What is the purpose of a bypass? What are the determining factors for installing one? Why do some jets have them and others do not?
How do I know if I need one? I am about to assemble a Jetlegend T-45 with a P-70.
Thanks for the help!!
Dom

A bypass tube serves multiple purposes. It keeps the engine from sucking up crud or debris from the airframe interior....you know, servo leads, UAT fill line plugs, a mouse that might have moved in last week and set up houskeeping, etc.

In a bypass installation, all of the airflow through the inlets goes down the bypass. Sure, some of it will leak out, but the airflow necessary for combustion and engine interior cooling is inhaled by the engine, and the excess has to go around the engine case for engine exterior and tailpipe cooling. Only about 25% of the throughput of the engine is used for combustion anyway...the remainder is flowing around the outside surface of the combustor to keep the circumfrential temperature average within limits. Need to cool the combustor, or we will burn it full of big, irregular holes.....just like Swiss cheese. This cooling air leaves the engine with the combustion gasses, and goes right out the exhaust nozzle, and out the tailpipe.

The airflow through the bypass aids in engine cooling, and is a drag reducer in flight. Most un-bypassed enclosed installations make a little more noise in flight, and some flyers think that noise is "cool" sounding. It may be, but the reality is, that "cool" sounding noise is the sound of "DRAG".

Why would you want to pressurize an airframe interior with ram air from the inlets? When the air goes down the bypass, it can be used for combustion and engine and tailpipe cooling.....not for pressurization of the airframe interior.

Here's my point:
The bypass tube might not make the engine any more efficient...say what you want....but it makes the airframe much more efficient, through drag reduction. The engine has to force the airframe though the air......why spend your life trying to make a very slick, smooth aerodynamically clean exterior surface, so it can slip through the air like a shark slips through the water, then throw out the anchor (or in this case the drag chute) by pressurizing the airframe interior with a plenum type installation?

Go to a major jet meet, and you will probably notice that the fastest airframes, with the quietest engine installations are bypassed.
Why doesn't BVM use non-bypassed engine installations? I can gaurantee, it is not to sell bypass tubes...it's all about performance.

Point of fact:
If the bypass cover is removed from a Bandit before flight, the ram air pressure from the inlets will blow the power system hatch off the airplane before it hits about 175 MPH.
Then, the canopy hatch will slide back, and then? The CG shifts aft, and the jet is a little harder to fly. You will spend some hard earned $$$$ with Uncle Bob to repair the damage. All because of ram air in the fuselage. Let's keep ithe ram air IN the bypass tube.

IMHO, those are my pros and cons and my .02 worth....what about yours?

Harley Condra
BVM REP
JetCat REP

Hi Dom, I would suggest just simply install a FOD guard on your P-70 and go fly without a worry....
IMHO a bypass is really not of any advantage on models at the speeds we are operating at i.e. sub 200 mph. For a bypass to offer any significant performance advantage ideally the intake duct shape/design, bypass itself and your tail pipe specification ALL need to be designed/optimized around your specific engine and airframe combo... I believe by simply adding a generic bypasses offers no real measurable 'performance' advantage per say especially in the 'slower' flying (sub 200mph) jet airframes and this to best of my knowledge has been proven to be the case, It really requires an optimized package intake/bypass/pipe overall for each engine/airframe I reckon regardless to show a measurable benefit. That said tailpipe design can dramatically effect thrust and cooling also.... but thats another story!

Cheers-

Wow thanks for the information Guys!!!!
Just what I was looking for.
Dom

On my Westbury Hunter which I'm building it has a fully ducted installation and a single walled pipe. The rear of the fuselage is completely sealed with a bulkhead at the rear of the engine bypass, so no cooling air can get into the rear part. Both my previous planes have been open installations which allows plenty of cooling air to flow through the fuselage.

I am concerned about heat buildup in the rear of the fuselage and causing damage. How do other fully ducted installations cope with the heat?

One option I have read is to cut some holes in the shape of the NACA inlets into the convergent section of the bypass to allow some excess air to vent into the rear fuselage around the pipe. I also believe that this may also improve the static thrust of the installation. Has anyone tried this?

Thanks

Paul

Paul,
In a fully ducted (bypassed) installation, it is a good idea to allow cooling airflow to exit the nozzle area by using an augmenter at the nozzle. Take a look at a BVM Bandit some time, and you will notice a gap around the nozzle that allows air to be sucked from the aft fuselage by the exhaust gas velocity. The tailpipe is actually positioned about .700 in front of the augmentor sleeve aft edge, creating suction in the aft fuselage to carry heated cooling air out of the fuselage.

The top of the aft fuselage will always get hotter than the bottom, so you can make a lightweight insulation blanket out of BVM ceramic heat blanket material (Fiberfrax) and cover it with aluminum foil. Make a template out of paper to fit the fuselage shape, cut the ceramic to size with scissors, glue the aluminum foil to it, and glue it to the upper fuselage inner skin with dots of high temp silicone adhesive, foil side out. High temp silicone is made by Permatex (and others), and is red in color, and works extremely well for this job.
With the turbine running, you will not feel any more heat from the pipe.

BVM "Heat Shield" is a water based silica compound (paint brushable) that will insulate any surface that you apply it to. Use three coats, with drying time between, and the surface will be protected from direct flames long enough for you to blast it with CO2.

IMHO, you should still allow the heated air to exit the fuselage, lest it becomes an oven.

Static thrust isn't all we are looking for....Dynamic thrust is what flies the airplane.
Measuring the static thrust isn't a total indicator of which combination will go the fastest.....The airframe flies in a dynamic environment..... Aero, thrust, total drag count.

harley Condra
BVM REP
JetCat REP