Benifit of counter rotating props
03-27-2010, 10:24 AM
#27
RE: Benifit of counter rotating props
Thanks for calling me brave. I think there may be another word for it 
The Duellist is actually an easy, fun flyer, although the new engine (yes, .46-AX) is a little stronger. That makes the set-up a little tough because this plane is a dinosaur: 15 years ago, I put a single throttle servo in center wing with bellcrank links to the engines.
I suppose I should install separate servos so I can easily match the engines with my 12X twin function, but to tell the truth, I kinda like keeping one ol' dinosaur around.
I think the old O.S. counter-rotator is a .46-FX(P), a step up from the FP, but I haven't looked at it since I re-tooled the fuel systems 3 years ago, so you might be right.
The new engine is an AX because I loaned the old standard engine to a friend to use in a Hobbico bipe and he broke it.
mt
The Duellist is actually an easy, fun flyer, although the new engine (yes, .46-AX) is a little stronger. That makes the set-up a little tough because this plane is a dinosaur: 15 years ago, I put a single throttle servo in center wing with bellcrank links to the engines.
I suppose I should install separate servos so I can easily match the engines with my 12X twin function, but to tell the truth, I kinda like keeping one ol' dinosaur around.
I think the old O.S. counter-rotator is a .46-FX(P), a step up from the FP, but I haven't looked at it since I re-tooled the fuel systems 3 years ago, so you might be right.
The new engine is an AX because I loaned the old standard engine to a friend to use in a Hobbico bipe and he broke it.
mt
03-27-2010, 01:25 PM
#28
RE: Benifit of counter rotating props
ORIGINAL: kram-RCU
Funny how we seem to have this debate every two months or so on this forum
I do think some very good theoretical points have been made here.
Two of my twins have counter-rotating props. Since I don't have the same models in iso-rotational variant, AND I've never had an engine out in either one ofthem, I can't make a real experiential comment.
Here's my practical philosophy:
1) Most of the fear (and the disasters) concerning twins revolves around engine-out situations.
2) An engine-out situation is gonna be difficult to resolve satisfactorily, regardless of which way your props are turning. It's a lot more about airspeed than torque!
3) Therefore, most (all?) of your time and energy should be sent preventing engine-out situations, rather than trying to gain some small (negligible??) advantage with a counter-rotator. There's a whole new level of art/science/skill/challenge to preventing an engine-out in this hobby.
4) Lastly, finding the exact counter-rotating prop you want can be a big pain, and you may find yourself making an unwISE compromISE!
Thanks,
mt
Funny how we seem to have this debate every two months or so on this forum
I do think some very good theoretical points have been made here.
Two of my twins have counter-rotating props. Since I don't have the same models in iso-rotational variant, AND I've never had an engine out in either one ofthem, I can't make a real experiential comment.
Here's my practical philosophy:
1) Most of the fear (and the disasters) concerning twins revolves around engine-out situations.
2) An engine-out situation is gonna be difficult to resolve satisfactorily, regardless of which way your props are turning. It's a lot more about airspeed than torque!
3) Therefore, most (all?) of your time and energy should be sent preventing engine-out situations, rather than trying to gain some small (negligible??) advantage with a counter-rotator. There's a whole new level of art/science/skill/challenge to preventing an engine-out in this hobby.
4) Lastly, finding the exact counter-rotating prop you want can be a big pain, and you may find yourself making an unwISE compromISE!
Thanks,
mt
Is better or worst, for an engine-out situation, just kill the other engine and perform a dead stick landing?
03-27-2010, 04:55 PM
#29
RE: Benifit of counter rotating props
Not a stupid question. It is a HUGE decision that has to be made within milli-seconds of an engine going out on a twin.
A decision often made without much solid data, and ALWAYS second-guessed by yer buddies, if things don't turn out well.
It depends on airspeed, altitude, type of plane, distance from pilot to plane, wind, skill of pilot, "cleanness" of airframe, off-field terrain, etc..
Bottom Line: If after you consider all the above in 0.05 seconds and have reasonable confidence you can fly with one engine, then do it. You will be applauded. If you don't have reasonable confidence, cut the other engine and glide it in.
If you glide it in off the runway, you will almost always have minor damage, but if you try to fly on one engine and fail, damage will be major, if not total.
Here's a couple of videos of me trying it both ways. Neither plane was damaged (that's why I'm showing these).
There are other videos.
www.youtube.com/watch?v=3tqliETnWOI
www.youtube.com/watch?v=BjXW5thFSbw
My decisions to fly vs glide were made in a split second based on:
The P-61 had good altitude, good airspeed, no flaps or gear down, the plane was pretty close to me when the engine went out, and I heard the engine sputter a little in advance, so I had time to get on the rudder before the attitude got bad.
The B-25 had flaps and gear down, not much altitude and the engine went out suddenly, violently yawing and rolling the plane, so by the time I hit rudder and throttle and ailerons to correct it, my altitude and airspeed were even worse. Plus I was over tall corn, which is amazingly gentle on a gliding plane if you bring it in level.
mt
A decision often made without much solid data, and ALWAYS second-guessed by yer buddies, if things don't turn out well.
It depends on airspeed, altitude, type of plane, distance from pilot to plane, wind, skill of pilot, "cleanness" of airframe, off-field terrain, etc..
Bottom Line: If after you consider all the above in 0.05 seconds and have reasonable confidence you can fly with one engine, then do it. You will be applauded. If you don't have reasonable confidence, cut the other engine and glide it in.
If you glide it in off the runway, you will almost always have minor damage, but if you try to fly on one engine and fail, damage will be major, if not total.
Here's a couple of videos of me trying it both ways. Neither plane was damaged (that's why I'm showing these).
There are other videos.
www.youtube.com/watch?v=3tqliETnWOI
www.youtube.com/watch?v=BjXW5thFSbw
My decisions to fly vs glide were made in a split second based on:
The P-61 had good altitude, good airspeed, no flaps or gear down, the plane was pretty close to me when the engine went out, and I heard the engine sputter a little in advance, so I had time to get on the rudder before the attitude got bad.
The B-25 had flaps and gear down, not much altitude and the engine went out suddenly, violently yawing and rolling the plane, so by the time I hit rudder and throttle and ailerons to correct it, my altitude and airspeed were even worse. Plus I was over tall corn, which is amazingly gentle on a gliding plane if you bring it in level.
mt
03-27-2010, 11:44 PM
#31
RE: Benifit of counter rotating props
Thanks!
Yep, that's what those videos show.
Here's a nice still shot of the first engine-out I ever had on a P-38: 2004.
2,000+ feet away from me, gear and flaps down in a holding pattern, dark cloudy day, pilot with low # of P-38 hours, plane was a P-38. One engine ran out of gas and plane was in a death spiral before I knew there was a problem. That day and today I still can't remember which way the plane spun. Audience was split about 50-50 (R vs. L) at the time.
mt
Yep, that's what those videos show.
Here's a nice still shot of the first engine-out I ever had on a P-38: 2004.
2,000+ feet away from me, gear and flaps down in a holding pattern, dark cloudy day, pilot with low # of P-38 hours, plane was a P-38. One engine ran out of gas and plane was in a death spiral before I knew there was a problem. That day and today I still can't remember which way the plane spun. Audience was split about 50-50 (R vs. L) at the time.
mt
10-03-2014, 10:23 AM
#32
I know this is an old thread, but I have a new question. Does running counter rotating props help with the ground handling of a taildragger twin? It would seem that it has been confirmed not to matter once airborne, but the take off roll can be tricky for a twin warbird with a tail wheel. I am planning on building an ME 410 with twin DLE 20cc engines. They are being offered as a set, with one reversed timed to run the other way.
Also, which way do you want then to spin? Toward the fuselage, or away? Or would that matter?
Also, which way do you want then to spin? Toward the fuselage, or away? Or would that matter?
10-04-2014, 06:29 PM
#33
You want to have the down-going blades close to the fuselage. This minimizes yaw in an engine out situation, although as discussed above we modelers may not be able to tell any difference from the other way around.
i think the best advice here is to take the time to set up the engines to avoid an engine out situation!
One could also get some practice with an old trainer bashed into a twin. My brother did this and can easily fly single engine circuits after a little practice. May not help with a p-38 but at least he will be prepared and will know what to do.
paul
i think the best advice here is to take the time to set up the engines to avoid an engine out situation!
One could also get some practice with an old trainer bashed into a twin. My brother did this and can easily fly single engine circuits after a little practice. May not help with a p-38 but at least he will be prepared and will know what to do.
paul
10-05-2014, 07:31 AM
#34
vertical grimmace
A tail dragger does swing on take off from a combination of the engine torque and the effect of the ground on the prop airflow and can reach serious proportions on planes with a really high power to weigh ratio..
Clearly counter rotating props removes this effect.
For example the DH Mosquito never had counter rotating props and the pilot just had to manage any take off swing, whereas the DH Sea Hornet (smaller, lighter, more powerful and flying of carrier decks) always had handed engines.
As ps2727 says inward or outward rotating has more to do with the effects of control with an engine out. Down next to the fuselage is the better option as the torque from the good engine will help counter the weight of the dead one.
To eliminate a buffet problem the P38 was changed to outward rotating props and as a result it had a minimum air speed required to maintain control on one engine.
A tail dragger does swing on take off from a combination of the engine torque and the effect of the ground on the prop airflow and can reach serious proportions on planes with a really high power to weigh ratio..
Clearly counter rotating props removes this effect.
For example the DH Mosquito never had counter rotating props and the pilot just had to manage any take off swing, whereas the DH Sea Hornet (smaller, lighter, more powerful and flying of carrier decks) always had handed engines.
As ps2727 says inward or outward rotating has more to do with the effects of control with an engine out. Down next to the fuselage is the better option as the torque from the good engine will help counter the weight of the dead one.
To eliminate a buffet problem the P38 was changed to outward rotating props and as a result it had a minimum air speed required to maintain control on one engine.
10-05-2014, 08:31 AM
#35
Having no experience with a twin model, I have a plan. I am going to build a .25 sized twin called the 50 caliber. Kind of a pattern design, so it should be pretty forgiving. Then I am going to get a buch of time with it, to learn the issues with engine outs and such.
I have read through this thread, and the consensus is that counter rotating is not worth the trouble, but with the engines available, the biggest problem would be getting reverse props. The taildragger aspect was never mentioned in the entire thread though, and I have heard that is a problem.
I am going to build my practice 50 caliber model as a taildragger, even though it was designed as a trike. The ME 410 is a taildragger.
I have read through this thread, and the consensus is that counter rotating is not worth the trouble, but with the engines available, the biggest problem would be getting reverse props. The taildragger aspect was never mentioned in the entire thread though, and I have heard that is a problem.
I am going to build my practice 50 caliber model as a taildragger, even though it was designed as a trike. The ME 410 is a taildragger.
10-05-2014, 09:39 AM
#36
VG,
you have a good plan. I do remember Dave Platt talking about a multi engine tail dragger and how counter rotating engines
would indeed help with the swing on takeoff. I have built a few multis and all were tri gear except for a Royal dc-3. I could never get the
engines to come up together and did have a bad yaw problem until both were up to speed. This model was very light on the tail wheel and
any airflow over the tail lightened it even more making for some wild takeoff runs. Eventually I just held the tail until the engines were up to speed
and all was well.
E power would take care of that problem but once again I don't know if having motors turn in opposite directions would be significantly better than both going the same way.
I suppose the only way to know for sure is to try it both ways on the same model. Practical experience is many times better than theory.
best of luck on your project!
Paul
you have a good plan. I do remember Dave Platt talking about a multi engine tail dragger and how counter rotating engines
would indeed help with the swing on takeoff. I have built a few multis and all were tri gear except for a Royal dc-3. I could never get the
engines to come up together and did have a bad yaw problem until both were up to speed. This model was very light on the tail wheel and
any airflow over the tail lightened it even more making for some wild takeoff runs. Eventually I just held the tail until the engines were up to speed
and all was well.
E power would take care of that problem but once again I don't know if having motors turn in opposite directions would be significantly better than both going the same way.
I suppose the only way to know for sure is to try it both ways on the same model. Practical experience is many times better than theory.
best of luck on your project!
Paul
10-05-2014, 12:29 PM
#37
Thanks for the info Paul. It would seem that electrics solve some issues, but have many of their own. I am an engine guy, and honestly, I have never had an engine out with the DLE 20 that I run now. For that matter, I have never had an engine out with any of my DLE engines that I run, so I hope I have that issue solved. I will research prop availability. I am hoping to be able to pull the trigger on the engine purchase soon. I am looking forward to the challenges that this area of the hobby present. Should be fun.
11-23-2014, 04:51 PM
#38
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after doing some research on MOST of the duallist i see they were run with standard rotating props. i am leaning that way too on this build. getting her ready for spring. electric motors on this one. got the kit partially started and just finished her off this month. excited and nervous as its my first twin and biggest heaviest plane in my hangar. wish me luck. Walt
10-28-2016, 07:38 PM
#42
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Hi Guys,
I am looking for a pair of 14x7x3, three blade props with one counter rotating puller prop. I have am building a P-38 with a pair of DLE20RA gas engines...with one having a reverse rotating spin. I need the props for the project. Can anyone direct me to where I can buy them.
Soft Landings Always,
Bobby of Maui
I am looking for a pair of 14x7x3, three blade props with one counter rotating puller prop. I have am building a P-38 with a pair of DLE20RA gas engines...with one having a reverse rotating spin. I need the props for the project. Can anyone direct me to where I can buy them.
Soft Landings Always,
Bobby of Maui
Last edited by Bob Paris; 10-28-2016 at 07:40 PM.
02-22-2017, 05:23 PM
#43
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First I ask what plane you intend to build. The first plane I know of to have counter rotating props was the P-38 and the only reason it was made so is because the torque from the props kept the plane from turning as sharp to the starboard side as it did port. The Germans knew this and until it was changed it helped them to target the plane. I too am getting ready to do a twin RCM Cessna T-50 63". I don't want the props counter rotating because the extra expense of the props and also on a RC plane you really wouldn't notice much if any difference.
02-22-2017, 05:42 PM
#44
My Feedback: (13)
Hay Oldbassard,
I plan on installing two DLE20RA engines into a P-38J 84" wing span. Master Air Screw makes three bladed props for this size engine, and I purchased three sets, along with the proper size P-38 metal spinners. I agree that I doubt it will make much difference in the flight department for this is my first counter rotating engines on a twin. I have built many twins in my life and never noticed the need accept for one model with four engines, that pulled hard right on take off if you were not careful with power management. It was a tail dragger too...but flew great once off the ground.
By the way...Master Air Screws were reasonably priced for three bladed props. Under $17 bucks a pop for CC and CR units.
Soft Landings Always,
Bobby of Maui
I plan on installing two DLE20RA engines into a P-38J 84" wing span. Master Air Screw makes three bladed props for this size engine, and I purchased three sets, along with the proper size P-38 metal spinners. I agree that I doubt it will make much difference in the flight department for this is my first counter rotating engines on a twin. I have built many twins in my life and never noticed the need accept for one model with four engines, that pulled hard right on take off if you were not careful with power management. It was a tail dragger too...but flew great once off the ground.
By the way...Master Air Screws were reasonably priced for three bladed props. Under $17 bucks a pop for CC and CR units.
Soft Landings Always,
Bobby of Maui
Last edited by Bob Paris; 02-22-2017 at 05:47 PM.
