willo jet

Hi guys,

I need some help on props on my b-25 bomber.

Im going to be using x2 .61 nitro's on it and i have purchased 2 master airscrew 3 blade props for it i know that the motors have too turn opposite
to each other but im abit confused on how to install the props ? one will go on as standard but for the 2nd one what do i do ?

Cheers
Will
New Zealand

sebo

This sounds like an advanced pilot question......I hope someone looks at the beginners post that can helpyou......you may consider posting this question elsewhere as well...IMHO

jetmech05

The engines will not turn opposite of each other...no you can't turn one around prop that is

JohnBuckner

My Feedback: (1)

What you are talking about is counter rotating engines on a conventional wing mounted twin. The purpose is to eliminate a critical engine caused by "P" factor when both engines operate in the same direction. For this to function it is neccessary to use a normal prop on one side and a Pusher prop on the other. This cannot work simply by turning one prop around.

The engines must be reversed and some older two strokes that are front rotary valve can be reversed that have a rotatable front case plate and in other cases only a reversed timed crankshaft will work. This severly limits the avalible practical engines for any project. This is only a crutch and will not eliminate the need to understand how to fly a twin on one engine or the need for proper and immediate action when and engine fails.

Counter rotateing engines are what every new twin flyer will always get hung up on rather than finding someone who can teach the method to continue to maintain controlled flight on one engine without the old saw of "shut the other engine off and crash wherever you can." The neccessary actions with an engine out is not intuitive for a single engine pilot so its best to seek help and that is why I do considerable training on multi conversion and developed a special airplane for this training.

CR engines are not the norm with RC multis and while useful are not neccessary. One big additional problem is simply the unavaliblilty of a suitable selection of pusher props.

John

opjose

And reversed rotation engines....

In the electric world this is much less of an issue. While you are limited in ideal pusher prop sizes, you can opt for the next closest match pair you can find.
Reversing brushless motors themselves is a non-issue.

skeeter_ca

Even though i have only limited knowledge of twin engine rc aircraft i do believe that counter rotating engines only help with counter the P-factor during take off and high thrust during slow climb. During an engine out it will make no difference which direction an engine is turning. If you go to the multi-engine forum they talk alot about adding outward thrust angles to the engines to counter the effects of an engine out problem. I have seen anywhere from 2 degrees up to 6 degrees recommended to counter the good engines thrust.

skeeter

JohnBuckner

My Feedback: (1)

Willo jets question is about glow engines and Willo jets that is my preferred way to power all my multi's as you will get the satisfaction with the right preparation and mentoring of actually learning to maintain maintain flight on one engine in all situations without crashing and that knowledge and practice never happens with electric multi's in most cases.

Yes Skeeter out thrust is indeed a usefull tool that I use on most all my multis. What it will buy you is time to make the critical decision as to which engine is out and make the appropriate response. That response is rudder into the good engine and maintain that rudder. Most everyone will respond with aileron which will only make things worse. Out thrust will only minimise the dysmetry of thrust with an engine out it will not eliminate it entirely.

The pictures show the airplane I use for short tanking to introduce new multi pilots to the proper responses that must be learned if the airplane is to survive a long life. The engines are fully adjustable for out thrust from zero to 10 degrees and I use this to taylor the airplane to the individual student.

Willo jets I just completed and tested a Mosquito with a pair of OS .55AX's and am also working on what I beleve to be a CMP B-25 arf and purchased OS .65AX's for that. I think my current fleet of multi's around 10 now and includes twins, triples, a pair of quads and a Six Pack.

John

skeeter_ca

That six pack must sound sweat on a fly by.

skeeter

JohnBuckner

My Feedback: (1)

http://video.google.com/videoplay?do...34645383078441

skeeter_ca

Like i said "sweeeat".

skeeter

willo jet

@ Johnbuck my question was what props do i need to buy ? because i have purchased x2 3 blade,master airscrew 10x7. from hobbyking.

MajorTomski

Will please identify the .61 engines that you are using. Then we can look at the MFG reccommended props.

willo jet

Well i was loking at using ASP S61A

MajorTomski

This page

http://www.nitroplanes.com/72p-s61a.html

Says an 11x6 Rule of thumb when going from 2 to 3 blades is go down one in either pitch or length. So your 10x6 three blade should work. HOWEVER, if they are new engines I'd break them in with the 11x6 two blades (if they'll fit between the engine and fuselage) until they are broken in. (one gallon of fuel through both of them) Then switch to the 10x6 three blade and see if the performance changes.

You are also going to need a tachometer.

And NEVER lean out one engine to match the other one's RPMs Start each one one at a time and set that one to its best performance. Then do the other. It isn't critial that they be spot on in sync on the RPMS.

willo jet

cool thanks for that but correct me if im wrong but ill need a pusher prop for one side and a normal pro for the other ?

willo jet

Im well aware of run in rules ect..

JohnBuckner

My Feedback: (1)

That engine is not a reversable type and can only be reversed with a reverse timed crankshaft which is unlikely avalible for it.

Therefore you cannot run Counter rotating engines (one in reverse). This means you must use the same propellor on both sides and both must be facing in the normal forward orientation.

The direct answer to your question is I would use use APC 11x7 two bladed props on both sides. In addition I would avoid three blade props simply because there are only a few sizes avalible and thats a bad place to be with no choices.

I do happen to have that engine on one of my airplanes (Schneider Sport) and the 11x7 two blade is ideal.

John

Sport_Pilot

You would need to replace the crankshaft with a counter rotating crankshaft. Not many companies offer them. I think Fox does, not sure who else. You would use a pusher prop on the reverse rotating engine. That limits your prop selection. So if you cannot find a pusher prop or crankshaft you would have to forget counter rotating engines. You can get a similer effect by adding right thurst to theright engine and left thrust to theleft engine.

Granpooba

My Feedback: (5)

I think that you are going through a lot of time and trouble for no reason. From what I have observed, counter rotating props only exist in the real world. And besides, the only reason for two engines is for when one engine quits the other takes you to the scene of the accident.

One other suggestion and that is as previously stated, do not lean one engine to match the RPM of the other. I personally would rather run both a little on the rich side while trying to MATCH rpm's. YES, I said MATCH rpm's. If you are to far off, you will be creating a yaw problem.

jaka

Hi!
You cannot use counter rotating props as the engines you have choosen isn't available with reversed rotation crank shafts.
Soo...use ordinary rotating two blade 12x6 props (RAM, Graupner Sonic or APC) or if you want less performance 3-blade 11x6 MA props.

MajorTomski

Post #3
Post #7
Post #17
Post #20
Hi!
THE

ANSWER IS NO! Both engines will turn the same way. You cannot reverse the direction the engines turn. Putting a pusher prop on one side will just result in air being blown the wrong way that is in your face. The engine will not start or run if you try to force it to run the wrong way.
NO you do not need a pusher prop!

rgburrill

Actually, Major Tom, the engine will probably start and run if he tries to turn it the wrong way.  It just won't get anywhere near the rpm or power needed.  But running them both correctly with a pusher prop on one might spin his plane fast enough to generate lift

willo jet

Thank you guys for all the input many thanks !!!!

JohnBuckner

My Feedback: (1)

One degree outthrust on the left side is insufficient to provide any elimination of the critical engine (aways the left engine when both rotate the same direction and normal engine direction). The critical engine is the the side that causes the greatest detriment to aircraft performance when that side fails.

Also one degree will not buy any additional time to identify and react with the appropriate rudder untill you begine to get up to around four degrees.

I always use equal out thrust on both sides and start out around four on all my conventional twin multi's. On my Quads I use two degrees on the inboards and four on the outboards. The triples typically do not need the out thrust at all and the six pack only has out thrust on the outboard pair.

John

willo jet

Okay well i have built it exactly too the plan so must be right.