warbird addict

After having seen a TF b-25 flown with a pair of os 70's I decided to peel the os 70 pumpers out of mine "unflown as of yet " and replace them with a pair of os 91's any thoughts or warnings from someone who has done this would be greatly appreciated, I"m just thinking the 70's are just a tad too scale and under powered for my liking.
Thanks in advance
Elton

c550

My Feedback: (16)

I don't think you will have any problems with the airframe, you will have large cut outs in the cowl. Also you might have a fuel capacity problem as the .91 will use more fuel. I have Saito .81s in mine and I wouldn't want any less power although mine has been glassed and painted so it weights more.

Hope that helps.

Dave

warbird addict

yeh thanks much, I'm not too concerned with the cutouts as much as I am with the 91's being too much motor.
I have heard alot about people using the saito's as the preferred motor over the recommened os 70's but the 91's are a sretch from that even.
I started making the changes for the 91's and the footprint is really not that much different than the 70's.
My impressions of the 70's left me really not wanting to use them as they seemed really underpowered with no "oh *****" room left on the left stick
the saito's were my first choice as they seem to be "proven" but good ol tower has the all too convenient 3 pay plan that seems to keep me plane poor LOL

c550

My Feedback: (16)

Doesn't os make an .80 four stroke now? The only real issue with a .91 is the fuel tank if your not worried about the cutouts in the cowl. I don't remember if there is more room in the nacels for larger fuel tanks, but if you are ok with a slightly shorter flight I think that would be the only real change.

Hellcat716

My Feedback: (2)

I am using YS 63 motors which are about the same power output of some 91 motors. I have plenty of power and fly about half throttle for scale flight. They have come out with the YS 70 now which would be even better. Fit in the cowl very well.

Dash7ATP

I love the B-25. It's a great looking plane and my ten minutes of stick time showed it to be a great flying plane with balanced controls. I"ll never forget that flight !

Now to the model... Increasing power in a twin is not always the answer to perceived problems. Just what makes you think you will need more power? This was a light bomber and not a fighter, so loops were not in it's flight envelope. I'm sure it would probably roll, but again, it's not something a pilot flying one woulld do very often, if at all. If you are concerned with single engine performance, more power just means a faster spiral into the ground. Huh?

If you are familiar with twin engine planes (as in full scale multi-engine rated) or models, then you already know that if you lose an engine, you must be at or above a minumum speed in order to have enough rudder available to compensate for the drag and loss of power on one side. If you are below that speed, more power won't save you as it just increases the need for more rudder. Trying to stop the roll that develops with aileron just increases the problem (it increases adverse yaw on the dead engine side). Airspeed is the answer and lowering the nose and MAYBE reducing power to bring the plane back into balance is often the answer.

Adding a more powerful engine will increase your speed slightly, but does require an awareness of the problems mentioned above. Adding more power in an engine out situation WILL require a higher airspeed to be able to control the increase in yaw (notice I did not say roll- that is the result of uncontrolled yaw).

Good luck and keep us informed...

Dash

warbird addict

yeh they do make an 81 but they are like $45 more than the 91?
I've got several planes with 91's in them and the fuel economy is extremely good.
I just discovered by googling the fs 70 ultimate that OS has been recalling and exchanging them for the 81's due to extremely poor runability/ tuning issues as well as issues with
the pump raising hell with the tank so I'll make contact with them monday and see what happens.
I really want to get this thing finished for spring and fly it , I've been puttin around with it workin on it a lil bit here and there for 2 years now, it's time to either finish it and fly it or get rid of it

warbird addict

My real concern with usind the 70's was the amount of runway needed to get the thing airborne as well as seemingly not having enough power to pull up after take off, the one I saw fly the guy had to fight stalling it at full throttle after taking off for quite aways till it built up enough speed , it just seemed to me it was grossly underpowered and the only way he could really climb was to make several circuits of the field gradually gaining altitude with each circuit and then putting it into a dive to gain enough speed to pull up and gain altitude with authority.
I am aware of the do's and don'ts associated with twins and what happens when you lose an engine.
My intention for going with bigger motors is purely to be able to fly it with a bit more authority and and not have to fight off a stall on take off, the guy I saw fly his said it was way underpowered and that he had to really be careful on take off not to pull back on the stick too early or face stalling it, I dunno It just seemed to me that even at full throttle the thing seemed to be just lumbering around and the guy was fighting off a stall all the time.
I did youtube it and found a guy that was flying one with a pair of 90 twins in his and it seemed to fly alot better and pulled altitude without first making several circuits of the field
I'd rather have the extra power and not need it than need it and not have it

Dash7ATP

Warbird,

That being the case, Ican understand your conderns. He may have just built heavy, but in any case, it doesn't not sound like a good situation.
Again, good luck. I'm sure you'll enjoy flying it.

Dash

Augie11

Yes, the new OS .81 alpha's are a bit pricey but they will turn the same prop as the old .91 Surpass motors and are much lighter and more compact.

telejojo

I don't know what the weight difference is but I have a pair of Magnum 52's on my H9 B-25 and it flys great.