Ehab

My Feedback: (44)

I have a Bandit ARF and a P80/120 CF bypass. BUT, I am installing a 3.5" diameter turbine in it which almost is a P60 size....Can someone who knows any negative/positive affects educate me regarding: take off acceleration, top speed, thrust losses and so on when this combo is used. The pipe pipe will be custom built to any size to optimize the thrust of the turbine. My concern is that the bypass being too large will cause more drag than a properly sized bypass for the 3.25/3.5" turbine....Thank you in advance for your educational comments....

jason

My Feedback: (1)

Ehab,

This subject I have often thought about. I have a Bandit Arf in build at the moment too and I have a P60 bypass and no P60 but I do have a 11lb Wren 44 and I am considering putting it in as I do not have spare supersport which was my first choice. The Balsa supers used to fly on the Ram 500 which were 11lb on a good day so my 44 should push it along ok (ish) If I feel the need for speed then I have a P120 powered one I can dig out and fly.

Anyway, to put a bit of input in about your question, my thoughts are that your 3.5inch turbine in a P120 bypass is just the same as putting an engine in a plane without any bypass i.e. the fuselage is the bypass. So I recon it will be fine. Does that sound right?

Jason

grbaker

My Feedback: (29)

Sounds right to me. You would not want to run a turbine in a bypass that is too small.

Shok

My Feedback: (28)

i first flew my cermark f-16 with a 120 sized pipe and a p60, it took 500-600ft to get off the ground.

changed out the pipe and now it gets off in 300ft

Edgar Perez

My Feedback: (13)

I though that a bypass make you lose static thrust... if so, how come it takes longer to takeoff?

I would have guessed if anything you will lose some top speed... Now i'm confused [sm=spinnyeyes.gif]

Shok

My Feedback: (28)

Sorry I dont know the science of it, just what I saw.

Top speed in flight was not affected.
Take off thrust was significantly reduced though

Ehab

My Feedback: (44)

a proper bypass and a proper pipe are very important to maximize thrust and minimize losses in an enclosed installation. A wrong pipe will cause you to lose thrust. That is why I am asking the question. I just do not know the scientific details....

jason

My Feedback: (1)

I WOULD AGREE THAT THE WRONG PIPE WOULD MAKE YOU LOOSE THRUST I HAVE FITTED A WREN PIPE IN A BANDIT AND ITS AWSOME. I ALSO DID SOME BACK-TO-BACK TESTS WITH THE STADARD BVM PIPE AND A WREN PIPE IN A BVM F86 AND THE DIFFERENCE WAS VERY NOTICABLE IN THE AIR. STILL NOT SURE THE BYPASS WILL MAKE ANY DIFFERENCE THOUGH.

JASON

slick121985

My Feedback: (3)

Not sure about the mathematics of it myself, but the added bonus to spending a little extra money and buying the correct smaller bypass is A LOT more space in the fuse with the smaller bypass. And we all know space is at a premium in a bandit. Greg

Ehab

My Feedback: (44)

BVM does not have a bypass for the 3.5" turbine. He has a P60, P70 and a P80 bypass for the Bandit. I know I can use the P70 bypass for my turbine but it will not be optimized. I was told by BVM himself a few years ago that the bypass/pipe has a huge effect on his jet's performance, so I am assuming he did his homework on his bandit's bypasses...I also wanted to know the technical details of the bypass/turbine size!!!!

EddieWeeks

My test show, the tail pipe is everything... It all has to do with engine temp..

A large tail pipe on a small engine will drop the engine temp quit a bit and result in lower static thrust..

I found a big bypass on a small engine works fine. It seems these engines are sensitive to back pressure. To much back pressure and you get tons of thrust
But it may over temp… For more common is, to little back pressure. Somehow big tail pipes will help
Suck the air out the back of a small engine, resulting in lower temp, less fuel burn, and less static thrust.

The way you maximize your tail pipe is this..

First run you engine on the test stand and take note of the temp at Max power.

Install you engine in you plane and run it again. Install a tail pipe that reduces the engine temp the least..

I found the best way to do this is taper the tail pipe just a bit to get the temp back up to something close
To the test stand temp..

Hope this helps..

Eddie Weeks
http://www.variablepitchhobbies.com/

jason

My Feedback: (1)

Ehab,

I have two bypass's in my workshop at the moment, the one for the P60/70 which I think are the same and one for the Ram 500 which was of course a 3.5 inch turbine.Both bypass's are produced by BVM. I have just measured the I.D of both. The P60/70 one is 4.5 inch and the Ram 500 one is only 1/16 less. If it was ultra critical then would they really use a common bypass for the P60 and P70? If you really want to get the right set-up then get BVM to supply you with a Ram 500 bypass but you will need a extended tail pipe because the p60/70 bypass is longer to cater for the 70.The Wren tail pipe is certainly better than the stock one supplied but its not easy to get it to fit in the BVM outer easy.
If you go with the p60/70 bypass make sure you ask for the p70 straps, I asked for the one suitable for the wren and they sent the p60 ones which are too short.

Jason

Ehab

My Feedback: (44)

Thx to all for the inputs. I may have to go thru a couple of pipe revisions to get the best results....we 'll see....

Miniflyer

I think soime people here are mixing around terms and expressions, which kind of makes it confusing....
There are 2 components that are talked about.

Number one being the bypass...i think this is what is talked about here mainly. Bypass is the ducting to and around the turbine, and aft around the pipe. The pipe itself is the metal tube leading the hot gases (and a certain amount of cooling venturi air) out of the aircraft.....

The pipe is most critical for static and dynamic thrust, most errors can be made here....too thin and your turbine won't be able to push out everything it needs to. Too large will slow down the airstream by overexpanding it.
The bypass is a neat way to get a clean airflow to and around your turbine, and getting a cool fuselage around the immediate turbine/pipe area. It will improve performance, just not by that amazingly much (it does more the faster you go, so for extreme speed a clean solid ducting and bypass do help more)...in fact, no ducting and no bypass at all also works perfectly.
Just be sure to get enough air to your turbine....which, with a too large bypass, should be no problem :-)
Best regards
Hank

Edgar Perez

My Feedback: (13)

Not sure I understand the over-expanding idea. If a plane does not have a pipe at all (exposed turbine, Kangaroo type, etc) the turbine do not lose thrust because of it. It should still put out the rated thrust. So how come a wider pipe make it lose thrust?

Thanks

Sheds

We have an old AD Exocet which produces an interesting effect in this area. It is fitted with an Artes JF50 Bee and has been very successful with this combination. It can carry up to 3 litres of fuel, but easily gets a 15 minute flight with only 2-2.5 litres. We used a Wren pipe for the Wren54 set at the distances between pipe mouth and jet exhaust established in RCJI during their Wright Pipe testing. The model does not have an internal bypass, so excess air at the intake is either sucked into the exhaust tube or passes out between the tube and the fuselage, providing cooling.

On its test flight the dorsal hatch was lost and we assumed that either the catch had not latched securely or had broken off. On the second flight with another dorsal hatch it too came off but was recovered, intact and undamaged. It became clear that the internal pressure inside the fuselage during flight was so great that it was blowing the hatch off. We reinforced it with carbon and the problem disappeared. Performance is impressive with large loops and excellent verticals, despite a supposed power to weight ratio of about 0.7. We have a near identical engine in a Baby Boomerang and the vertical performance is markedly inferior, despite being a much lighter model (although also much higher drag obviously!).

The Exocet was I believe originally designed for ducted fan and has large intake ducts and a relatively small tail eflux, so we believe that this is in effect supercharging the air at the point of entry to the turbine, with the Wren pipe then providing the optimum exhaust shape. With the Bandit and a Wren 44 you may find a similar effect, particularly if the ratio between air intake versus output cross sections on the fuselage are less than one.

You could experiment easily with this by using the Wren 44 exhaust pipe and gradually reducing the clearance between the outside of the exhaust pipe and the inside diameter of the bypass, enabling you to gradually increase the pressure at the front of the engine. I would be wary of enclosing it completely in one go though, as the drop in intake cross section to exhaust cross section on a 44 pipe would be large and I have no idea what the effects would be!

Measuring the increase in performance will also be tricky unless you have an onboard speed sensor or unless the performance increases are dramatic!

Worth a try though and good luck with it! I've heard that people have flown the CARF Eurosport on the non-gold Wren 44, so you should have no problems with the Bandit if that is true!

cheers
Sheds

EddieWeeks

The main reason thrust is lost with tail pipes is, the air expands quickly in any tail pipe.
But it does not slow down near fast enough.

The result is a large low pressure in the middle of the tail pipe. This low pressure
does 2 things. It sucks large volumes of air from the front of the tail pipe near the engine.
Second, and the most importantly. It changes the back pressure on the engine.

These engines run constant RPMs not constant mass flow. If you drop the pressure behind
the engine it just not work as hard and thrust goes down.

Static thrust is correlated very closely with engine exhaust temp. If your engine temp is low
your thrust is lower also...

Engine temp is correlated to exhaust velocity. Higher temp means more velocity witch increases thrust.

I made a tail pipe once where the thrust went up 1-2 lbs static.. Worked really well till the aluminum foil around
the tail pipe melted off.

Its very easy to make tail pipes.. 0.008" SS shim material and a drill and nuts and bolts... A bolted tail pipe
is far stronger than many spot welded one and cost about $20

All tail pipes drop engine temp and lower thrust. Some more than others. This is not allways a bad thing
cooler engines and far less fuel is burned is a good thing.

Eddie Weeks
http://www.variablepitchhobbies.com/

WrenTurbines

"If your engine temp is low your thrust is lower also... "

That isn't true. The more efficient the engine, the lower the temperature will be for any thrust.

"All tail pipes drop engine temp and lower thrust."

The pipe designed by John Wright with the tapered first section actually increases thrust, as many RCU users can confirm.


Sara Parish
Wren Turbines

EddieWeeks

my mistake.. I should have said all the pipes I have seen..


In theory.. you can make an injector pump increase thrust by a lot.
But you need a something close to 5:1 nozzel to pipe ratio to work best..


Eddie

Miniflyer

First as a rule: effective thrust is created at the exit point of the exhaust gases. If a turbine pushes 50lbs, and looses 10lbs through a pipe (exagerated), the turbine in front will most likely still be producing around 50lbs. The 10lbs can be found along the path of the pipe, pulling the other way.... (trying to paint a very very rough picture: the turbine pulls forward with 50lbs and the pipe pulls aft 10lbs, thus pushing the airframe 40lbs in total).

The theory behind it is not so very much difficult if abstracted a bit: take a long straw and blow into it. Air will exit at the other end very quickly. Now take a large diameter pipe and blow into it. Exhaust velocity at the other end will be miserable.
Energy, and thereby thrust, is defined with mass x velocity². You see, by lowering velocity you severely hurt your energy. By increasing mass you slightly increase thrust.
When the air exits through the nozzle and finds no pipe to keep up pressure and speed, it will expand more and more. Air surrounding the nozzle will be ripped along also (venturi effect), slightly increasing the mass, and at the same time slowing down the larger the pipe gets.
Most air is ripped along right at the nozzle, the further out you go the less the effect....so the larger you get, the less your gain in mass is. At the end of the airplane, where the effective thrust is created (total amount of air x velocity²), there will be less remaining to push ahead.
In an open air situation, thrust is also created right where the airplane ends: in this case right at the turbine nozzle. So you get exactly what the turbine is producing without losses.

The reason why the wren pipe "increases" thrust is also very simple: to a certain point venturi air cause little loss in speed but relatively high gains in mass flow (remember the part about most air being ripped along right at the nozzle?). The losses in velocity are more than accounted for in the gain in mass within this amount of expansion.
Slightly expanding the pipe as Wren does allows just this to happen in a controlled environment, thus gaining STATIC thrust. Dynamic thrust (moving airframe) will decrease though.....so basically you are buying more static thrust for less top-end performance. Since the exhaust velocities are so high, most modeling uses will not notice this effect at the models top speed.

Hope to have shed some light on this mystery...
Best regards
Hank

Ehab

My Feedback: (44)

Great info from all angles, still hungry for more data.....

Ehab

My Feedback: (44)

My goal is to minimize losses in the bypass and the pipe, maximize take off thrust and top speeds. I know any reasonably designed bypass/pipe will "work" in this installation but to optimize it is a totally different story. I will share my findigs with you....Again, keep it coming!

EddieWeeks

In practice, the tail pipes actually change the design of the engines !!!

Say an engine is running at 30 lbs at 650C.. Install this engine in a plane and it puts out 27 lbs and runs at 600C.

The reason it only has 27 lbs is not because of 3 lbs of skin friction of the pipe. Its because the pipe has caused a 50deg

temp drop. Has very littel to do with skin friction or tail pipe velocity. I have mounds of test data that show this.

Eddie Weeks

EddieWeeks

I was talking about one engine at a constant RPM, not different engines with different efficiencies.

Kevin Greene

My Feedback: (85)

From the information given in this thread I've come to this conclusion.....

A) Thrust can me maintained or even augmented to some degree with a cone shaped pipe vs a straight pipe....AKA the Wren pipe...

B) Thrust is lost through temperature decreases in the pipe....



My idea to reduce thrust losses is to come up with a pipe that is cone shaped and retains most of the turbine efflux heat without transferring excessive heat to the airframe.....

How about this....A cone shaped pipe like Wren Turbines has developed wrapped in a ceramic blanket instead of being of dual wall construction. The ceramic blanket would actually help retain heat in the pipe but provide enough of an insulating value to not transfer excessive heat to the airframe----

How about it?????

Kevin