Learned something today about twins

simtech

simtech

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Simtech
Flew to memphis today..well olive branch. Was waiting to leave and I saw a twin pull up. I have not paid attention to twins much but while he was at idle I can see the right prop was clockwise and the left prop was running counter clockwise. Makes sense to me why for p factor but just never noticed that before.
 
Yes, several light twins have counter-rotating engines and props, where the top blades of both props rotate "inward".

But there was a famous twin in which the top blades of both props rotated "outward". Anybody know what it was?
 
Pilawt said:
Yes, several light twins have counter-rotating engines and props, where the top blades of both props rotate "inward".

But there was a famous twin in which the top blades of both props rotated "outward". Anybody know what it was?
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For a short time, the Twin Mustang :D
 
Pilawt said:
Yes, several light twins have counter-rotating engines and props, where the top blades of both props rotate "inward".

But there was a famous twin in which the top blades of both props rotated "outward". Anybody know what it was?
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Yup, the P38
 
simtech said:
Flew to memphis today..well olive branch. Was waiting to leave and I saw a twin pull up. I have not paid attention to twins much but while he was at idle I can see the right prop was clockwise and the left prop was running counter clockwise. Makes sense to me why for p factor but just never noticed that before.
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Some do, more don't.
 
What was the maneuver a P-38 pilot could use to exploit that fact to advantage over their SE pursuants?
 
Some of the Aerostars did also. Seems like the 700P when Piper bought the company the top of the props turned outward also.
In fact I can think of no other GA twins except the Piper Senaca and and Navajo that have counter rotating. Must be a Piper thing.
 
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I believe it's expensive to make counter rotating engines, vs just one copy. Have significant time in both. The good thing is the procedures (SE) are the same.. it's only the performance that's different.
I have significant time in Barons, Senecas, Navajos, but mostly 310's.
 
Might be but, in the Senaca, Navajo, and 700 P the left and right engine cost the same from the mfg.
 
ronnieh said:
Might be but, in the Senaca, Navajo, and 700 P the left and right engine cost the same from the mfg.
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lol!! Never bought one! Thank you for the info should I ever decide to do so.
 
ronnieh said:
In fact I can think of no other GA twins except the Piper Senaca and and Navajo that have counter rotating. Must be a Piper thing.
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Some twinkies do also.

ronnieh said:
Might be but, in the Senaca, Navajo, and 700 P the left and right engine cost the same from the mfg.
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I was told that the unpopularity of CR planes was because the commercial operators had to keep double inventory. True? I have no idea.
 
Kritchlow said:
I believe it's expensive to make counter rotating engines, vs just one copy. Have significant time in both. The good thing is the procedures (SE) are the same.. it's only the performance that's different.
I have significant time in Barons, Senecas, Navajos, but mostly 310's.
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The left turning engines are less common as fewer of them are produced but I don't see any reason they are drastically MORE expensive to make. In fact for most engines, there's hardly any difference in the L version. It's not the case that the whole engine is a mirror image. The ignition timing is different and there may be some minor internal lubrication changes. The accessories need to be designed for the opposite rotation (but those are typically available with both rotational directions anyhow). Obviously the props have to be twisted the other way.

The answer is not just p-factor but all those things that pile up with right turning engines. In normal operations it can pretty much be accounted for statically. The big issue is in single engine operation. When they both turn the same way one will have much worse performance characturistics than the other, hence known as the "critical engine". Despite jokes, it's not always the one still running.
 
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The Beechcraft Duchess had CR engines also. CR engines were put on light twins in the '70's targeting multi-engine training making it easier and safer. Piper also had a CR version of the Pa-30 Twin Comanche, the Pa-39.

Noah W
 
ronnieh said:
Some of the Aerostars did also. Seems like the 700P when Piper bought the company the top of the props turned outward also.
In fact I can think of no other GA twins except the Piper Senaca and and Navajo that have counter rotating. Must be a Piper thing.
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Only some of the Navajos, some of the Twin Comanches as well.
 
The vast majority of Navajo are counter rotating. There are a few of the 310 HP out there but, they were not popular. The Chieftain and the smaller "CR" are actually quite popular. Even the popular Colemill conversion stuck with the counter rotating engines.
I would think with the exception of the cam most internals would remain the same.
As stated the "thinking" was to offset the critical engine situation. With either engine out the rotation of the remaining engine helps the adverse yaw due to P factor. Never understood the 700P as this configuration actually makes it worse in that either engine becomes the " critical engine" so to speak.:dunno:
 
ronnieh said:
In fact I can think of no other GA twins except the Piper Senaca and and Navajo that have counter rotating. Must be a Piper thing.
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Piper had the PA-31 Navajo CR; PA-34 Seneca; PA-39 Twin Comance C/R; the short-lived PA-40 Arapaho; and the PA-44 Seminole.

There were also the Cessna T303 Crusader; Beech 76 Duchess; and Piaggio P.180 Avanti.

And oh yes, an earlier one ... the Wright Flyer. :D
 
flyingron said:
The answer is not just p-factor but all those things that pile up with right turning engines. In normal operations it can pretty much be accounted for statically. The big issue is in single engine operation. When they both turn the same way one will have much worse performance characturistics than the other, hence known as the "critical engine". Despite jokes, it's not always the one still running.
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So again, I know nothing of twins, but to have them both turning inwards seems like a better option. Why are fewer made like that then?

We couldn't id the plane, it did have a T-tail if that helps.

Oh and we got chastised for going to Memphis and not eating BBQ. :D oh well.
 
simtech said:
So again, I know nothing of twins, but to have them both turning inwards seems like a better option. Why are fewer made like that then?
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The XP-82 was to be powered by two Packard-built Rolls-Royce V-1650 Merlin engines. Initially, the left engine was a V-1650-23 with a gear reduction box to allow the left propeller to turn opposite to the right propeller, which was driven by the more conventional V-1650-25.

In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control. This proved not to be the case when the aircraft refused to become airborne during its first flight attempt. After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area.

The engines and propellers were then exchanged, with their rotation meeting on the downward turn, and the problem was fully solved.
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http://en.wikipedia.org/wiki/North_American_F-82_Twin_Mustang :D
 
simtech said:
So again, I know nothing of twins, but to have them both turning inwards seems like a better option. Why are fewer made like that then?

We couldn't id the plane, it did have a T-tail if that helps.

Oh and we got chastised for going to Memphis and not eating BBQ. :D oh well.
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It the better way to do it, but the difference in practice is marginal. The primary reason is the ability to order engines and all the accouterments in single series bulk. There is a small cost penalty to be paid for doing it this way. Any cost that does not realize a profit at the end of the day is not spent. CR would not exist except for the marketing potential of "Ours is safer". The actual percentage of safety itself is small due to the exposure rate at which it comes into play, although when it comes into play, is relevant. It only comes into play in situations you shouldn't have gotten yourself into.
 
simtech said:
We couldn't id the plane, it did have a T-tail if that helps.
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Probably a Beech Duchess or. Piper Seminole, two of the most common multi trainers, both with counter-rotating props ...

be_76_1981.jpg


pa-44-180t_1981.jpg
 
Bobanna said:
I like your last sentence best. The CR concept evolved from the idea of eliminating the "critical engine" concern in light twins: truth is, now they're both critical, but some of the aerodynamics become more favorable. Of course, if you're only running one hydraulic pump or whatever from the engine that fails, maybe that's a "critical" engine (Aside from older Apaches, I can't think of an example of such a set-up. I think all the Apaches were pre-CR, anyway).
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I think one of the Geronimo kits gave an Apache CR, but I'm not sure. I'm pretty sure a lot of Aztecs have only one pump as well.
 
Pilawt said:
Probably a Beech Duchess or. Piper Seminole, two of the most common multi trainers, both with counter-rotating props ...

be_76_1981.jpg


pa-44-180t_1981.jpg
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I would say it would be the Seminole. It did not have that bubble look to it like the beech.
 
simtech said:
Duh..hahaha okay. So what makes the critical engine?
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The difference in thrust between ascending and descending blades (P-factor) and the position of the lever moment of the descending blade from the center of gravity. This effects the yaw moment of that engine. The further out the blade is the more leverage it has against the rudder. So if you lose the engine with the inboard blade descending, you will lose control of the aircraft and roll at the predicted Vmc when operating at gross. If you lose the engine with the outboard blade descending, you will lose control and roll at a speed below predicted by Vmc under the same conditions.

With counter rotating as typically applied with tops rotating inward, both inboard blades are descending so both will cause loss of control at the same predicted speed.
 
Also with an inboard turning prop, P factor counters offset thrust, the outboard turning prop the P factor augments the offset thrust.
 
Henning said:
What was the maneuver a P-38 pilot could use to exploit that fact to advantage over their SE pursuants?
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Nobody huh? Right Turns. The P-38 could out turn anything to the right. A little tidbit from Uncle Lloyd....
 
Henning said:
:lol: It's one of each I believe, top Dutchess, bottom Seminole.
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Hahaha yeah I figured that. I assumed the Seminole was the bottom one since the way he worded it and the top one looks more bubble like in the front glass than the bottom.
 
Henning said:
With counter rotating as typically applied with tops rotating inward, both inboard blades are descending so both will cause loss of control at the same predicted speed.
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Which is why I'm surprised they are not all CR..one less thing to think about..
 
Henning said:
Nobody huh? Right Turns. The P-38 could out turn anything to the right. A little tidbit from Uncle Lloyd....
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"In a P-38 if we were trying to escape an enemy single engine fighter, we would normally make a turn to the right, with counter rotating props we could turn equally well in both directions, but remember the enemy single engine fighter didn’t turn quite as well to the right as to the left. That is why we would turn right. "

Diary of a P-38 Pilot
 
Subject line is misleading. Thought it was about 'twins', not twin engines.. :)
 
What is it that makes a piston engine ALWAYS rotate one way or the other? Is it something about the crankshaft?
 
simtech said:
Which is why I'm surprised they are not all CR..one less thing to think about..
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I'm not surprised at all, there is MONEY TO BE SAVED by having both rotate the same way. It really is pretty irrelevant in practice. Don't get below Red Line and you will never realize the difference.
 
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