Operating below the Green - slow cruise

[Doggtyred]

5pm your time?

 

[Ted]

You can not harm any engine by removing fuel or manifold pressure. all it will do is quit, when too much is removed.

Unless you manage to get into a zone of detonation (removing fuel).

You want to use as low of an RPM as possible - but there is a point where you go so far behind the engine's power curve that you actually shoot yourself in the foot.

Keep in mind that you also need to consider torsional vibrations and propeller stresses. Running at too low of an RPM and too high of a manifold pressure will cause significant stresses on the crankshaft and propeller for certain engines and certain propellers. This doesn't mean you need to operate at or below "square." There are no shortage of engines that are very happy operating with a higher MP than RPM. However, my Aztec as an example has manifold pressure restrictions at or below 2200 RPM. I don't know what the 182 has, check the POH.

 

[Tom-D]

Unless you manage to get into a zone of detonation (removing fuel).

You can not cause detonation by removing fuel, it is impossible.

Keep in mind that you also need to consider torsional vibrations and propeller stresses. Running at too low of an RPM and too high of a manifold pressure will cause significant stresses on the crankshaft and propeller for certain engines and certain propellers. This doesn't mean you need to operate at or below "square." There are no shortage of engines that are very happy operating with a higher MP than RPM. However, my Aztec as an example has manifold pressure restrictions at or below 2200 RPM. I don't know what the 182 has, check the POH.

The situation you are describing is a condition where the piston speed down the bore is not fast enough to relieve the pressure caused by too much fuel and MAP.

This is why we have restrictions on Over Square operations, but you can run at any RPM by reducing the power setting all the way down to Idle and not harm the engine.

That doesn't mean you are going to fly that way.

 

[skidoo]

Unless you manage to get into a zone of detonation (removing fuel).



Keep in mind that you also need to consider torsional vibrations and propeller stresses. Running at too low of an RPM and too high of a manifold pressure will cause significant stresses on the crankshaft and propeller for certain engines and certain propellers. This doesn't mean you need to operate at or below "square." There are no shortage of engines that are very happy operating with a higher MP than RPM. However, my Aztec as an example has manifold pressure restrictions at or below 2200 RPM. I don't know what the 182 has, check the POH.

My POH charts acceptable combination at 2000 rpm anywhere from 18" MP to 28" mp, so I don't know how this "square" comes into play on this one...

I haven't taken it out to make notes yet.

 

[Skylane81E]

My POH charts acceptable combination at 2000 rpm anywhere from 18" MP to 28" mp, so I don't know how this "square" comes into play on this one...

I haven't taken it out to make notes yet.

I know that I got a chuckle when a CFI asked me when I was going to square up to 25/25 for climb, my response was to tap the redline on the tach (2400)

 

[Henning]

For most NA planes the issue is a moot one anyway. As soon as you gain some altutude you loose enough MP that you are forced "undersquare" just to maintain speed. My typical cruise is wide open 20-22" and turning 2450. She sounds fat dumb and happy at that. That gets me 175-180 kts 10* LOP on 21 GPH. I could still turn up faster and richen up, but it nearly doubles my fuel flow for 15-20kts and is a lot tougher on the equipment.

 

[Let'sgoflying!]

Be sure to report back if you try it and anything unusual happens with the engine! Few have done this, so I think you are going to be providing some good data for us.

 

[skidoo]

I went out today and made some checks with some interesting results. First, I leveled out at about 6000 MSL and applied the autopilot to maintain altitude, then set for 2000 rpm and 18.0" mp which is the lowest level in the POH. I then leaned and it came in at 8.2 GPH. I waited a little while and observed that everything was stable at these settings, and I measured 105 KTAS, and 340 CHT. Outside temp was 64F. This compares to 103 KTAS and 8.2 GPH listed in the POH at standard temp of 3C. I was less than gross weight (solo), so I figured the numbers match close enough for this experiment.

Next, I reduced the MP to 16.0" and the GPH dropped to 7.2. Allowing some time to stabilize, I noted 95 KTAS and 345 CHT.

Then, I reduced the MP to 15.0" and the GPH dropped to 6.8. I allowed some time to stabilize, and I noticed it did not! First the airspeed gradually slowed, and then as it approached 65 KTAS or below, the ball on the rudder seemed to go one way and then the other while the autopilot started wallowing left and right. The slower it got, the more pronounced this wallowing. Eventually the stall warning came on, so I disconnected the autopilot and went back to 16.0". This time I allowed more time and it seemed to stabilize at 88 KTAS (78 KIAS). I think this is more accurate than my first notation at 95kts.

So, in my conclusion, I think I understand why the POH stops at 18" mp (43% max power) at this altitude. If I want to putt around at 15" or less, I will need to allow some altitude loss. But, I could maintain altitude in the 16" to 18" range, as long as I keep an eye on things often.

 

[jesse]

A common profile I fly in the 182 loaded up with four skydivers plus me:
10,500 FT
15" MP
Prop full forward
Result: 80 MPH indicated

I takeoff, prop full forward, cowl flaps open, climbing out at Vy. I'll then climb to 10,500 feet and pull the power back to around 15". That'll slow me to 80 MPH and I maintain this profile on the jump run. The jumpers climb out and sometimes you'll have them four of them hanging off the side of the airplane. I'll slowly dial the power up to full power to maintain altitude (which you can't). They'll eventually release from the airplane. I then dial the MP back to the bottom of the green, dial the prop back to the bottom of the green, close the cowl flaps, and go into a steep descent.

The only time the engine is operating below the green is when I reduce power abeam the numbers and go flaps 40. The power then comes back to idle and I land. Load up with more divers and repeat.

 

[Ted]

You can not cause detonation by removing fuel, it is impossible.

As I've told you before when you've made this claim, go try that on a Navajo at climb power and tell me what happens. Yes, by leaning the mixture on certain piston engines you do have a concern about getting into a detonation zone. Keep leaning you'll get out of it, the question is where, and whether the engine has good enough mixture distribution to run that lean.

My POH charts acceptable combination at 2000 rpm anywhere from 18" MP to 28" mp, so I don't know how this "square" comes into play on this one...

I haven't taken it out to make notes yet.

Point is that there's an OWT about engines needing to be "square" or below for safe operation, which is not true.

 

[Doggtyred]

[/B][/COLOR]

As I've told you before when you've made this claim, go try that on a Navajo at climb power and tell me what happens. Yes, by leaning the mixture on certain piston engines you do have a concern about getting into a detonation zone. Keep leaning you'll get out of it, the question is where, and whether the engine has good enough mixture distribution to run that lean.



Point is that there's an OWT about engines needing to be "square" or below for safe operation, which is not true.

I think he meant by closing/pulling back the throttle, not pulling the mixture. Even if that's not what he technically posted.