I would be shocked to see my air filter drop the MP by 2-3 inches. If I open the alternate air, I see a rise in MP of about 0.3 inch. A 2-3 inch MP loss would translate to a massive loss of high altitude capability, equivalent to 2000-3000ft in the operating ceiling, and there would be big pressure (no pun intended) to do something about it.
Some years ago a group of TB owners in the USA organised an aftermarket filter manufacturer (K&N) to get an STC for their filter for the TB aircraft. The promise was that the filter was easy to clean (the current foam filter has to be thrown away, but costs only about €15) and that it dropped less pressure (but the foam filter drops only about 0.3 inch on the TB20). Anyway, I bought one and immediately discovered that it didn’t fit because the STC holder did not read the TB IPC properly and didn’t realise the air intake changed in later aircraft. I eventually managed to get my money back, after about a year… Maybe it works better on a TB21 on which you have the full MP much higher up, but I can’t see why it should.
I would be shocked to see my air filter drop the MP by 2-3 inches.
Filter&induction system together. Ambient pressure at sea level under ISA conditions should be around 30inHg. Full throttle, on the typical aspirated Cessnas and Pipers I expect to see 27-28inHg indicated.
I expect a loss of an inch per 1000 foot of altitude from sea level. For my Bonanza, there is about an inch and a half loss in the induction system over ambient, so I get about 28.5 inches at sea level and reduce from there.
On my 30 SMOH IO360 with full throttle at sea level I only see 97% power. I have gone to great lengths to re-design the air intake but it didn’t make the slightest difference!
Either I have a calibration issue or inlet duct losses are indeed significant.
The “don’t operate over square” thing is an old wives’ tale.
http://www.avweb.com/news/airman/184483-1.html?redirected=1
The “don’t operate over square” thing is an old wives’ tale.
Of course it is nonsense. There is no physical connection between MP (expressed in inches) and RPM. They are different units. It’s like saying a car should not be driven at a speed which when expressed in MPH is greater than the length of the car in inches.
This stuff comes, I believe, from the FTO (ATPL) training environment. I don’t think it comes from the PPL system because they rarely operate variable pitch propellers.
I think it comes from the days of large radial engines. I hear that it could do some damage to them.
Radial engines are often supercharged to relatively high MP and have large cylinders, which are both potential causes for detonation problems if operated improperly. A 150 HP Lycoming is for sure a different situation and the operating manual allows sea level full throttle MP at anything above some modest rpm like 2300 (I forget the exact rpm). A 180 HP Lycoming or the six cylinder version of same needs a little more rpm for full sea level MP due to slightly larger cylinders, but it’s still not greatly constraining per the manual.
I’ll remember the 1 inch per thousand feet rule of thumb, I’d not come across that before. I have a few power settings memorized, with higher rpm for the same power with increasing altitude, but mostly I don’t shoot for exactly 65% or whatever – the engine doesn’t care about such things.
The “don’t operate over square” thing is an old wives’ tale.
Of course it is. But if you do not know better (read: have not read the manual) you will probably not hurt anything by abiding it.
Of course it is. But if you do not know better (read: have not read the manual) you will probably not hurt anything by abiding it.
You can by exceeding CHT. For TB20 WOT in combination with full-rich ensures enough FF to keep CHT within limits. Closing throttle (lowering MP) in this situation is not recommended.
