Anybody have a suggestion wrt how much loss in thrust from rated values should one compensate for while selectin a turbine for an airframe.
For eg BVM suggests a 12lb to 17lb thrust turbine for the Bobcat , this would be the ideal thrust for the Bobcat at sea level and temp from 20-25deg C , however if the temp and altitude conditions changed to eg 2000ft above sea level and ambient temp 35~38 deg C there would be a substantial decrease in performance.

My Kangaroo with a 14lb PST is adequate but as ambient temp reaches 35~36deg C at a 2000ft elevation I start feeling the loss in performaance and find myself flying at almost at full power most of the time.

Im building a composite bobcat and most of the flying will be in similar conditions and am wondering how much additional thrust needs to be planned for to compensate for loss in performance due to high temp ?
I am considerin the following turbines
PST600R - 14lbs thrust
Jetcat P70 - 16~17lbs thrust
Jetcat P120 - 27 Lbs
PST1300R - 28 Lbs
Also Ive come across numerous examples of the Bobcat flying with 120 power not sure however if it is the new Bobcat[:-], is there any difference betn the older Bobcat and the new composite bobcat ?

xantos

Xantos,

I cannot answer your question. However, I have seen the Composite Bobcat fly with the Wren Super Sport, giving 18lbs of thrust for the same weight as that standard MKIII engine.

I know this engine uses an advanced diffuser to achieve such, and seeing this combination together, appeared to me to be the ideal composition.

I would suggest you look at this engine, the performance and price quite appealing in my view, and contact Wren directly, to see if they may advise on performance issues in climatic conditions such as yours.

I wonder how the diffused air may be effected by the design in such conditions?

I guess the upside is that is normally sunny to be able to fly in unlike the UK, which just happens to be sunny today!

Gazzer

Xantos

I have just obtained a Bobcat XL kit and intend to power it with my Wren Supersport which is giving 19lbs of thrust.

We fly at 500 feet ASL and the normal temps are 50 - 70oF so we have cool dense air which gives optimum operating conditions. I suspect the Bobcat would fly very well on a 14lb thrust turbine in these conditions.I am currently operating a Boomerang Intro on a Wren 54 Mk3 (15lbs thrust) and a Jetco on a Ram500 (12lbs thrust ) and these models perform very well. Of course my idea of very well may not be somone elses, there are always pilots who want the biggest possible turbine in any airframe

While a P120 will compensate in the power department for any reduction in performance due to ambient temps and altitude remember it will be heavier and need a bigger fuel load which will decrease performance and make it a bit harder to fly due to increased wing loading especially in marginal conditions.

I think the Wren Supersport would be a good choice as it is very light and fuel efficient. Alternatively the latest Jetcat P80 gives around 20lbs thrust and is very well priced at the moment, hope this helps you, I am sure you will get some more good advice.

John

Not another kit Mr Agnew!!!!

Thought it was flying weather in Scotland?!!

Gazzer

Hi Xantos

Just ran a couple of quick numbers here on your temps and altitude. I wouldn't consider any engine under 25lbs thrust. The temperature is more a factor than the altitude. To get 17lbs of thrust at 38C, you'd need to start with about a 22.5 lbs thrust motor. So that would limit your list to the P120 and the PST 1300R. You may also want to consider either the Simjet 2300(25lbs) or 3000(30lbs).

Although the supersport is a great engine, it's thrust output would probably be in the 13-14lb range at your temps and altitude. Which would still fly the Bobcat well, but may not give you the power margin that you'd like or possibly need.

Good luck with your choice.

Ed

Gazzer

The Scottish Bandit is almost ready for gear install therefore one must have another model ready to go onto the building board

I hope David Gladwin is able to respond to Xantos's query as he operates two Bobcats in high temp conditions in Oz and has a lot of experience on the airframe. I still think a Supersport would give adequate performance due to its very low weight. By the time you take into account turbine weight and fuel load on a P120 I don't think the power / weight ratio would be all that different assuming the loss in performance is equal percentage wise on both turbines.

There is also a big difference in cost.

John

Hi John,

Wonder how well two SS would go together!!!!?

Be a bit different I guess but probably not a good idea![X(]

Gazzer

I've flown the Bobcat XL, Kingcat, and Comp Bobcat. Comp Bobcat flies like the Kingcat, just lighter and lands the best of all three. The composite Bobcat flies better than the XL, is lighter than the XL, lands better than the XL, bigger than the XL, more room inside, and requires no gyro. It is overpowered with a p120, but going down to a P80 or P70 would make no sense for the weight issue. The only real gain would be to go to a P60, but in your instance I think you are going to need more thrust. BV has a Comp with a tuned in P60 that flies awesome, but thats at sea level. I have no experience with the wren. An engine that weighed the same as a P60 with 18lbs of thrust would be the ultimate for this airplane.
You will love the airplane, it goes together very, very quickly and alot of fun to fly.
I think if you go with the smaller engine, I would leave out the cockpit kit, smooth stop, air gauges, and use duralites.
Scott

Scott

My point exactly! The Wren Supersport gives a minimum of 18lbs thrust ( mine gives 19lbs) and the manual start version which does not need a heavy battery weighs 1 1/2 lbs less than the P60. Result happiness

John

I have flown many Bobcats with everything from Ram 500-750s, Jetcat P-70-120's and Pst 600 all flew well and the only differance I could tell was top speed and rate of climb they all landed about the same speed. I personally would go with the larger engine and turn it down as required for top speed limiting.

My Bob-Cat was powered by a RAM 500, till the turbine blows up! Turbine-wheel has broken.
Then I install a Behotec JB 165 with 174 N, rated down to 100 K.
Need full power only for TO, in the air the throttle sets at max. middle position, which result in power of approx. 35% !
In this config the jet flies great and very quiet.
BRG Fritz
www.jetmodel.ch

Hi Scott, John and Ron

I would agree that a Bobcat would be vastly overpowered by a P120/Simjet 3000 under STP conditions. But at the temps and altitude Xantos is flying at it's output would be about 17-18 lbs, which is the top end of the recommended thrust. A Wren supersport would be putting out about 13-14 lbs thrust, which is the bottom end of the recommended range.

Keeping the aircraft as light as possible, or at least making the weight in the aircraft functional instead dead, will help as well.

Ed

Thanks ED ,thanks Guys
for all the useful info.
Unfortunately all turbine manufacturers rate their turbines at STP and I just wasnt sure how much thrust I would get in REAL conditions.
It seems my choices are clear either P120 or the PST1300 derated to 96K~100K RPM.
Xantos

Yes, I fly my two Bobcats in temps of upto the high thirtys (centigrade !) One has a PST 600r the other a P70. Both cope fine with the temps., but obviously performance is down somewhat and when its really hot the P70 has a better reserve of power. I used to fly one BobCat with a RAM 500 of only 12 pounds of thrust and even that was absolutely fine in reasonable temps. I fly from a short grass field and the model's take off performance is more than adequate, I just don't think you need big engines in BobCats, the wing loading is low enough to take care of that, and the smaller engines have a lower fuel consumption for longer flights. I think a Wren Supersport in any Bobcat would be more than sufficient for any temps. except for those who want outrageous performance.
Your choice.
Regards, David Gladwin.

consider the Wren supersport. I have one on a Composite Bobcat and it is great. I only needed to fit the two kevlar tanks and left the plastic one out and still get a long flight time.Its also really light weight compared with other 18lb engines.

jason

Xantos,
I have been flying an XL for about three years now.... P-120 with the pitot tube for airspeed control. My home field is 5,400 feet MSL.. The Bobcat handles great up here, lands in reasonably short distances. But the real neat thing are the takeoffs. If I want too (and I do offen).. I can go vertical after about a 200 foot takeoff roll... and climb straight up until I can't see it anymore.. the 120 is a great source of power and reliability. My vote is for the JetCat P-120 all the way!
Regards,
Lee H. DeMary
AMA 36099

I am flying my bobcat composite with a p120 and it flies great and lands very slow I had a bobcat xl with this same turbine and the difference I found was the xl was much faster on both top end and landing speed. I have a smoke system installed the extra room inside works great for this.

Tom