RE: Nitro models pusher Bobcat?
 

I flew bobcat today with a new ultrathrust sport muffer on an os 55ax 11x 7 apc and it makes a significant difference.Top speedhad a small increase but there was a big increase in vertical performance.now I can make giant loops without loosing all my speed .If your fying an os 55ax I think its worth the 65 bucks

RE: Nitro models pusher Bobcat?
 

Flew my new Bobcat today......... truly a snooze to fly and not nearly as fast as everyone on this thread had claimed.....
I did all the mods and even a few nobody has done yet....

Im running a Jett .60Lx with a tuned pipe turning a 10-6P at 21800 static. This thing couldnt get out of its own way.. Flies nice though.. I guess I was expecting it to be more like my Gripen. Any suggestions on props or anything else I can do to get this thing to come alive????

Thanks,
NP

RE: Nitro models pusher Bobcat?
 

NP,

How fast, or slow, was it? You should be near 95 or so with that combination. Is your prop facing the right way? This is not a speed plane so I can understand how someone with a Gripen would be disappointed.

Mark

RE: Nitro models pusher Bobcat?
 

wow....what the heck?......that thing should really be moving with that setup. With a Tower .75 and 11X7 APC I've had mine on the radar gun at 110 mph. (This was coming out of a dive tho to be honest).
Its even faster with a Graupner 11X8.

RE: Nitro models pusher Bobcat?
 

try a 11x8?

RE: Nitro models pusher Bobcat?
 

Thanks for the advice guys I will try stepping up to a an 11-7 and 11-8 APC. I had a wood prop that came with the kit on. I double checked and it was on the right way... or wrong way however ya look at it. I even built servo covers to clean up the drag around the aileron and it has spring air retracts......

It flies very stable, no aileron trim was required... Although, it did require every bit of UP trim I had and I still had to fly it adding in UP elevator... Yea, it was balanced per plans 10.3 or 260 mm. I guess I will move the battery from on top the front retract to behind the N/W steering servo.
NP

RE: Nitro models pusher Bobcat?
 

Pics

RE: Nitro models pusher Bobcat?
 

How are you guys getting the T.H. .75 engine tuned with the 11x7 APC prop?

I tried with a new GMS .76 and a reliable T.H. 75 and I am having EXACTLY the same problems.

The 11x7 APC does not seem to load the engines down enough.

As a result the idle is extremely sensitive to the low end setting. Once I can get a reliable idle any attempt to run the engine up causes it to REV like crazy before cutting out.

If I make the engine richer, it cuts at mid throttle and above when I remove the glow plug, even though the engine is upright.
If I lean it out to the point it begins to run above mid throttle, then the engine tends to cut out as I run it up faster no matter what I do.

I took a reliable and already broken in T.H. .75 and put it on the Bobcat, and it went from a stable engine to having the same problems.

I had several people comment that two things were probably happening ( mind you the days were at 98+ degrees each time I had the plane out... )

1 - Since there was no airflow over the engine while it is on the ground, it would overheat as I attempted to tune it.
2 - Since the 11x7P APC prop is so much smaller than what is normal for this engine it would overrev, build up heat and cut out.

People were recommending that I go back to a .50 engine that would be more suitable for the prop.

Comments? Suggestions?

RE: Nitro models pusher Bobcat?
 

hmm,...no tuning problems with my Tower .75. I was running the APC 11X7 for several flights until I went to the Graupner 11X8. Didn't fly it today, but did last Sunday. Temps here are over 100 about middle of May thru most of September. Last flight was with Byron fuel...10% nitro 18% lube...before that 15% nitro. Seemed to be no difference in how it runs with different fuels. Mine can sit there idling for several minutes then transition right up to full throttle without even a sputter.
Notice any foaming/bubbles in the fuel line?

RE: Nitro models pusher Bobcat?
 

Who cares? Don't you ever get tired of asking the samequestion. If it bothetrs you that much, call Bob Violett.

RE: Nitro models pusher Bobcat?
 

So you didn't have any problems tuning the T.H.


A few questions if you would:

Was this engine previously tuned up with a regular prop?

If so did you then just switch to the pusher prop and leave things as is?

What kind of revs are you getting?

The GMS .76 is a ringed engine. Would this be unsuitable for the role as a pusher?



Re: Foaming

I would get an occasional bubble in the line. My fuel tank is suspended in foam and I have it reversed ( opening towards the front ).

When I had problems originally I redid all of the lines just in case.

RE: Nitro models pusher Bobcat?
 

I have the Tower 75 on mine. It idles great and transitions great as well until you go to take off. Then it would go rich. I would have to bring it back and lean it a click or two. Once I got it set right it would fly perfectly. This setting was only a click or two off of peak but that is where it needed to run. It never went lean in the air with this setting. I used the APC 11x7 and though it was turning over 15,000 it ran fine. I have purchased the Graupner 11x8 props but have not run one yet. I tore the landing gear out the last time I err......landed. :( We have a pylon race this weekend so the Bobcat is on the back burner right now.

Blessings, Terry

RE: Nitro models pusher Bobcat?
 

Hi opjose

My engine was new when I mounted it on the Bobcat...broke it in with the 11X7 APC pusher prop.
If I remember correctly, I was getting somewhere around 14,500 rpm with that prop, ( I might be wrong with that number)..
If you're having EXACTLY the same problem with 2 different engines I would think it would have something to do with fuel delivery.

I can't answer your question about using a 'ringed' engine,...I've never owned one,..know nothing about them.

RE: Nitro models pusher Bobcat?
 

Re: Fuel delivery

That was exactly my thought.

I was about to pull the engine yet again, but given what you've told me I'm going to give a try breaking in the GWS .76 right on the plane.

I was concerned that it was simply too hot.

Given what you've said however, it is unlikely that heat is as big of an issue as I may had thought.... given that I don't lean the engine out too much.

Thanks for the push in the right direction! Just what I needed.

RE: Nitro models pusher Bobcat?
 

I vote for fuel delivery as well. Check your lines for kinks. I replaced the kit tubes with larger ID. Also check your pressure tap to make sure that isn't clogged. Oh, and limit ground running to short bursts.

Mark

RE: Nitro models pusher Bobcat?
 

Usually I check fuel first. Make sure youar filtering it well when you fill the plane, and make sure your fuel lines dont have anything in them. Pull your stopper and flush it out.

Maybe go to a closed system, where you put a one way check valve on the muffler to tank line.

They tend to go rich when you cram the power in and a surge of pressure hits the tank raising the head pressure in the tank and making it easier for the engine to suck fuel. the one way check valve equalizes the pressure and doesn't let it bleed back under low power.

I am doing this to my keecat. There are two types of closed systems, 2 line and 3 line. either will help. if that doesn't, then use larger line. You could have a simple kink in the brass tubing, inspect for that too.


I want you to try something. Put a conventional tractor 11x7 or 11x8 prop on it, and try it then on a static runup, to see if you get the same issues. It should not overheat either way. I was wondering if you were getting issues with the carb spilling. A velocity stack on the carb will cure that one if it is the case, but fuel delivery hits my mind first. Also, put a new exaust tip on it, that isn't beveled with the wind flow, that creats suction, it could be killing backpressure. experiemnt and see.

RE: Nitro models pusher Bobcat?
 

Heh, every time I've tried this I get SOOO much pressure that fuel will spray out when I refuel and fuel will flood the carb after I shut off the engine.


Thanks.

I had thought of putting a conventional prop on to tune it. I was concerned that the 11x7 or 11x8 prop would not provide sufficient load on the engine to get it run in and tuned properly.

At first I couldn't get it to run rich as pulling the glow plug shut the engine down until I leaned it.

Then I got it to run for a bit as I leaned it out, and the RPM's were REALLY high at 70% throttle. It was screaming.

After a few moments the engine seized up with the heat. I couldn't even turn it over for a good 8-10 minutes. Strangely it seems none the worst for wear... inspection from the muffler port reveals no scoring that I can see.

RE: Nitro models pusher Bobcat?
 

Because of the heat buildup?

RE: Nitro models pusher Bobcat?
 

I got a leaf blower at a yard sale for $10.00. I direct the air flow on the pipe and the engine head. I have been able to tune the engine with a pusher prop and not overheat.

RE: Nitro models pusher Bobcat?
 

Yes, short bursts so it doesn't over heat.

Check valves without regulators are hard to set properly and in most cases will add to your problems.

You mentioned the engine is over revving, what RPM is it turning? What fuel are you using? Which plugs? Have you checked all of the bolts on the engine to make sure they are snug? Which direction is the muffler exhaust pointing? Is the baffle still in the pipe?

I am still leaning toward a fuel delivery issue because you said this happens on 2 different engines while running on the ground (there are a whole new set of variables once you get in the air).

RE: Nitro models pusher Bobcat?
 

It is quite possible that the 11x7 or 8 may not load it right. I asked the same question to Bax at SuperTigre about the .90. He said the same thing about the ST.90 with the 11x8. so its certainly possible about not loading it enough. There is also a graupner 12x8 3 blade out there. Thats what we gonna try on our ST.90

RE: Nitro models pusher Bobcat?
 

Yeah I just finished cutting down a 14x7x3 to about a 13x7x3.

I'm going to try that to break in and tune the engine. It only gives me about 1" of ground clearance though...

I've run the TH .75's with 15x8x2 props on planes w/o problems.

I was surprised to see that everyone has run 11x7's and 11x8's on these engines.


MMallory:

I don't know the revs as I could not get it to hold the speed long enough for me to check.
It sounded as if it was screaming though. With the glow plug in place it would hold revs and I backed it off peak, but it was LOUD!!! About the loudest 2 stroker I've ever heard.

I was using 10% cool power with a Fox Miracle Plug. The bolts are tightened down ( I also always re-tighten them after the first tank fulls too ). The muffler exhaust was pointing forward with a Dubro diverter, using the stock muffler. There is no baffle in the stock mufflers.

RE: Nitro models pusher Bobcat?
 

Guys,

A friend of mine ordered a RETRACT ready bobcat 50, and we need to know:
1. what size wheels fit (main and nose) ?
2. anyone tried these wheels ? http://www.airpowernet.com/twe/produ...roducts_id=620 they look like a bargain deal with the brakes and all for 60$ !

Please advise.

RE: Nitro models pusher Bobcat?
 

Guys, can anyone measure the wheel wells and let me know about wheel size ?

RE: Nitro models pusher Bobcat?
 

Question for guys that have fit turbines. How much down thrust on the engine? I was planning on 5 degrees. Does that sound about right?