keveaux

it takes off perfect about 75' of runway nose is up 2 deg lands good to I fly off of a grass field so need large wheels btw the engine says supretigre on the block its been around for awhile [8D] I have the CG at 11'' the wheels are dead on where the factory ones were they are off a sig 4 star (may it rest in peices)I had the same enging on it airframe couldent take it

Stick Jammer

Well the pics are kinda dark and its really hard to get a good look at that engine but from what I can see it doesn't look like any DF engine I've ever seen. It appears to be side exhaust and front carb just like any normal 2-cycle glow prop engine. Maybe you can get some better pictures of it.

Stick Jammer

Original: Craig-RCU

O S .55AX specs:

Practical RPM: 2,000-17,000
Power Output: 1.75 ps/16,000 RPM
Recommended Props: 12x7-8, 13x6-7

No, it's an APC 11x7p. Throw that ThrustHP calculator in the recycle bin.

keveaux

kind of a cheep camera but it says super tigre 60 the 60 is in a large G

Meekskin

Still not able to tell what engine this may be. But if its a super tigre, its an old one....I don't think the G series is made anymore and if you are flying with the wood prop that's in the pic, the RPM will be even lower than the APC.

keveaux

the apc one i had on it kinda came apart at the tips the one on it now is a 11/8 zinger the apc was a 11/7

Craig-RCU

Well, I guess as always, one should take whatever what "some guy on the internet says" with a grain of salt and "trust but verify."

Meekskin

Since we have a lot of talk going on,,,,someone tell me why there is a difference in RPM when using a tractor prop vs a pusher prop. On my Magnum 52 I could turn 16,500 with a normal prop and could only get 15,500 with a pusher prop...

opjose

I'll bet there are multiple reasons.

I'd imagine that having the engine sitting in front of the "inbound" airstream to the prop, is going to reduce RPM's.

The engine has to work harder to move the already distrurbed air around the engine in effect.

Stick Jammer

My honest opinion for any of those THRUST/HP/RPM/SPEED calculators is to maybe use them as a basic guideline but not gospel. This also holds true for using prop pitch and RPM to determine flying speed, it simply doesn't work.

Meekskin

Well, what is the best was to caculate speed if these don't work.

Craig-RCU

Without more information about how ThrustHp calculates speed, I take the speed calculation to be "pitch speed" that doesn't take into account aerodynamic drag of an airframe. However, I do find the ThrustHp calculations of horsepower and static thrust to be consistent with the horsepower ratings of manufactures and my own experience. I have never measured an rpm from an engine that ThrustHP calculates a greater hp rating than the manufacturer's listed hp as Stick Jammer seems to have done. I think it is a good idea to follow the wisdom of believing none of what you hear and only half of what you see. Therefore, I have to assume that Stick Jammer has just made an honest mistake in measurement or memory until I too experience measuring an rpm that ThrustHP calculates a greater hp number than the manufacturer's ratings.

Meekskin

Craig-RCU,,,sounds good.....anyway, here are a few more pics of my Bobcat 52.

DaveJohansen

My Feedback: (8)

Meekskin.... What engine and muffler are you running?

Dave

Meekskin

Dave, Its the Magnum 52 with a Weston Genesin tuned Pipe. Without the pipe the Magnum will turn 16500 and close to 18000 with the pipe. I was expecting my radar gun today but UPS failed me. Should get it tomorrow and will check actual speed Thursday when we fly. I will post speeds of three different Bobcats...one with a supertigre 61, one with the Magnum 52 stock muffler and mine.

Meekskin

Dave,

Forgot to say that I am running a 10/7 prop and the Bobcat should fly around 110 mph....If my caculations are correct.

Stick Jammer

I'm sure there will come a day when my eyes and my memory are not up to par but for now they are still working just fine thank you. You seem to overlook that the OS specs list the upper operating rpm's at 16,000 and their recommended prop size is 12x8 - 13x6. They also never state what percentage of nitro these figures are based on. I am under the recommended prop size with the 11x7 and using 15% fuel so I'm not sure why you think 14,000 rpm's is unrealistic. If I used 30% I'm sure the rpm's would go even higher. In fact, someone else has posted several times who is running 30% heli fuel in an OS .55AX and said the difference is quite significant.

DaveJohansen

My Feedback: (8)

Thanks... I look forward to your test results.

Dave

Meekskin

I think I agree with Stick Jammer on the RPM issue...14000 on the ground sounds about right when using 15% nitro. It would probably go to 15-15500 if he used 30% heli fuel. Like I have said on previous post, my 52 gets 16500 on the ground and is rated for 18000 and I can get close to 18 with 30% fuel.

moto-x

Hello to all who have posted on this thread. I have used many ideas from reading all 112 pages of this thread. No two guys build the same so I just used some of everyone's ideas. Here's what I have. Bobcat 52 with a super tiger 51 ringed, APC 11/7 pusher prop. Futaba 3004 servos on alerons, throttle, nose gear and a single 3004 for elevator and rudder with a #440 rod to tie rudders together. I used a forked spring nose gear and drilled a 3/8s bolt for the stem to slip into and welded it together. I epoxied try stock to the front gear former for strength. I built some custom aluminum for the mains that are swept back and mounted to the fuse just under the fuel tank. I epoxied a 1/8 inch plate in the fuse for the blind nuts to bolt on the gear. The width is the same as the stock gear but it is 1 1/2 forward of stock to help with rotation on take-off. I used a 6v battery and velcroed it as far forward in the fuse as possible. The CG is at 10.4 and all up weight is 8.3lbs. My futaba 7c does not have spoilirons set-up so I used the air-brake function. With a 3 position switch, first position,nose gear and rudder on. center position,nose gear off so it is not acting as a canard. forward position is nose gear on and alerons up 1/8 inch and elevator down 10%. I have not maidened this plane yet due to whether here but I hope to soon. I will post a report on how it goes. I just want to thank all of you who have posted there experiences with this plane here on RCU. Man, what a wealth of knowledge that goes on here. Thanks again.

[IMAGE]http://i6.photobucket.com/albums/y214/jinglet/000_1537.jpg[/IMAGE]

opjose

After seeing you guys with the tri gear, this seems like the way to do the fixed gear, as opposed to the provided wire gear.

I like it.

Stick Jammer

moto-x,
Looks great, nice job on the gear. Be careful of the 4-40 rudder tie rod, at about 13" long it has some flexing potential. I used carbon fiber tube, much stiffer and a lot lighter. It's hard to tell for sure in the pics but it looks like your exhaust is pointed right into the oncoming air stream. This can cause the engine to go very rich in the air.

BillS

1. Measuring a distance and putting a stop watch on the airplane is an easy and proven method to measure and calculate speed.
2. Garmin has the potential to provide speed information if it's limitations are observed.
3. Radar guns work if you can afford the cost.

I fail to see how rpm and prop size can ever be expected to give a good indication of flight speed. The prop manufacturer is inexact about pitch, the engine manufacturer fibs about HP, and the coupling between prop and air is loose at best.

Bill

Meekskin

Well, UPS delivered my Radar gun today so will use it Thursday on 3 different Bobcat...will post results.

moto-x

Thanks stickjammer. The #440 rod seemed to be pretty stiff to me. I'm hoping I won't need alot of rudder input at high speeds. We will see. As for the exhaust pipe, it has a small hole in the end of both tubes but a hole lot of small holes out the side towards the end. If the motor goes rich on me I will install some exhaust deflectors on them. Thanks for the input though.