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[Yellow Snow]

The "oversquare stuff" most certainly did not apply to radial engines, most all of which were turbocharged, supercharged, or turbocompound boosted. Most large radial engines are operated with manifold pressures well in excess of RPM values. There is NO correlation between manifold pressure and RPM.



A myth, perpetuated by inexperienced flight instructors who don't know better.

Particularly in a normally aspirated engine, just how are you going to overboost it?? You can never develop more than barometric pressure...if you're operating above sea level, you're always developing less pressure in those cylinders than the engine was designed to handle.

The highest pressure you can develop with the engine running at full throttle is whatever you would develop with the engine shut off, no matter what position of the throttle. Think about that.

Av Bug that is the most ignorant thing you could possibly say about an internal combustion engine. You are confusing manifold pressure with cylinder pressures. Yes manifold pressure is lower than atmospheric pressure because in a normally aspirated engine there is suction pulling the intake charge into the cylinder. However when the valves close and the cylinder starts its compression stroke the cylinder pressure is most definately MUCH higher than atmospheric pressure. If you open the throttle all the way and pack the biggest charge possible into the cylinder while restricting the rpm of the engine with the prop governor you will most certainly destroy the engine.

Yes most of the later radials were turbo and/or supercharged but the square power settings came from older engines which were sensitive to high CYLINDER pressures due to their designs (read early Wright engines). As most every one knows All turbocharged or supercharged engines do in fact run positive manifold pressures however the engines are designed to handle the higher cylinder pressures that result.

Trust me if you run too much cylinder pressure on any engine turbo or naturaly aspirated it will come apart.

 

[VNugget]

Trust me if you run too much cylinder pressure on any engine turbo or naturaly aspirated it will come apart.

The point is, what is "too much?" Is it a value determined by the people who designed the engine, equipped with specialized equipment and years of engineering experience?

Or is it the relationship between a unit of length based on the length of a barley seed and a unit of time based on Earth's rotation (all in a number system based on the number of fingers humans have)? What does the latter have anything to do with anything? Ladies and gentlemen of the supposed of the jury, that does not make sense! If Chewbacca lives on Endor, you must acquit!

P.S. The poor little C-65 on the Taylorcraft L-2 I fly that makes only 2100 RPM on takeoff should have disintegrated into atoms long ago since it's run oversquare, no?

 

[Yellow Snow]

Too much would be more than the settings recommended by the engine manufacturer. I never said that engines shouldn't be run oversquare. If you look at my original post I said they are more efficient when run that way and it is common to see oversquare settings on cruise charts. I did say however if you shove the throttles open with the prop governor set for a lower RPM you may grenade the engine. I have seen it. I was backseat on a training flight guy did a go-around and shoved the throttles forward before the props and cracked two cylinders, melted a valve, and blew a hole in a piston. That is overboosting.

 

[FlyinTim]

hey man...look in the poh

 

[lawfly]

POH & GUMP. And when you fly turbo charged engines, you'll see higher MP values, of course. At least at cruise and above. And overboost can be a consideration on takeoff in those airplanes. When I got my Citation type, they at first had to get on me to unhestitatingly advance the thrust levers (non-flying pilot usually sets pre-calculated takeoff N-1, after Captain/flying pilot initially advances the levers, but I was advancing too gingerly at first) as I had been flying a Seneca II, where ramming the throttles full forward on takeoff can potentially overboost.
As for the normally aspirated Arrow, full forward for takeoff, but consult POH as to possibility of leaning mixture for high density altitude takeoffs.
In the Twin Otter, we called out "props to go" on our pre-landing checklist, because, as in the Arrow, advnacing the prop(s) early will result in an apparent surge of RPM. And why bother the passengers with that. Just some random comments.

 

[A Squared]

Im a little confused though on landing. Throttle comes back to 15"( I get that) but prop to full forward? Is this done just in case of a go around?

Yeah, as has been mentioned, it's to be prepared for a go-around. Consider this, instead of automatically putting the prop control in at 15" MAP, how about putting the prop control forward when the RPM starts to dropa as you reduce power and slow down. This will prevent an increase on prop RPM.

and I cant remember what to do with the prop/mixture would I damage anything or get myself into trouble just by leaving everything full forward?

You'd be fine like that. I wouldn't embark on any long cross country flights like that, but then hopefuly you wouldn't embark on any long cross countries with your instructor incapacitated in the right seat.

 

[DaveJ]

Depending on the model of Arrow you're flying, you might have a 'avoid continuous operation' range marked on the tach. In my 1973 Arrow, it's 2100 to 2350 rpm, which means I either cruise at 2100 rpm or 2400 rpm. The POH cruise setting chart has several over-square settings in it. I most often cruise at 24/2400, 24/2100, or 21/2100 (75%, 65%, and 55% respectively).