The International Cessna^® 170 Association

Hi Dave, 10 MPH won't matter on a good setup. On an ILS at about 90, I'm still showing around 4.5 hg. Russ Farris

I have another wuestion, with the ventury set up how long does it take to properly spool up the instruments?

I doubt taking off with a 300ft ceiling is going to be very smart, I'm wondering what a good set of minimums would be? Maybe you sit on the end of the runway for a few minutes at high rpm to get some airflow, I don't know.

Dave

There's generally not enough airflow to to really spool them up just sitting there and running the engine. When I do my mag check I usually see around 1" on the suction gage. The wear and tear on the engine wouldn't be worth the effort.

I find I'm usually in good shape by the time I reach pattern altitude, but then again, I'm not IFR.

Dave: The minumum ceiling you are willing to take-off in is a function of your experience, comfort level and how well your venturi system is set up.

No doubt about it, venturi systems are all over the map, with varied degrees of performance. They are less forgiving of old collapsed hoses,
dirty filters, and worn out gyros. Some guys report their gyros aren't up to speed until they reach cruise speed. Some don't seem to work well even then.

I went to great effort to maximize the performance of my set-up, since I fly IFR. I replaced all the hoses with aviation grade. I replaced the wallowing ADI with a new RC Allen unit. The DG was made by long-defunct Standard Precision - it has a 1969 overhaul date stamped on the case! Works flawlessly. I adjusted the regulator to read about 5.0 hg in cruise - I have one of those later model Cessna peanut gauges. My airplane made it all the way from 1952 to 1990 with only an electric turn and bank; the previous owner added the gyros and venturi.

If I set my DG at the end of my runup at 1,700 rpm it is up to speed enough to be lined up with the runway after turning 90 degrees. By 200 feet at 90 mph, they both seem to be working normally. My 500 foot personal limit is a fudge factor on gyro operation AND the fact I wouldn't take-off in ANY single lower than that.

Take the time to get it right and I'm sure you'll be very pleased...Russ Farris

Russ's words are all good, wise, and correct.
Let me add that modern "pictorial" (3-1/8") gyros require more vacuum (ideally 5.5") than the original "AN" series (which required only 3.5"). The original AN gyros had larger, more massive rotors which were easier to get spinning with low vacuum because the jets were directed at a larger outer circumference, and once spinning they were more inclined to keep spinning. (A large water wheel in a small stream will do a lot more work than a small water wheel in a river.)

Ok, that annual from he!! is finally done.

I test flew the airplane and the suction gauge is reading 7 inches in flight! I now am wondering if I need a regulator. I also wonder if too much suction is a bad thing for the instruments.

Dave

If you're getting seven inches of vacuum, you need a regulator or an adjustable bleed valve to set it at not more than the amount recommended for the type of instruments installed. If the vacuum is too high it will cause the gyros to overspeed their normal range and will cause excessive wear on the bearings and perhaps over sensitivity of the gyros.

If you do need to buy a regulator, I found some in the spruce catalog. The Airborne brand regulators (4 different models) list for $600 & up. A single Rapco is listed at $147, sez it's a direct replacement for Airborne 2H3-12 & is "FAA approved". Also four Tempest regulators which are FAA-PMA & list for a bit over $200.

Eric

dkalwishky wrote:Ok, that annual from he!! is finally done.

I test flew the airplane and the suction gauge is reading 7 inches in flight! I now am wondering if I need a regulator. I also wonder if too much suction is a bad thing for the instruments.

Dave

You might want have your suction gauge tested. I have found most aircraft vacuum gauges to be very inaccurate. just a litttle more HE!!
I only read the last page of this post so I do not know if you have already done this.

The gauge is brand new. I am looking into a regulator, in a shallow dive to descend from cruise alititude the gauge winds up to 8+ inches. Probably too much. I'd rather have too much suction than not enough.

Dave

While the excessive vacuum won't cause increased "sensitivity" it will cause increased wear and precession. (You may have to reset your DG more often.)

Thats to all who posted here. I am going to have some kind of regulator instaled. I was going to get one from Cheif aircraft but the shop seems to think they can do something pretty inexpensively.

Dave