with the figures in the POH .

 

Edited November 15, 20205 yr by Glenn Fitzpatrick