mikes68charger

My Feedback: (34)

So I'm trying to decide on what motor I need for my 1/4 F86 wood project I'm building. So the original designer used a Beotec turned down around 40 pounds of thrust. I know in most of my jets I jump to the biggest motor I can get (ie even thow my Diamond can fly with a K160 I got a nice K210 in it) But this is for sport use. But In this big F86 its more of a big floater than a heavy scale bird. I have seen a few Tomhawk models F86 which is bigger than mine 123inX123in vs mine at 112X112in flying with K210, and they tell me they fly at 3/4 stick as a big floating jet.

So I was looking around and was surprised to see how many K170 are for sale on this site used, then looking at my stock, I got a Merlin MK2 160 sitting on the shelf.

But what struck me as odd while looking around is the small amount of thrust difference between these larger motors.

IE:
Kingtec 160 = 35 pounds of thrust
Kingtec 170 = 36 pounds of thrust
Kingtec 180 = 40 pounds
Kingtec 210 = 46 pounds
Merlin 160 MK2 36 pounds ???

This seems like such a small amount the would it even be noticeable?

In my Carf Eurosport I had a P120sx at 31 pounds then put a Jetcat Titan at 34 pounds while this motor only lasted 3 months before it exploded but I have to say I could not really tell much difference, a slightly shorter take off. but this is a very draggy delta wing jet.

This is my exsperace, what's yours?
Is it really worth buying another motor when I have 36 pound Merlin ready to us vs the required 40 pounds recommended.
Its odd the cost difference between K170 vs K180 with only 4 pounds more thrust.

Henke Torphammar

Money talks. How much are you willing to spend to get that last bit of performance? How much performance in terms of thrust, spool time, weight and fuel consumption would you gain or loose by adjusting the budget?

With the new K70 out would you go for the K60 or K45 at a lower price? I bet you can find good samples where it makes perfect sense keeping the old engine, I also think there are many applications where a new engine and maybe whole new setup would make the plane fly so much better.

kevinthoele

My Feedback: (1)

I remember flying the BVM F86 on an 8 pound engine at 200 mph. I think the F86 is a good one for smaller thrust engines. I think your and the shelf motor will fly it fine. It wont have unlimited vertical but neither did the original. Its all up to you. I would put a 210 in it, if it were mine.

Good luck which ever way you go

mikes68charger

My Feedback: (34)

LOL thanks, it always seems like 210 is the answer for almost all jets LOL I always love exstra power, but I m worried this being a wood build vs a composite build, I don't want to push it to hard.

FenderBean

I think what has happened is just updates, as these companies continue to refine their turbines with small tweaks they add it to the list. With this in mind they still have the older products to sell since not all can be upgraded. I wish they would work on becoming more fuel efficient, we always over power our stuff so I would give a couple lbs of thrust for a more fuel efficient turbine. If that’s even possible with this type of turbine.

SECRET AGENT

My Feedback: (18)

Why not fly it with the 160 and if you don't like it then start looking for a new motor. Who knows, you might love it with the 160.

erbroens

IMHO small differences on static thrust are less important than user familiarity, presuming you are happy with your choice of turbine brand. :-)

erh7771

My Feedback: (30)

My experience is 50% more thrust past a certain thrust level doesn't push the plane 50% faster but you do get shorter take offs or higher verticals.

If neither one of those things are of importance then the +/- 20%'s don't make that much of a difference except in turbines that have a ... dependable ... faster spool up.

The other thing of importance is the pipe which needs to be built for higher efflux speeds if you want a faster jet.

Flow loss stinks I'm all for rear mounted turbines like a couple of the skymaster jets.

olnico

Yes. Speed is all about efflux velocity.

Flow loss is a vague term. If you include induced aerodynamics drag, rear mounted engines without any intake duct all the way to the powerplant result in a much bigger flow loss than a properly designed pipe, which increases efflux velocity.

Generally speaking, a full intake line with full bypass and properly designed pipe dramatically increases aircraft performance.
Here is a video of the Crusader at 41 lbs takeoff weight with a B-140F engine ( 31 lb thrust ). Flight performance does not look like it has a power-to-weight ratio of 0.75.
This is because in-flight efficiency of the powerplant and airplane internal aerodynamics are properly designed.

<iframe src="https://player.vimeo.com/video/247979404" width="640" height="360" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>

1/7 Scale Crusader flight display from Oli Ni on Vimeo.

Dave Wilshere

External aerodynamics ruined by that dangly leg ;-)

erh7771

My Feedback: (30)

Head loss or the more general phrase friction loss through pipe or ducting

My understanding is most of efflux out of micro turbines without pipe is around mach 1-ish IINM, seems like plenty enough speed for a scale model jet but I'm old school in flying scale jets a little above avg scale speed.

FalconWings

My Feedback: (57)

BAM! Couldn't agree more Oli.. The key to smoother engine performance on models is mostly hidden at the inlet. And this is no coincidence. Generally speaking, a good pipe with a good bypass is easy to develop, so keeping that exhaust flow "flowing" is not too complicated (and fairly easy to optimize). Problem is when you install an engine in the tail, you have almost zero major losses on the exhaust side (but a lot of momentum drag), but the losses on the airflow path to the engine inlet are pretty horrible. For our hobby.....designing a proper inlet with proper pressure recovery (to minimize performance losses) is mostly unfeasible....it requires significant CFD modeling knowledge that is typically not available to the model developer in general (by modeling I don't mean CAD). SO....the overall best approach is to minimize the inlet complexity, and compensate with a decent bypass and exhaust design (which us far less complex!).

Ironically I learned this on my first turbine jet model, years before I became a propulsion engineer. It was a BVM Maverick with a turbine and stock DF inlets. It sucked. A friend changed the inlets to a pair of BVM turbine inlets and BAM! totally different model.

David