I have heard there is a way to test for leaks in a 421 or other pressurized Twin Cessna using the pressure side of a shop vacuum but I have not seen it explained. Do any of you know the steps involved to do that?



Thanks

Rocky

I have not tried this - - but I suspect that a shop vac is only going to be able to create pressure on the order of a few tenths of one PSI.

That MAY be enough to hunt for leaks - - but I doubt it.

It can be done by pumping air in the stub wing hose near the heat exchanger. I'd prefer doing it from inside the engine compartment. I agree with George, it will be hard to get any differential. This especially depends on your serial number and the type of door seal you have. Mine, for example, is sealed with cabin differential which will never happen with a shop vac.

Better solution.....take a passenger and a box of Kleenex. In flight, have the passenger move about the vessel holding the Kleenex against the cabin walls. Obvious areas are windows, doors, etc.

Also, make sure you check the plumbing for the heater under the floor of the nose. Inside the plumbing is cabin air and outside is ambient. If your hoses are old and crusty, they will leak like crazy right through the hose. You can close the heater valves and pump up these hoses from inside the nose while spraying soapy water on the hoses and fittings. This is where my leaks were.

Best,

Contact Peter Danto he knows how to do this. 1-925-788-9010

As a one time Flow Bench designer and manufacturer(used mainly for Cyl heads) which on a NA engine sucks on In and blows on Eh I can give you a Kentucky windage answer

The so called "standard" for testing was for years 28 inches of water which is very close to one psi.
To turn a blower capable of doing this required about a 7 1/2 HP 220v motor.
Frank

i have tried it, and a huge shop compressor. Neither will supply the volume of air to do the correct testing.. Get a huffer on the plane. The in-flight tissues and the shop vac will find the large obvious leaks...but just barely. Only a huffer can find the wing root leaks , fore/aft bulkheads leaks, and overhead antennae and belly leaks. It saves money and time to head to a shop that can blow the plane to full diff...in a cage, if necessary (as on reworked doors)

Fred is absolutely right, find a shop that has a huffer cart (pressurization test cart) and go there and hunt for leaks. In addition to everything already mentioned, the other techniques don't really get you to the problem while you are standing on the ground watching/listening and/or with bubble soap solution or tool in hand to really isolate the issue. Do it right is the way to go, get the tool for the job (i.e. a huffer cart).

Thank you all very much. I guess the vacuum hose deal was one of those Cessna myths.



Rocky

No myth Rocky...many have tried it. It just doesn't mean it works very good.

Probably would do better going to the party store and see if you could rent one of the blowers used for kids bounce houses. Not as good as as a huffer, but better than a shop vac.

Rocky

My mechanic recently fixed a leak on the 421 and the 414 I owned prior to that this year.
He did use a shop vac to to test the heater hoses, mine was recently replaced but there was still a leak on the one hose due to improper instal.
The shop vac gave about .4 psi but that was enough to detect leaks in the heater hoses but won't be much help for anything else, i.e. cabin leak.
He checked the door seal, emergency exit seal etc with an ultrasonic leak detector, that he bought of Ebay for I believe less than 1 AMU

The end cause of the pressurization leak in both planes ended being in exactly the same area: the "inflow" hose from the right engine in the wing root. This can be easily checked in the air by alternatively dumping the right and then the left engine's pressurization. Look at the cabin psi while doing this and you will likely see a much bigger rise/climb when the functioning engine/side is dumped.( or you will get i.e. 4psi on the one side and 3 psi on the leaking engine, theoretically they should maintain full pressurization on one side only, but I have heard most Cessna twins will only give about 3.5-4 psi on one engine)

Hilgard

Hi Rocky,

Look up the postings by your namesake about Dukes.

Rocky started with the shop vac but has since switched to electric leaf blowers - one on each inlet host to the intercoolers. I will be trying his method this year on our Duke.

Interesting the one post suggested a 7.5HP motor well each of the electric leak blowers is about 3 to 3.5HP on 110volts - tight in line.

Ken
Dallas,Tx

Actually I asked that question two years ago. I finally took it to a shop that had a pressure machine made for finding leaks. They found it in about 10 minutes. It was a boot in the front landing gear that was broken open. I never did figure out how to get enough air pressure on the ground to find the leak. the whole think cost me about 1,200 bucks which in the whole scheme of costs is not that much and now I get full 5.5 pressure. Corporate Ait Technology CAT in San Jose did the work.

At 120V, a 1HP motor will draw between 8 and 10 amps depending on the efficiency of the motor. About the most you could run on a standard 20A 120V circuit is about 1.5 HP. A lot of equipment with electric motors will often advertise "3 HP (or higher)" motor, but this is the max for the motor if it was wired at another voltage. The only way to tell the actual HP of the motor would be to measure the Voltage x Amperage x efficiency / 742. Some industrial motors can get efficiencies as high as 98%, but most smaller 120V motors are between 60 and 80%.

Vince

That would almost be tolerable, but the real reason is specmanship.

Consider this:

https://www.shopvac.com/wet-dry-vacs/va ... ?vacId=427

Shop Vac that claims "6-Peak-HP".

In the specs, listed as 11.8 A, 120 V, which is 1.9 HP electrical input. 1.5 HP would be the highest actual power output you could generate from the motor.

The web site explains:

Horsepower Rating
Peak horsepower is the maximum output horsepower of a motor determined from a laboratory dynamometer test. Since peak horsepower is outside the normal operating range of a vacuum cleaner, it may not be indicative of actual air power differences when comparing two cleaners.

So, basically, it is the maximum power they can get from the motor by severely overloading it for a few seconds before it blows up.

In any case, there is no 6 HP the user can get from this machine.

They also list "Peak Air Watts" at 275, or 0.37 HP. I'd assume this means the true actual power of the airflow at the best power operating point (pressure x flow maximized) but they don't explain that clearly. Assuming this is true, then this particular model produces 0.37 HP of air power for 1.9 HP of electrical power. Hmm.

Mike C.