Engine experts opinion required
 

Hi folks,

I plan installing my brand new MVVS.49 (rear exhaust + mvvs tuned pipe) into my Balsa USA Force One.

My question is the following one : how much power loss (due to exhaust gases flushing forwards) shall I expect from the pusher configuration ? 1-2 % ?

My guess is that when aircraft speed is constant or decreasing, then no power loss shall happen. The problem shall only happen on 'aircraft axis' positive accelerations, I mean when accelerating... That's what I 've learnt during my physics classes...

What do you think ? I bet you're going to tell me it won't be noticeable, but just curious to know if someone has a real good answer...

Thx.

RE: Engine experts opinion required
 

Is this a "ducted fan" application?? Or a "push prop".........I think your complicating matters here! You cant compare a F-16 or even a real MIG 21 to a .49 MVVS RC engine and model! If your so concerned about power loss then move the .49 up to a .52.....the Magnum XLA .52 ABS "Blue Head" is an excellent application! Or buy a Rossi .45 at 2.1 HP.....it will still outperform MOST .75 engines! Even if there is SOME power loss (1 to 2%)... I serioudly dought it will be noticable!! How much could it be?? NOT MACH 1.....that's for sure!!

The Slip!

RE: Engine experts opinion required
 

It is a pusher prop.
I actually don't care losing 1-2%, not even using a more powerful engine, but just interested in theory vs. practice....

RE: Engine experts opinion required
 

Strykaas,

Your estimate of the power loss should be about right at most.

The main problem will be engine cooling on the ground.


Enjoy

Jim

RE: Engine experts opinion required
 

Why not put a "U" bend in the header?

I think you will be pleased with that combo. ;) Airgar has one with a GMS-47 and it flies REALLY well!

RE: Engine experts opinion required
 

If I understand correctly you're wondering about the affect of the aircraft accelerating on the exhaust gasses which are travelling forward from the exhaust port? If you've got a 1:1 thrust/weight then it'll accelerate at 1G horizontally (until drag builds up) so there's a 1G force the exhaust has to work against. Now given that all the exhaust has to exit the cylinder in roughly 1/1000 of a second (depending on revs and the time the exhaust port is open) then I'd say your power loss would be on the order of 0.000000000001% :D

RE: Engine experts opinion required
 

Good hint, hadn't thought about that !! I'll take care.

Actually, the mini pipe would be too long for that. Moreover, it would increase drag and disturb airflow coming to the prop... It shall be manageable though, it's not a "no go"...

Any pix of his ship :D ?

Yeah, but 250 times per second @ 15000 rpm... Anyway, aceleration phases are not that long, therefore power difference will be very hard to notice I think... There have been many designs with pusher configs having exhaust ports facing forward with no problem... so...


Last, I forgot to say that I plan adding something (some kind of cowling) just in front of the mini pipe exhaust in order to avoid adverse dynamic pressure on the pipe exhaust. The only effect exhaust gases will have to fight against is aceleration...
We will see anyway ! There are other serious mods I have in mind... You will all see that later :D