Higher RPMs than ususal

[Frank DeBartolo]

Today the Outside Air Temp was 95 degrees F. The Density Altitude was 3,500 feet. During climb after climb configuration is established, the tach shows a couple hundred RPMs higher at full throttle than during normal or cooler conditions. I don't remember if this is a normal occurance or if there is something the matter with my tach. All feedback is appreciated.

Thanks

Frank D
N43684

[Pearson]

Re: Higher RPMs than ususal

Frank,

I don't think this is normal. With higher density altitude you should actually see LESS rpms. The question in my mind is, were you seeing higher indicated airspeeds, was the engine using more fuel per hour? If the answer to those questions is no, it is probably the tach. I would try to do a max rpm check when the temperature is cooler, to see if it still indicates high. I would next check the linkage to your carb, to see if the stop screw has come off, or backed out to allow higher rpms. If that isn't the problem, I would suspect the tach.

[lfries]

Re: Higher RPMs than ususal

Can you borrow an electronic tac that counts the prop revolutions. THOUGHT i HAD THE WORLDS FASTEST 150 ONCE BUT AFTER BORROWING THE TACH FROM THE LOCAL A&P DISCOVERED THE RPM was about 400 HIGhER THAN REPORTED

[M Towsley]

Re: Higher RPMs than ususal

I use a tach from my r/c modeling days. Works great.

[Hank Jarrett]

Re: Higher RPMs than ususal

VibraTach or Heath optical? ;-)
Hank

[M Towsley]

Re: Higher RPMs than ususal

I've got one or two Heath opticals around, (I remember now I was going to dig that out and take a picture of it for you). The one I use is a more modern Hobbico tach.

[Frank DeBartolo]

Re: Higher RPMs than ususal

Thanks for all the responses. I flew it again this evening and am 80% sure it is the tach which is only approximately 6 years old. I have a friend that has a device that counts the rpms. I will let you know what we find out. It will be a few days before we can conduct this test. One other thing. At engine start the engine idles at approx 500 RPMs and take off rpms are normal. On entering climb rpms are normal untill they start to come up to over red line. on engine shut down, I cannot get rpms down below 700. After the refuel and restart after the plane sits on the ground for 10-15 minutes the idle rpms are 500.

Again--thanks and I will let you know what I find out.

Frank D

[resojames]

Re: Higher RPMs than ususal

Thanks for all the responses. I flew it again this evening and am 80% sure it is the tach which is only approximately 6 years old. I have a friend that has a device that counts the rpms. I will let you know what we find out. It will be a few days before we can conduct this test. One other thing. At engine start the engine idles at approx 500 RPMs and take off rpms are normal. On entering climb rpms are normal untill they start to come up to over red line. on engine shut down, I cannot get rpms down below 700. After the refuel and restart after the plane sits on the ground for 10-15 minutes the idle rpms are 500.

Again--thanks and I will let you know what I find out.

Frank D

There is a quick and dirty to tell if you tach is accurate. Take the plane out on the ramp after dark, crank the engine and point the nose at a security light or any AC light. This is a poor mans strobe light. At multiples of 60, you will see the prop look like it is stopped. You won't see a perfectly stopped prop unless you can get a static of 1800rpm (2 blades X1800 = 3600 blades / min) which = 3600 electric cycles / min. It will be obvious though when you match the engine speed to a harmonic of 60HRZ. 900 rpm will be your best shot on a C-65

[M Towsley]

Re: Higher RPMs than ususal

Maybe one of the old A&P's has one of these: http://vb.taylorcraft.org/showthread...highlight=chek

[M Jones]

Re: Higher RPMs than ususal

Hi Frank,

I've haven't tried this one, but a friend sent me the link below some time ago. You could download and install the application to a laptop and give it a try. I don't know if the newer Windows OS supports this, it has been around a while.

RPM from sound

[Hank Jarrett]

Re: Higher RPMs than ususal

If you do the light trick for 60 cycles it works better looking into a fluorescent light
Hank

[Frank DeBartolo]

Re: Higher RPMs than ususal

Thanks to all for the replies.

A friend has a device that measures the RPMs and we met last night. It looks like my tach is approximately 100 RPMs higher than actual at low RPMs and approximately 200 RPM higher than actual at the higher RPMs. We went up in his Tri-Pacer afterwards and his tach is within 30 rpms. Since my annual is due this month, My IA is removing the tach and sending it in for calibration. So the annual will be done this month so I can make it to the Antique fly-in at Blakesburg. I wondered why my trim was more nose up than normal and since red line bothers me, why I was having to throttle back in climb. My question is this: What causes the tachs to go out of calibration? Probably time and use I imagine.

Thanks much for all the responses.

Frank D
N43684

[NY86]

Re: Higher RPMs than ususal

A tach is a magnet pulling against a spring. If the spring or the magnet gets weaker, the calibration will go off. Also, wear and friction can affect it.