SALMONBUG

My Feedback: (1)

How can you be so sure that the thrust remain the same ?

for an rc turbine the parameter the ecu regulate for a giving throttle stick position is rpm. usualy you are not max egt limited before reaching max nominal rpm.
In the case your air intake is partialy obstructed, your rpm (either he is nominal won't deliver you the same trust
Simply because there is less massic flow trough the engine.

it's late here in belgium and will be happy to explain you that more deeply tomorrow. After having flown for an belgian airline 10 years I returned to school to study more deeply the developement of high emperature gas turbines.
and the fact is that a rc turbine IS a gas turbine.......... So I think I could explain you that easily

regards

Helijet

By using my old and trusty fish / scale sliding thrust meter at max rpm, I found no noticeable change in thrust from running the engine on a bench or on the
Reaction. I thought at first that the tapered boat tail may create some turbulence or pressure difference effecting the smaller engines.
The RAM 500 was particularly susceptible to venturi effects of poorly designed inlet ducts. I found no such issue with any turbine
placement on the Reaction. The compressor while " partially obstructed " by the starter motor, can still easily acquire it 1.5 X inlet area
for operation - especially in its open air configuration on the Reaction.

Anyway, that has been my experience and I wish you good luck. Its a wonderful design by a very talented engineer!

Cheers
Dean W.

joeflyer

My Feedback: (48)

Moving the engine forward is common practice among R54 builders. The mounting will allow only about 1 inch of the starter to protrude into the "boat tail" (depending on the engine). That's how mine is and the plane is not lacking for power. The "boat tail" allows for a smooth transition well in front of the turbine. Turbines will pull in air from wherever it can, and as long as there isn't a severe restriction thrust won't be affected.

You can tell by just looking at it that it won't be a concern. If you look back through this thread you'll probably find several pictures of others that have done it. Here's a typical one that I just found.

SALMONBUG

My Feedback: (1)

And I guess you didn't forgot to apply the corrections to be able to compare your results accuratly. I mean you made the correction for the static air temperature, the static pressure and the humidity at the moment of each test.........

as said in the post just above the difference is neglictable, but don't say it doesn't exist........

I really go to bed now

Woketman

My Feedback: (6)

Dean is correct. There is plenty of free intake area in a normal install on this plane. It ain't gonna make any difference in thrust on any turbine. No shortage of intake area at all.

SALMONBUG

My Feedback: (1)

the main discussion was not about the design of the intake aera of the R54 but about the effect of partialy mask the air intake in general. The discussion started around the fact some modelers drill the R54 boat tail to be able to correcti the balance by moving the starter inside the boat tail , this move the air intake closer to the boat tail.

Maybe my wrong english makes that what I writte doesn't reflect what I want to say, so I will writte very simple phrases that reflect what I want to say .The two next lines :

Some people says that the for a given rpm on a given turbine engine, the shape of the inlet has no effect on the thrust........

What I say is simply that this is not thrue.........


In some case the difference can be unsignificant ( and I am sure this is the case on the modification we are talking about on the particular R54)
What we should be aware about when discussing about this is that rc jet engines are extremely primitive designs and the way we install them is also extremely empiric.
For exemple of the ruticity of our engines I will simply say for example that the name "reaction 54" is because the plane prototype's engine was build around a 54 mm centrifugal compressor coming right out of a car turbo.
by comparison I would say that the bmw003 ( engine fitting the first real heinkel he162 during ww2) was already fit with a 7 stage axial compressor.
What made possible the developement of RC turbine engines is not the engine concept itsfel, it's the developpement of the small electronic component that made possible to build ECU smaller than a shoebox
The engines are primitive because they are cheap, they are easy to operate, the maintenance is unsignificant and also nobody care about the cost of the jet A1 for a sunday flight session and also nobody would be ready to pay for the developement of rc engines fitted with compressor stall protection ( like variable stator vanes and so on)
We buy them, we install them and they do the job as soon we follow some general rules and we don't care about the limit of the performances, we fly.and we have fun !!!

If the engine, surge,, broken or flameout, we crash and we start a new winter project. We don't kill anybody

This discussion is deviating from the main subject of this thread, that's my last post about it. I'll be happy to discuss this subject in PM

Enjoy building and flying your r54

Woketman

My Feedback: (6)

The shape of the intake can indeed have a huge effect... when there is ducting. But in the case of an R-54, the engine is out in the open air! THERE IS NO CONSTRICTING DUCTING! The engine is able to breath with no significant constrictions, therefore there is no difference, it is out in the open with no intake issues. Dean was absolutely correct, beyond doubt.

SALMONBUG

My Feedback: (1)

Some last words on the subject

Let's talk about the r54 configuration and particulary about the boat tail end and the compressor.
let your imagination work a bit and imagine a jetcat p60 or any kind of similar engine on a testand with . a dozen of sensor monitoring and recording all the possible data about it . Now imagine a volunteer holding a nice R54 boat tail in his hands and slowly approaching it from the compressor runing at max Rpm. the engine parameters will vary for sure..........
Quantify and determine the variation and the reason of it is complex.
If I had to predict what will hapen I would say ( but without any certitude) that either the engine receive enough airflow,it will be affected by the fact that the angle off the air attacking the compressor blades will vary and then reduce the compression ratio .

The first example that come in my head about people who have been disurbed and had to solve that complex problem is when lockheed and douglas went in competition to build a 3 engine long haul airplane.
lockheed decided to place the engine in the back of the fuse and build a long curved air intake going from top of the fuse to the engine. that was the lockheed tristar
douglas decided to go with a apparently more difficult challenge and place the engine on top of the fuse ( requiring to design a complex strong vertical fin). The dc10

lockheed losted the competition simply because they encounterd unexpected enormous problem with the design of the intake and also because in flight at high body angle, the fuselage was maslking the intake and the plane came on the market a bit too late

Woketman

My Feedback: (6)

Nonsense. The front of a normally mounted R-54 engine is unobstructed, totally. The angle of incidence of air striking the compressor blades is rendered mute due to the symmetry of air coming in from both sides, again, unobstructed. Dean is 100% correct.

SALMONBUG

My Feedback: (1)

so you say that having a boat tail just in front of the compressor is the same that having nothing in front off it............

that's your opinion, not mine

Woketman

My Feedback: (6)

Since the engine still has plenty of cross sectional area to draw from at low R-54 velocities, YES. It is the same as having nothing in front of it. Absolutely, without doubt. Dean is correct.

SALMONBUG

My Feedback: (1)

if I extrapolate what you say, you can have anything in front off an engine, without consequences...........
I explain myself

a boat tail is nothing else than an "object". his shape is optimized to not perturb the airflow entring the engine, but unfortunatly a 100 procent efficency is something that simply doesn't exist., so it perturb the airflow ( very very slightly)
according to what you explained this very small perturbation has NO effect,, this mean that a huge perturbation has also no effect........ ( multply 0 by anything you want, this should remain 0).......

thank you for this interesting demonstration

end of the discussion

Woketman

My Feedback: (6)

Whatever dude. the fact remains that Dean is correct.

SALMONBUG

My Feedback: (1)

according to what I read, Dean made test using a fish scale, I don't think that's the best way to quantify the nearly unexistant loss off performances of an rc gas turbine runing in front of a R54 boat tail

the fish scale test must be something extremely difficult to perform, in fact the turbine is on a "carrier" (a airplane or something else.and the carrier should pull on the fish scale, great !!!!
but how do you determine the amount of friction between the carrier and the ground ? that's important data for the test

mr_matt

My Feedback: (10)

Hey SALMONBUG, you got bigger issues to worry about!

Did you know your battery weighs more after you charge it? So you better double check your CG, it might be moving around a lot. A femtogram here or there can really add up!

SALMONBUG

My Feedback: (1)

I edited this post to remove my answer, see reason below

CARS II

My Feedback: (7)

Like I said earlier, why reinvent the wheel? Bruce did an excellent work on the design and a bit more drag, weight or less or more trust won't make that big of a difference, build it, fly it then fly it some more

You get my point, right?

​​

Woketman

My Feedback: (6)

Salmonbug is starting to remind me of Tracy Jenson! Hey buddy, tell Travolta I said hello when you deliver his Playboy jet!

SALMONBUG

My Feedback: (1)

either I don't understand exactly what you say ( lack of english speaking) I clearly understood you don't like The way I ended up our discussion, I admit I have been offensant and I apologize for that ( I am not in a good day, my mother is 87 and entered the hospital this morning with a one way ticket)
I go work a bit in the workshop to change my mind. and then to bed

Keep up good job on your planes

why_fly_high

My Feedback: (19)

I have been privy to a bunch of testing by a University. Our engines pull air from wherever they can get it. You can put a plate at the end of the starter. If there is enough area around the plate to get enough air in it will run fine, temps normal and thrust normal. Doesn't help drag, aircraft speed, etc. but will not effect engine. No way the boat tail messes with the engine.

Check out this one. A bunch of testing. for non aerodynamic reasons the inlet was best on top. The engine front was centered on the hole on top.

Woketman

My Feedback: (6)

Exactly!!!!

causeitflies

By moving the starter motor into the boat tail we may actually be making the compressor more efficient by getting the starter out of the air stream. So go ahead and stick it in there. I did and it works fine, especially with inlets.

SALMONBUG

My Feedback: (1)

Midnight here, not sleeping.

I was in my workshop framing the r54 and thinking "why am Icertain to be right and all other people say I am wrong ?"
why members claim the compressor will work just fine either there is a wall in front of it, regardless the true fact (for example) that a tail mounted engine in a real airplane has special protections that will discharge the compressor at high body angle because the fuselage mask air intake and that test made at sabca five years ago with a old jt8d showed engine performances degradation in front of a group of students when the air intake was partialy masked .......
Why?
Well the answer is that what I claim is based to the fact I think radial compressor......
and you think centrifugal compressor.
Because rc jet engines are fit with centrifugal compressor, you are right, they can accept a lot.........
I'll sleep better now

CARS II

My Feedback: (7)

They are not saying you are wrong, what they are trying to tell you is that the blockage, airflow disturbances are so minor that no one is concerned about it

We appreciate your opinions on the subject but let's stick to the building, set up and flying of the sport rc jet and please keep posting pictures of your work and install.

Thank you.

causeitflies

Yes, you mean "axial" compressor. Radial and centrifugal are the same.
Sleep well.