Nylon Screws: NO WAY! Replace with Metric metal screws, or better yet, glue the booms on. I will do that on the next one. You can still unscrew the stabilizer if you have to take it apart, but so far, I've not taken my wings off for any reason.

Exhaust: Get a Jett Pipe - tell Dub that you're using it on a Bobcat, and he'll modify it for you to send the exhaust out the back. I posted a picture of it previously - don't ask where, cause I don't remember...but it's somewhere buried in this thread. Put it on the OS 55 AX and you'll be VERY happy. My Bobcat (with the pipe and 30% heli fuel, APC 11x7) was clocked at 133.8 mph a few weeks ago. (I also tried the Weston pipe, and I do not think it perfomed as well, but plugs and fuel make a difference, but after flying with the Jett pipe, I had no desire to experiment. My Weston pipe (1 flight on it) is for sale: $50 + $7s&h if you'd like it).

TOWER 75: The Tower 75 probably does not generate too much more power than the 55AX, and it doesn't want to turn as high an RPM, and it's heavier. Yes, it's $70 less, but IMHO it's not a good buy. You will need to add weight to the front of your plane to make it balance, and I truly do not believe it will go any faster at the end of the day, and you've raised your wing loading for no good reason.

Bottom line: you get what you pay for. I've heard a lot of good and bad on the Tower motor. I've only heard good about the OS 55. PS. I just bought the Cessna 310 Twin from Top Flite, and purchased 2 more OS 55 AX's for it. I would not trust my $1800 investment to a "cheap" motor just to save a few bucks. This is only my opinion, and we're all entitled to one, right?

Good luck!
Mark

Dead link...but thanks anyway.
Mark

Stickjammer: Good temporary fix...I can do that!

I ran the numbers on the Tower .75 earlier in this thread, the 55AX will not beat the .75 in performance. The reality is the OS .75AX engine is only 10% more efficient than the Tower .75 and cost 2X. There is no replacement for displacement. Don't get me wrong the 55AX is a great motor but those who think it will keep up with a Tower .75 are not being realistic.

http://www.rcuniverse.com/forum/m_45...anchor/tm.htm#

Post # 2016

I just went and ran the numbers on the 55AX as well. @16,000 rpm (optimistic) the 55AX will produce 1,287 watts.

The Tower .75 produces 1,640 @ 15,600.

The Tower motor produces 21.5% more power than the 55AX, the Tower .75 motor is 19% heavier than the OS .55 AX but that is probably a benefit if you are looking for speed.

And I would have to disagree with this.

The Tower IS a VERY good buy.

I've well over 7 of them under my belt now, and all work flawlessly.

They DO need a bit of experience to set up, so they may not be suitable for novices.
But if you have someone with experience around, or someone who has worked with one of these, they are wonderful.

The biggest problem with these engines are the assumptions which the novice make make... e.g. assuming that they have them properly tuned when they are actually far off.

As far as "trusting" the investment...

I put CHEAP $6.78/ea high torque SG5010 servos on my bobcat's ailerons and a myriad of other planes.

I also put in the more expensive Hitec's for the rudders & elevators on the Bobcat, and Hitecs and other brands on otherplanes...

Guess which have the highest failure rate and DOA record? It's not the less expensive servos!

I even have a couple of the "better" high end Hitec's which test just fine, but put them near a running 2C engine, and they go nuts. ( Maybe they don't like the neighborhood? )

None of the "cheap" SG's have ever done this. ( The JR servos have done a fine job too, btw. )


Likewise the Bobcat is a "cheap" plane, which we are putting our expensive gear into... so putting in the "cheap" Tower may be a better match than an AX.

Meanwhile I've saved the cost of a couple of planes by not throwing O.S. engines into everything I own, much as I DO like them.


I'll often put together planes "on the cheap" so that I can feel better about pushing them hard and not caring if they fall apart. Inevitably I tend to have the most fun with those as I end up flying them more often and much harder.... and they end up lasting far longer than I ever expect.... so you don't always get what you pay for...

... after all the aim of any business is to maximize profits... so the dollars paid may sometimes not reflect reliability or value for the money, merely the product the seller can make the largest margin on. I may also be bypassing a middle man or name brand and be purchasing a better product for the same amount.... but it is always DIFFICULT to tell either way.

Original: marktur

I just went back a few posts and caught this. You don't want a pusher prop if the engine rotation is reverse, which is basically the reason for doing the engine mod. I think you meant; Don't forget to turn the prop around. A standard prop has to be installed backward from the norm.

airhog1391,
The tailboom screws are 4mm and the stab screws are 3mm. Match up the lengths with the supplied nylon screws, I think they are 25mm but I can't say for sure. I know some have glued the booms, and/or stab, and/or wings, but I would rather have the option to dismantle everything for maintenance and/or repairs. I had a mishap with the safety fence at our field and the repairs were much easier with the pieces taken apart.

The older ST .90 made in Italy had a removable front housing. Larger ST's also had a removable front housings. I think the sizes were 2000, 2200,2500 and 3200. Also Moki has a removable front housing.

Measured intake timing on ST .90 (standard rotation):
Intake opens: 135 degrees BTC
Intake closes: 50 degrees ATDC

With rotated housing and reverse rotation the timing will be:
Intake opens: 140 degrees BTC
Intake closes: 45 degrees ATDC

Intake timing indicates the engine should run successfully in reverse.

Measured weight ST .90 with torpedo muffler = 25.3 oz.

A 12 inch diameter prop might be somewhat small.

Bill

I put a os91 on mine and its way overpowered. I cut a 3 prop 16x12 pusher to see if that would work and it doesnt. still not happy. someone on our field has a 61 and it flys great. so now I got a jett 50 with a leftcrank and going to try that.

Do you know the all up weight?

Bill

Sorry for re-opening an old discussion on the Tower motor...we've been there and done that. Brain-fart on my account.

I actually have a Tower 46 on my kid's trainer and it works just fine. Reliable and strong.

Let's end it here before I make enemies out of friends!

Love you guys, maaaaaaaaan!

Mark

Mark,

I have been curious about the weight of your Bob Cat and the prop that you are running. If the thrust has been measured it would also be helpful.

UPS will likely be playing with mine until next week.

Very little information exists about the all up weight but engine selection and weight are related. The manufacturer's statement of 5 1/2 pounds cannot possibly be correct. In addition I have an opportunity to purchase a completed Bob Cat less engine that has been sheeted but is seriously overweight. It was flown once with a turbine.

Bill

Guys,

What is the best set-up for the nose wheel when using retracts....I already have my mains installed but need a Robart "something" for the nose retract....what do you guys recommend and what is the best pull-pull system??

thanks

Bill,
AUW of mine with an O.S. 55AX, stock muffler, fixed gear with 3/16" wire is 7 pounds 2 ounces.

Thanks Stick Jammer. The information gives me a much better idea of how to plan and what flying traits to expect.

Bill

Video Bobcat ??
Video Super Falcon ???

[link=http://video.google.com/videoplay?docid=-7350356995110526328&hl=en]Bobcat Video[/link]

Interesting video.


It gives me a good point of comparison.


It looks like you come in much steeper on landing than I do.

I usually bring it in lower, stabilize the altitude, then as I approach the threahold start backing off the throttle to let it settle in.

Yours comes in steep and is almost completely stalled by the time the wheels touch down.

I'm afraid to bring it down that quickly for fear of hurting the retracts.


It's easy to see in the video that this plane is fast.


I swear that I'm turning much higher RPM's on the GMS .76 engine though.

At level full throttle flight, it sounds much more like your plane does in a dive and the sound is - LOUD - .

opjose,

I should clarify that this video was done when I completed the review of the kit. You can find it by following the rating link at the top of page .

In this video the Tower .75 was not fully broken in, this was the 3rd or 4th tank through the engine and I was using a 12x6 zinger prop. The 12x6 I believe is actually faster than what I am flying now 11x7 apc and using a pitts muffler, although the 12x6 does not reach the high rpms like the 11x7.

That landing was actually a fluke, I think it was the 2nd time I landed the plane, in other words I got lucky. It was actually a bad landing because I dropped the plane like you noted.

The way I land this plane now is by turning final and bringing the throttle down to about 1/3 to 1/4 and establish a glide slope, when I reach the end of the field I let the nose come up and add a little throttle to keep it from stalling, as I come onto the field I begin reducing throttle while keeping my forward momentum up until the mains touch down. Note: this is the way I do it now with spoilerons and no elevator compensation, before I had to point the nose down and bring it in hot and try to pull the nose up in perfect timing so the plane will not bounce.

Here is a video of my friends Falcon 120 with a wren 54 and my 50 size falcon with a OS 55AX on the back

http://media.putfile.com/Two-Bobcats-over-England

Dennis

I have a tower 75 on mine and I am just a few oz more, but I am still under 8lb. Also nice video bigplumbs

About 100 pages back someone mentioned canard. Most airframes have need some up front weight and a canard shifting the weight forward might be a solution.

Food for thought. Deserves experimentation at a future repair. Doesn't appear to be a difficult project.

Bill

I finally got an accurate mid range scale and weighed my plane.

8lbs 12oz.

Hmmm... Maybe this is why I'm having some difficulty getting it down.

I have to come in hot to keep it flying.

Who really cares. Don't you ever get tired ofasking the same question over and over again. Call Bob Violett if it means that much to you.

I am not all that surprised at a weight of 8 pounds 12 oz. Does the airplane stall abruptly or stall to the right or left? Are you using spoilers or flaps?

The beautiful airplane below flew once with a turbine and was retired. It also came in way too fast. The airplane is sheeted and painted. The weight is 6 pounds 8 oz without gear, without engine, without rudder servos and without other equipment. If an all up weight of 9 pounds (31 oz/sq ft) or so can be achieved some experimental flights will be attempted. I anticipate using snubber brakes, a slight amount of spoiler, and flaps that are tied down.

Bill

Original: opjose

Why so heavy?