i3dm

My Feedback: (51)

Hello guys,

I read somewhere that it is possible to reverse the rotation of a glow engine like Super Tigre or Webra, which have a front case cover with bolts, by rotating the front of the crank case (with the carb) by 90 degrees - is this in fact possible, and if so what should be done ?

this is in order to reverse rotation on a glow engine to make it a "pusher" engine in fact, with a standard prop.

Dr1Driver

It is very possible. Rotate the fronk crankcase piece 90 degrees to the right - looking at the engine from the prop shaft (front).

Dr.1

i3dm

My Feedback: (51)

Here it is shown to the left when looking from the front ?

http://www.rcuniverse.com/forum/m_58...tm.htm#5878804

Cobra1

My Feedback: (42)

You have it right, from the front of the motor, rotate the front housing 90 degrees counter-clockwise. The engine will run backwards and be within a few hundred rpms of before the mod.

Dr1Driver

The picture shown by downunder is correct.

Dr.1

Campgems

Just curious, what do you gain by reversing the engine vs using reverse pitch prop. I guess I'm assuming that it is an aircraft engine.

Don

Campgems

Something just came to mind. With either a reverse engine, or a pusher prop, I would plan on replacing the front bearing about twice a season as it will now be taking the load of the thrust vs the larger rear bearing.

Don

i3dm

My Feedback: (51)

Campgems, pusher props are available only in very specific sizes, if you reverse the engine you can use any standard prop.

victorzamora

What if someone were to rotate the front housing on a .46AX, and then use a pusher prop on it. Stick it on the right (correct me if i'm wrong) side of a Dual Ace (for example) with a normal .46AX on the other side. Just richen until at identical revs. It would be very risky running one engine that lean, but I think it'd be cool to see a Counter-rotating R/C Twin-engine plane. What do y'all think about that??

Jazzy

Vic, Ideally that is what you would want. The torques (via rotational inertia) created by the engines would cancel each other out. Ever notice that many models have firewalls angled to the right? With counter rotating props the angling would not be necessary. If the angling of the mounts/firewalls was desired, the counter rotating engine's firewall would be angle to the left.

OS .46 AX engines have a one piece crankcase and cannot be made to counter rotate without serious modifications to the crank.

Why would one engine be run lean?

I think I'm generally correct; if not, others may offer corrected explanations or go into greater detail.

victorzamora

Well, it's good to know that I can't reverse the 46AX's direction....now I won't go downstairs and try it. Also, I'd like to correct myself when I said that it'd be dangerous for one engine to run lean. I meant for one engine to run that much richer than the other. If reversing direction drops a couple hundred revs, then you'd have to richen the other to overcome that difference. I forgot that dropping a couple hundred revs is no big deal if its just by richening...so cancel that. The one thing I can't remember is if you'd want the engines rotating inwards or outwards. If one engine goes out, then one case is better than the other...i just can't remember which. Now, wouldn't it be nice to have an R/C CR plane?? Most full-scale twins don't have counter-rotating props. My dad's Beechcraft Baron B-55 doesn't, Seneca's don't, and Twin Commanches don't. I think Navajo's come with that option. Anyway, I think it'd be a fun project for someone with the thumbs to try it.(pretty sure, correct me if I'm wrong)

Jazzy

I think I'd want the CR engine on the left/port side. The top half of the prop arc would be swinging away from the fuse.
Yeah, I think it would be fun to try!

DarZeelon

Jazzy,


You would normally want both props with the top of their arc turning inward.

In this configuration, the spiral slip-stream from each prop, would hit the rudder on the same side as the prop.
This will, in case of the other engine shutting off, reduce the amount of rudder deflection needed to maintain heading on the remaining engine.

Please see the Piper PA-31-350 Navajo Chieftain, which for this reason, has one Lycoming TIO-540-J2BD on the left wing and one LTIO-540-J2BD (L - left hand rotation) on the right wing.

The Grumman Mohawk, on the other hand, had both props spinning outward, but also the triple rudders, which provide a different lateral control solution and the engines both have marked out-thrust.


On a normal model you would want both props turning inward and so the counter-rotating engine belongs on the right/starboard wing.

Jazzy

Interesting point Dar. I hadn't thought of that. Not having any experience with full size leaves me at a bit of a disadvantage.
I chose that configuration based on the torque created by the pair of engines. If the standard rotation engine has a tendancy to want to "pull" to the left, it stands to reason that a CR engine would want to "pull" to the right. Having them "pull" towards each other would seem to be beneficial. In my mind I was adding thrust/torque vectors.
Being able to control a single rudder aircraft might have a slightly higher priority though!
Starboard it is.

The OV-10 Bronco is different animal.

CrateCruncher

My Feedback: (1)

This is certainly a timely discussion for me. I am planning a Duellist MKII build with counter rotating props. I have a pair of K&B .40's that I plan to use. They have 3-piece cases and I understand a reverse-rotation crankshaft is available from somewhere but rare. Would the crank-swap eliminate the need to clock the front 90 degrees? Would I still lose WOT rpm?

victorzamora

First off, good luck with that Duellist!! I want a twin...and a kit, but don't have time or space right now. Anyway, this is from what I know about these engines and general physics/mechanics. Correct me if I'm wrong, anyone.
Well, theoretically, the only thing dropping the WOT rev's would be the crankshaft being designed for maximum CW(as seen from back of engine) performance. If you get a reverse-direction camshaft...then there should be no reason for it to drop WOT rev's.

By the way, I was thinking about what side you'd want it on. DarZeelon and Jazzy, y'all were focusing on the wrong aspects of the counter-rotating effects. Why do 3D planes have so much rudder throw? In stalled flight, the only air passing over the rudder is produced by the engine. A trainer has VERY little rudder throw, but it still responds in a dead-stick landing...right? Most of the air that effects the lift of a control surfaces (which is how they work, differential lift...jic) is provided by the plane's movement. It's going to be pretty long, but that's mostly because its hard to explain via forums. Now, on to the actual explanation:
>
1) \ <------------ 2) |
\ <----------- ^----|----v
\ <------------ |
<
When you lose an engine, you have to increase your AOA due to the loss of power. The "diagram" above will help me explain what I mean. In the first one, the slashes are the prop in profile view, with the nose of the plane pitched upwards to maintain altitude despite the loss of power...and the arrows are the wind. In the second picture, it's the prop seen from the back...the "arrows" are the direction of rotation. In this case, clockwise. Torque is defined as the sum total of the forces applied multiplied by the distance between the center of the point of application and the specific center of gravity of the object. So, if one engine quits...you want the least torque possible, because you must use rudder to compensate for it. Assume that the bottom of the prop is coming out from the screen and going in the screen at the top in the first diagram. Also assume that the wind is coming out of the screen in the second diagram. The left side of the prop will be traveling upwards (and a bit backwards), partially WITH the wind (kind of like a tail wind) and will therefore produce less lift than the right side of the prop, traveling downwards (and a bit forwards), partially AGAINST the wind(kind of like a head wind). So, there will be more thrust produced by the inside edge of the prop than on the outside. Since the distance between the General Center of Gravity (GCG) and the inside edge is less than the distance between the GCG and the outside edge, you want more the side producing more lift on the inside. Torque=force x distance. Force doesn't change, and you want minimum torque...the relationship between torque and distance is linear and positive. The less distance, the less torque. If you had the props spinning outwards, they'd create more lift on the outside and therefore more torque...the plane would nearly flip the second you lost one engine. The Mohawk uses the outward-angled thrust to compensate for it. Ex: If the left engine goes out, the plane would pull EXTREMELY HARD left. The outwards thrust will compensate for it and pull the plane right (or at least less left). It's less fuel efficient, but reduces a lot of things that the military doesn't want on an airplane. They did that for a whole different set of reasons. So, my little mental process has taught me that (I THINK, correct me if I'm wrong) you'd want the counter-rotating engine on the right side of the plane, so that both engines spin inwards. What do y'all think???

Jazzy

Vic, I'll be honest - I can't make heads or tails out of your "drawings." Sorry.
I do understand what you're trying to say though (excluding everything having to do with the "drawings").
I can see your reasoning and how it might pertain to full scale regarding engine and fuel efficiency but, models? nah.
Flipping over? I'd have to see it.
It sure would be fun to experiment!

Man! I wish I had more time & money. Come to think of it, I'll add calm, warm, sunny days to the list too!!!

Cratecrucher:
If you can find or can turn/CNC a reverse rotation crank there would be no reason to rotate the front housing 90 deg to the exhaust side and the R's, in theory, wouldn't change.
Best of Luck!

victorzamora

.................................................. ..>
1).\.....<------------........2).............|
.....\....<------------................^----|----v
......\...<------------........................|
.................................................. ..<

Sorry about that...I was in a rush right as I was posting it, and decided I wouldn't preview it. I guess I should have checked something that complicated. Ignore the dots, they're just so that it reads the spaces. I definitely don't blame you for not understanding the last post. It's still pretty hard to understand....considering how annoying the dots are, but it's hard enough to explain in real life...so I won't be offended if you don't get them. A guy here built a HUGE B-25 from original plans...had 24 servos and 2 g62's in it. Like a 15 foot wingspan, though I can't quite remember. It was almost ten years ago, and I just turned 17 so...I was little. Plus, I wasn't in the hobby yet...so it was just "big" to me. He got up and flew a couple laps around the pattern when an engine died. This guy was a great pilot, he was actually the first person here in El Salvador to fly R/C and was devout to flying every weekend. First person there, last person leaving. He was doing a low pass, when the engine died. He gave it full throttle because he was at the end of the runway and he had to clear the electricity cables. When he gave it power, it torqued it into a full Snap-Roll headed directly at the ground. On the maiden take-off on my Kyosho Spitfire .46, i gave the throttle a QUICK boost from like 50% throttle to full...and it snapped. His plane became splinters. Mine was perfectly safe (my plane that is, my pants had a wet stain in them and my heart decided to punish me by not beating). And, eventhough full-scale planes are worried about efficiency, one big reason for the inwards spinning props is because most military planes have a bit of help for the pilots that commercial planes don't have.

Isn't that what everyone wants?? time and money. Calm, warm and sunny days would be nice, but if every day were calm, warm and sunny, when would we ever get a chance to build?? Weather here has seemed to be pretty windy, so I guess I'd sacrifice my lovely rainy-building days to get some pretty-flying days.

DarZeelon

Victor,


Please try copying your model using a sketching program (Windows 'Paint'?).
Then enter that drawing into into your post as a .jpeg, .jpg, or .gif file. It will make things clearer.

Torque has nearly no effect... Virtually none at all.
What flips a plane over (ground loop) when too slow a take-off is attempted, is the prop's slip-stream.

I have elaborated about this issue in a past thread.

With the prop spinning in the normal direction, with a tail-dragger, at the beginning of the take-off run, the tail is low and the prop's slip-stream is deflected up by the ground. As a result, the bottom of the slip-stream hits the rudder and the vertical stabilizer, from the left. This requires some right-rudder to keep the plane going straight.

...But as speed is gained and the tail goes up, the top of the slip-stream hits the rudder and the vertical stabilizer, from the right, which requires some left-rudder to keep the plane going straight.

If a three-point take-off is performed, the pilot keeps right-rudder as the plane becomes airborne... and the effect is abruptly reversed...

Torque? Let's assume a .61 engine, producing 110 oz./in. of torque, in a 6 lb. plane. The main landing gear wheels are about 8" out from the engine's center-line and that mean the left wheel is pressed to the ground with 55 oz. of force and the right wheel is pressed to the ground with 41 oz. of force.

That would have very little affect on the drag this wheel produces. Not enough to cause the plane to veer off-course.


CrateCruncher,


If the reverse rotation crankshaft is machined identically to the normal crankshaft, there shouldn't be any real power difference.
If the K&B crankcase is turned around, no flow differences will occur inside either.

victorzamora

I didn't mean rotational energy, or torque of the engine...what I meant was Mechanical Torque, like a see-saw. A 200-lb man sitting two feet from the fulcrum has the same torque as a fifty pound child sitting 8 feet from the fulcrum. In a twin-engine aircraft...two engines pulling the same force at the same distance from the centerline (fulcrum) cancel the yaw-torquing but add forward momentum. Now, if you lose an engine, the plane will yaw towards the LOST engine. P-factor there will cause the plane to want to roll even more to that side. Have you ever noticed that, unless a plane is a 3D monster, when you input rudder in straight-and-level flight that it will yaw and roll? P-factor, accelerated slipstream, and many other things make that happen. The accelerated slipstream also increases lift in the wing, at the point behind the engine, on the still-functional side...rolling the aircraft FURTHER in the opposite direction. On a non-counterrotating twin-engine plane, losing an engine creates EVERY ASPECT OF AERODYNAMIC FLIGHT to fight to roll/yaw the plane to that side. If the side of the prop swinging downwards is farther away than the side swinging upwards, it will create more torque (again, NOT torque from the engine....mechanical torque, like a crow-bar or a see-saw. you know, leverage??) than in the opposite case. So, if the props rotate inwards the unequal leverage is less than if the props rotated outwards due to a larger distance from the centerline. Just, read the links below.
Wikipedia isn't a guaranteed place to look for correct information, but this happened to be EXACTLY what i was talking about. And the pictures are OBVIOUSLY not as good as my pieces of art, which deserve to be in the Louvre ASAP, but they'll do just fine. I think it does a pretty good job of explaining what I mean. By the way, this is not where I got my info from the first time.

The site for Critical Engine is: [link=http://en.wikipedia.org/wiki/Critical_engine]Critical Engine[/link]
The site for p-factor is:[link=http://en.wikipedia.org/wiki/P-factor]p-factor[/link]

DarZeelon

Victor,


The effects you described are all true and were previously discussed.

Torque is torque, whether it is applied by an engine, or on a lever with a fulcrum and in any plane (geometric...).

Torque has a measurable effect, which in my example, was calculated for a single engine.


When flying a lateral twin engine plane (not one like a Cessna 337), there is also the VMC (Velocity of Minimum Control), which essentially means the minimum speed, at which full opposite rudder will fully counteract, full power asymmetrical thrust.

The plane will not be able to maintain heading on one engine at a lower speed, unless power of the remaining engine is reduced.
The VMC for the critical engine is higher than for the non-critical engine.

Jazzy

Vic,
Yep, that is what I understood from your first post (#16). The important word in the links you mentioned was "high" with regards to angle of attack. What is considered "high"? 15 deg? Someone that yanks back on the stick with only one engine running is asking for trouble regardless of which engine is out and yes, if the operating engine is spinning toward the fuse much care needs to be taken ! The guy with the huge B-25 made two critical errors. The first was applying full throttle. The second was too much up elevator. Another mistake was doing a low pass in the vicinity of power lines (safety issue).
The farther apart the engines the more difficult the aircraft will be to control if one dies. Controlling the yaw as an engine dies is critical. If it isn't controlled quick enough the yaw will cause more lift in the leading aerodynamic edges of the side with the operating engine (as discussed earlier) inducing roll and pitch. A cascade effect ensues and catastrophic failure is inevitable.
Lets leave out the scenario of "high" angle of attack for a moment. In this case, I believe yaw (and the pilots ability) to be the deciding factor of the aircraft's fate.
Believe me, I DO understand what you are saying and in no way am I trying to discredit you. I just don't see a severe problem if relatively level flight is maintained - even through turns.

I admit I'm stubborn,, and maybe the effects of inward spinning engines is greater than I imagine. I concede these things .
Given the things I have recently learned I probably would not want CR engines spinning inward towards the fuse. Apparently twins are harder to control than I originally anticipated!

I don't think the horse is with us anymore... we should let it rest in peace.

Best to All

CrateCruncher

My Feedback: (1)

Sorry, I don't mean to change the subject (cough), but I just picked up an APC 10-6 pusher prop and will try the K&B both conventional and counter-rotational using the 90 degree trick discussed. Lets hope I get the same RPM. I'll post my results....

CrateCruncher

My Feedback: (1)

I had a chance to annoy the neighbors today with my engine tests. After not having run for 20 years my old K&B 4011 fired right up but would not hold an idle. After replacing the dried up carb seal with an O-ring I was back in business. In standard configuration with an APC 10x6 the K&B ran wide open throttle at 12,200 RPM. In the configuration shown below with an APC 10x6P (pusher) the engine ran wide open throttle at 12,100 RPM. That small a difference is so small its negligible. Throttle transition in both configurations was better than my OS40SF's! I gave up on the K&B years ago because of carburetor issues but have to say I'm impressed with this little engine.

victorzamora

Well, that's great to hear!! I'm glad you've run the tests and found that. I'm guessing that's NOT using the reversed crankshaft, right? Have fun with that!!