Low Static RPM on 1978 C-172

Re: Low Static RPM on 1978 C-172

The best indication of whether or not you actually have a problem is performance. I suggest you ignore the RPM and concentrate on rate of climb and TAS. Check what you get against the book, compare fuel consumption etc., If RPM is the only indicator that seems excentric, it's a tach issue. If your actually not getting the performance the book predicts, send the prop out. My concern would be that an IA returned this thing to service and it wasn't turning up to spec. Whats up with that?

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Re: Low Static RPM on 1978 C-172

Dear Sir/Madam

We Sindel Pilot School have 1 1978 model C-421C  (S/N:0513), 1 1982 model C-152II (S/N:152-85300), 2 1978 and 1979 model C-172N (S/N:172-72170 and 172-69863), 1 1982 model C-172P (S/N:172-75167) and 2 1977 model R172K (S/N:R172-2577 and R172-2708)  aircraft.

The pilots are complaining about “Low Power” problems on one  of those 1979 model Cessna 172N aircraft (S/N:172-72170) with O-320-H2AD engine on it  A/C flight hrs:TSN 3759,00 Engine flight hrs.:TSN:3392,00 and TSO:340.00

We have the following information for your attention.

1.    Magneto timing checked
2.    Cylinder differantial compressions tested
3.    Replaced carburator
4.    replaced propeller
5.    Engine runs smoothly
6.    Static RPM range at full throttle is 2150 RPM  (at 3125,00 feet)
7.    Cruise flight 2300 RPM
8.    Climb 2200 RPM

Q: What should be the minimum RPM on ground, climb and cruise ?

Q. Would you consider that this aircraft has low power ?

Q What else should we do to bring power high ?

Q. Do you have “Test Flight Form” of those aircraft listed above with their S/N’s that show the minimum and maximum values of RPM, PRES, TEMP. Etc. And is there any possibility of getting these forms.


Best, Regards,

Atalay Agdasan
Tech.Manager
Sindel Air Pilot School

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Re: Low Static RPM on 1978 C-172

Hello Atalay;

Please see my post from 6/25/2006. According to the POH and the Type Certificate Data Sheet for the 172N, the Static RPM should be between 2280 and 2400, with maximum cruise RPM of 2700 (Red Line). The Type Certificate Data Sheet is available on the FAA web site.  The symptoms you described are what I experienced, which turned out to be the camshaft to crankshaft timing problem.  The camshaft to crankshaft timing problem can be determined using the following procedure from the Sacramento Sky Ranch web site:

Crankshaft to camshaft timing off. Note: It is easy to assemble the Lycoming O-320H2AD with the internal engine timing 1 tooth off.

This condition may be checked by first disconnecting starter. Remove top spark plugs and rocker box cover on #2 cylinder. Turn engine to TDC on compression stroke #1 cylinder. Engine timing is checked by first observing the number two cylinder valve rocker arms. Both valves should be closed or nearly closed. Move the propeller slightly in one direction. Rocker arm motion should be seen as one valve starts to open. STOP. Now rotate the engine back to the original position with the #1 at TDC.. Now move the propeller slightly in the direction opposite from the first movement. Rocker arm motion should again be seen as the other valve starts to open. If the two valves started to open as described with only a small amount of engine movement in each direction, the engine timing is correct. If movement in either direction exceeds twenty degrees of engine rotation before motion of the rocker arm occurs, the crankshaft to camshaft timing is not correct and the engine was assembled incorrectly.

Hope this information is helpful.

Jon

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Re: Low Static RPM on 1978 C-172

Hello Jon,

Thank you for your support.I asked the engine overhaul center that overhauled our engine to check the procedure with me.They agreed that crankshaft to camshaft timing is off. They decided to open the engine and correct the timing at no cost.

The information you sent me was very helpful.
Thank you so much.

Atalay

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Re: Low Static RPM on 1978 C-172

Let me tell you another way that your static rpm can be low. My 67 172 was giving me "doggy" performance. Since I had moved down from a 182, I did not give the 172 performance much thought. It just is a lot less than the 182. Anyway, I had a shop fly it for their opinion, and they said it was definately underperforming. I thought it good that I was using only 6 gallons per hour at 2400 rpm. To make a long story short, all of the accessories were removed from the back of the engine. After taking off the cover on the back of the engine, it was discovered that the engine timing gears were 1 tooth off. When put back into the proper setting, the engine has performed so much better. It amazes me how this got past the engine builder, whomever that may have been. So, for 1100+hrs. this engine has been running a bit retarded.
Also, this correction is a bit costly!!!
Glenn

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Re: Low Static RPM on 1978 C-172

We have the same problem with our 1978 172N.  WE just bought the plane about 2 months ago with 1300 hrs SMOH.  On climb-out we get 2150 - 2200 and can only get 2350 to 2400 on a good day at cruise.  We had assumed it has a cruise prop because the IAS is 110 - 115 kts (which is pretty good) at 7.3 gal/hr..  Engine runs good and strong so, guess we'll continue until overhaul.  Do you think there is any problem in expecting this engine to reach expected TBO?

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Re: Low Static RPM on 1978 C-172

Recently , on an aviation forum there was a rather lengthy thread on low static rpm on the H2AD engine. As I recall from the thread a mistimed cam shaft will cause the problem you are experiencing. I wouldn't expect a prop to cause a low static rpm unless it is improperly pitched. Luckily the cam shaft cam be retimed without an engine tear down. Removing the accessory case will allow access to the timing gears. you might want to check Sacramento Sky Ranch's web site for info on checking the cam timing.Good Luck

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Re: Low Static RPM on 1978 C-172

H2AD engines have an intregal accessory case, (part of the maincase) and require the case to be split to change the camshaft to crankshaft timing.

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Re: Low Static RPM on 1978 C-172

Hi All;

I started this thread two years ago, and it took several months for me to identify the timing problem as the cause.  As noted above, the H2AD does not have an accessory case.  However, One Stop Aviation, the shop that did the overhaul, was able to reset the timing without splitting the case (and at no cost to me).  They removed the oil sump, the oil pump housing, and the magneto and were able to remove the bolts from the gear, rotate the engine and reattach the gear (I don't know if it was the camshaft or crankshaft gear).  Rick at One Stop in Oceanside, CA knows how to do this, but I don't know if he will share it with your mechanic.  If you are willing to fly your airplane to Ocenaside, he might fix it there for you.  My engine has been operating normally since he reset the timing (normal static runup and full power cruise up to 2700 RPM).

As far as how long the engine will run with the problem, most of the opinions I got was that this arrangement actually results in a smoother running engine and less stress on the parts.  My experience was that the engine ran smoother and cooler before I fixed the problem.  However, there is a noticeable improvement in performance with the engine timing set properly, especially when climbing above 5,000 ft and climbs on warm days.  My mechanic also believes that if the airplane won't perform to the FAA Type Certificate limits (static RPM of 2280-2400), then the airplane is not airworthy and should not be flown.

Jon Plaxton

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Re: Low Static RPM on 1978 C-172

Hello, Jonathon. I have a '65 172 that shows a lower (still acceptable) static rpm. The further into the takeoff roll the more rpm's I get and when I lift off, I'm getting max power. Climbout is in the 400 fpm (Hot day) and better range. I can't account for it, other than I checked the tach and it shows about 50 rpm low. The engine has all new cylinders and runs very good. It hasn't caused me any problems and I'm not going to throw money at it. I think it's fairly typical for static to be lower than max power, based on the Skyhawks I've flown. I know theoretically static you should get max power, but I haven't seen it. I'm sure many will disagree. Good luck,
Mike

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Re: Low Static RPM on 1978 C-172

Jonathon Plaxton Wrote:
-------------------------------------------------------
> Thanks, Barry;
>
> I went to the web site and read John Deakin's
> articles, which were very informative.  I'll post
> my results after the prop overhaul.
>
> Jon

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