Pete737

Buying your first Turbine ARF without ever having seen one, these questions are inevitable [sm=red_smile.gif]

I'm curious as to the purpose of the turbine bypass. Intuition tells me it is designed to direct the incoming air directly to the turbine. Thats what it looks like it would do anyway. If so, Why is it called a bypass?

Another gentleman I spoke with briefly on the phone told me something that seemed a bit counter intuative. He said it DID bypass the air AROUND the turbine, In order to prevent any turbulence within the engine compartment. How is that possible?

Again, I realize this is a very easy question for you guys but Im a bit confused due to conflicting data I have recieved so far.

I did run a few searches on the subject, all pictures I came up with seem to show what I thought originally.

Please excuse my ignorance, I just want to set this straight.

Thanks, Peter

DelGatoGrande

My Feedback: (23)

Hey Peter!hapy new year

bypass control the entrance of air and exit of air and hotgasses in a model.
It will give better performance since there is less buildin "airbrake" in the model.cose the air will flow more "clean" without produsing torbiulence from objects in the way..
Like in ductedfans..
Also will cool down pipe and engine better...its called bypass cose it pass the "rest"air over the turbine and in the pipe.

Bottom line..dont use it unless you need it my friend!simple thinks work better..My 2c

Sorry for the bad English boys

P.s. Send by iPhone and it's dam hard

Harley Condra

My Feedback: (4)

Pete,
This question has been asked and answered many times on RCU.
Here's a synopsis of what the bypass tube does for the installation.
1. Air from the inlets goes directly into and around the engine without pressurizing the fuselage. A pressurized fuselage is going to be more draggy than a bypassed installation.
A properly designed bypass tube installation is a good drag reducing device.

2. Not all of the inlet throughput is used for combustion. What the engine doesn't need for combustion and internal cooling, is directed around the engine exterior and
is used to carry away excess heat.
This air flows around the engine core exterior and is mixed with the exhaust gasses in the tailpipe, helping to cool the high energy exhaust gasses.

3. Engine suction and/or ram air cannot FOD the engine with servo leads, loose nuts and bolts, or anything else that might come loose inside the airframe.
An unbypassed installation (a plenum installation) should use an FOD screen to prevent ingestion of foreign objects.

4. In the unlikely event of a start-up fire, the bypass tube will give the operator a few extra seconds to extinguish the fire without damaging the fuselage. I would rather replace
the bypass tube than replace the fuselage. A good bypass tube is constructed with carbon fiber and hi-temperature resin, and will not burn as fast as the fuselage will.

5. The fuselage skin on a bypassed installation will be slightly cooler than an airframe with a plenum installation.

6. Noise. Correctly designed bypass installations are much quieter than plenum installations. Next time you see a BVM Bandit at a meet, take notice of how quiet it is
coming toward, and abeam of you. Some plenum installations sound neat, but that noise is the sound of drag. This drag reduction is one of the reasons that Bandits are fast.

7. Airflow to the compressor is unobstructed....no equipment in the way to obstruct the airflow.


Hope this helps....

tamjets

My Feedback: (2)

Let me add few tips to bypass.

Full bypass restrict air flow in the pipe at low airflow speed. This cause the pipe to run hot and great power loss.
Running bypass is good idea to open some holes at the rear to allow air draw in the pipe.
This help you accellerate faster and kept the pipe running cooler at idle.
Not worry about air escape from bypass. The airflow around the engine is not enough. There is more air suck in the pipe than air flow in the bypass.
We had done this with our bypass and got great result.
Here is the picture. The bypass we had 5 holes around the back. The holes is the size of the Nickle.

Pete737

Thanks and happy new year to you!

That was an awesome explanation!

Only thing I will look into now is exactly how much MORE noise does a plenum installation produce? This will probably determine whether or not I use the bypass. I want it to be LOUD!!

Thanks, Peter

Pete737

Not challenging you, Im just curious as to how that mechanism works. Shouldnt the engine pull in what it needs from the intake? regardless of having holes in it?

Thanks, Peter

Pete

causeitflies

The holes are for the air being pulled into the pipe, not the engine.

Pete737

The bypass? How could air be pulled into the bypass and not utilized by the engine?

Is heat propagated forward of the turbine inlet? Is the end of the bypass subject to heating by the turbine? I thought most of the heat is moving rearward with the airstream?

Pete

causeitflies

The holes are at the rear of the by-pass, not the entry end.

Pete737

But dosen't the entire bypass lead to the engine inlet area?

I guess I just don't understand [].

Pete

causeitflies

Pete,
I'll try to draw you a picture in a little while, Have to run an errand now.

Pete737

Thank you so much.

Pete

JMCJET

again... there is negative pressure inside the exhaust pipe. the higher the speed of exhaust gases, the higher the negative pressure... so, we need air inside the pipe so that the pressure inside the pipe equalize the pressure outside ( atmospheric pressure) or we will get a collapsed pipe. Cooling is another question, but uses the same law: Bernoulli`s law.
Making it easy, it says that the addition of pressure and speed remains unchanged: speed+pressure=k. If the speed goes up pressure goes down, because the addition of the two items remains unchanged. The air finds is way to reach the inside of the exhaust pipe

Pete737

Thank you sir but I am somewhat educated in fluid dynamics, I studied it briefly in college.

I understand pressure gradients. Unfortunately Bernoullis pricipal is somewhat inaccurate in this application since the fuse is not considered an ideal resevoir. Also the gas system is not isobaric due to the combustion process..

Regardless, my problem was in understanding cooling at the rearmost end of the bypass and its relation to the holes being drilled.

Thank you, Peter

KC36330

the exhaust gas creates a venturi on the holes drawing in air which is cooler and denser then the hot exhaust gas, this cools the pipe and actually produces more thrust. on a non bypassed installation too large a gap at the bellmouth can draw in too much air and cause chocking and reduced thrust becase of it.

Desertlakesflying

My Feedback: (28)

Looks like you all have it covered.....lol Less heat, more thrust, a little safer. When they finally produce a low bypass axial flow turbofan turbine for models it will be a great day.

JMCJET

So, Peter:

If you study it briefly I can only suggest you to study it deeply. Remember, that what is written here, is read for many people. If you disagree or do not understand, you can ask again or expose your facts. We can always learn something... The answer for the holes is well explained in this thread. And in other threads in this forum. You would be amazed if you know what Bernoulli did for us. From the intake of our jet engines to the exhaust, passing by the wings and disturbing our landings, everything is related with Bernoulli.

Happy new year.
JMC

Pete737

Indeed and thank you.

I feel I knw enough about physics in general to understand.

Bermoulli's work would obviously be the dominant mechanism here, especially for a conceptual understanding but, As I previously stated It can be somewhat decieving in this particular aggregation.. Bernoulli's principal is only partially valid if the gas (airflow) involved is used to do work, Such as is observed during the combustion process.

Since energy (kinetic) of the gas flow is utilized in the expansion of the gas, Bernoulli's application here is somewhat tenuous.

I don't disagree with you, Nor did I fail to understand you. I feel you may have misunderstood my question, I apologize if it was written somewhat ambiguously (I was never a good writer).

Thanks! Pete

invertmast

My Feedback: (23)

Lets see...

A bypass is basically like sticking your turbine inside a 2 liter plastic bottle w/ a hole on either end (inlet and exhaust). Their is typically a gap between the bypass wall and the turbine itself (about 3/4" to 1" i believe). The air the turbine needs for combustion and running is drawn into the turbine. What air the turbine does not need "bypasses" the turbine in the gap between the turbine and the bypass and into the tailpipe. Some setups have both walls of the dual wall tailpipes inside the bypass, so the "bypassed" air goes into both the wall of the tailpipe. Some only have the thrust tube wall of the pipe inside the bypass. So the holes in the back (like in tams pic) allows more cooling air to get to the pipe to help cool it on the ground and slower airspeeds...

AVIOJET

And for the people at the other side of the Atlantic Ocean, what is the diameter of the Nickle? The five holes are located at 72°x5=360°?

Thanks in advance and Happy New Year from Spain

Jesus

jetnuno

Hi

In fact there is one kind of set-up that is not covered.

I had the same doubts that the author of this thread has. That's why after reading all the posts in the herunder link, I have decided for half by-pass.

http://www.rcuniverse.com/forum/m_58...tm.htm#5867962

Main reasons:

I like too see the start-up of the turbine.
More thrust reported for the F18F airframe from several users. I haven't notested nor mesured the thrust difference.

Downside:

As stated before in the event of a fire, the full by-pass will give you the extra seconds to put the fire down (or maybe not)
Full by-pass is reported to reduce noise.

I have started flying with full by-pass and the chaged to this set-up

Regards


Nuno

tamjets

My Feedback: (2)

It about 3/4". Sorry about that.

ps2727

I have a question for the pipe experts here. I've seen setups ( BVM type ) where the ducting is continuous from the intake to the exhaust with no openings into the fuselage. My understanding is that this is "fully bypassed". Other setups such as the one posted above show the engine in an enlarged section of duct but it is left open to the inside of the airframe. Is it correct to describe the latter type as a bypass since the open "hatch" area does not seal the airflow? Seems that since the duct is open to the fuselage it's more of the plenum setup described earlier.

Any comments on the benefits of either setup?
Thanks,
Paul

causeitflies

My explanation for the holes:
The high speed/low pressure exhaust flowing into the pipe draws a huge amount of air by venturi effect. This air cools the pipe and can through expansion increase thrust (with the proper design!!).
With no by-pass the engine gets the air it needs and the pipe gets the air it needs.

With some by-passes the restriction doesn't allow the pipe to get enough air and/or the engine and pipe are competing for the same air. This is especially true at low speeds and high throttle where very little air is rammed in the intakes. Performance of the engine will suffer.

Cutting holes in the aft of the by-pass allows extra air to be sucked into the pipe so it and the engine get all the air they need.
Some sketches below may help to explain:

tamjets

My Feedback: (2)

Good drawing Mark.
That is proper way for using bypass.