I posted a similar question in the general forum, but what nobody was able to explain concisely was what the percieved advantages of using an inverted motor over a sidewinder.

Reading over the replies I got I'm starting to think that, apart from looks, it doesn't really matter, unless you're using maybe a Fox 35 or a larger PAW diesel.

Why is the concensus that inverted motors run better, and what exactly is "run better".

I've heard the arguements about the fuel head position relative to the needle valve in a sidewinder, but surely a similar arguement holds for the needle valve feed in an inverted setup, since the hole in the needle valve is located in the centre of the venturi, generally on the centre line of the motor.

Just to explain where I'm coming from, yesterdays flying session was a lesson in frustration. On it's wheels, the motor was either too dry or flooded, with no obvious in between, but when turned over it started in 3 flicks...

I posted a similar question in the general forum, but what nobody was able to explain concisely was what the percieved advantages of using an inverted motor over a sidewinder.

Reading over the replies I got I'm starting to think that, apart from looks, it doesn't really matter, unless you're using maybe a Fox 35 or a larger PAW diesel.

Why is the concensus that inverted motors run better, and what exactly is "run better".

I've heard the arguements about the fuel head position relative to the needle valve in a sidewinder, but surely a similar arguement holds for the needle valve feed in an inverted setup, since the hole in the needle valve is located in the centre of the venturi, generally on the centre line of the motor.

Just to explain where I'm coming from, yesterdays flying session was a lesson in frustration. On it's wheels, the motor was either too dry or flooded, with no obvious in between, but when turned over it started in 3 flicks...

Hi

Yes typically they will run better than a side mount engine in full fuse Stunt ships. The main reason is that the fuse is much stiffer in the up and down direction than it is side to side. Many stunt engines shake ,and the shake in the same axis that is bad for them to be mounted on the side and better mounted either right side up or inverted.
 You need a really stiff front end to make many engines have good  motor runs.
Many people have had massive problems with sidewinder engines. Bill Werwage for example had one that was side mounted and kept getting harmonic vibrations and it drove the engine runs..and him crazy for a while. He also had the exact same plane same motor mounted inverted with no problem. This is just one example of the many many people this has happened to.
If you look at a profile you can see the engine shake or oscillate side to side from the vibrationtal forces generated by the motor. If you take the same profile and make an upright mounted engine..like Al Rabe did with the Mustunt.. it will have a stiffer front end and many times a much better motor run.
This is also the reason adding cheek cowls to a profile model will make the same  profile stiffer ,and it will reduce vibration and many times help with better motor runs.
As far as you not being able to crank an inverted engine that should not be a problem, People have been doing that for decades with 1 flip starts. You just need to learn the sequence that will prime the engine, then flip it through 6 or so times very fast to vaporize the charge then it should crank right away.

Regards
Randy

Pretty obvious Randy and I see things from a different view. I have seen and personally experienced poor engine runs caused by making the nose too stiff even with inverted mounted engines. The most notable was Paul Walkers Bomber. The other example was building a really stiff nosed twister and not being able to get an OS 35s to run right when hundreds of stock twisters have been built with 35s power without problems.

This is one of the engines in Paul Walker's bomber, as flown at the worlds and the NATS.. Side mounted on an RC mount...

Hi Bob

This is not a very good example of what I just wrote about, I was generall speaking of the  typical stunt ship.
 Paul's B-17 is not a typical stunt ship, it doesn't have a typical 4-2 engine run, and does not use engines that shake very bad.
Pauls  4 engine B-17 has almost nothing at all to do with typical stunt ship nose construction.
It is a fact that it is easier for an engine to shake or oscillate the nose side to side as opposed to it shaking the nose up and down, in most all typical stunt ships construction.
 Paul ran the little 15s  in a  2 cycle and they are smooth engines, compared to say an ST 60 or ST 51 or most any other  stunt engines used on the typical stunt ship.
There are many planes that people have built that have RC type radial mounts, mounted at many differant angles. I never said  these would not work.
I was very specific to talk generally about typical stunt ships and  typical (4-2) stunt runs.  These don't really include 4 strokes  OS 15s  or  electric motors.
Although I think you are much better off with a 4 stroke having a very very solid nose.
I know that other type mounts work, I have seen many examples of that. Some even  mount engines at the 7 O Clock position.
I am sure you have good runs with you 4 cycle mounted sidewinder, However place a ST 60 there and it will most likely be a different story.
I have personally seen very bad problems with ST 51 and ST 60s  on side mounted  profiles. They would have been  much better off to do as you have shown  and  built a better frontend on the plane and used an RC type mount.

Randy

Hey, all I'm doing is trying to tell the guy to go for it, he doesn't like starting inverted engines and the odds are pretty darn good a sidewinder will work just fine. Maybe the right answer is go buy an electric starter.

Back in the day Bill Hopkins produced a line of excellent kits - the profiles of which all had their sidewinder engines mounted inboard. He used plastic tanks for these.  Rational had to do with the fuel not being slammed up against the glow plug due to centrifugal force (or something like that).  I have seen some excellent runs w/engines mounted in this manner...but it's not a practice I follow.  RE starting:  Many stunt planes have to be started upside-down (w/engine upright).  It's not a big deal however if you are recruiting a new helper talk about it and practice the "hand-off" at least once before trying it with the engine running.  Not recommended solo.

EI168,

I suspect that inverted installations started mainly for a sleeker look. The engines, once running, should act the same upright and inverted, since we fly both ways in the stunt pattern. Tank height may - usually does - need tweaking to get it to do that, but once you have that, it should be dependable. Sidewinder tank mount height for most engines is usually close to centered on the venturii (or shaft) centerline height, but Fox 35s on profiles DO like the uniflo inside vent around 1/4" higher (when right side up.)

There is a difference between fuel head conditions for upright/inverted mount and for sidewinders. The tank width in a built-up fuselage model (upright or inverted engine) is generally centered on the engine crankcase/shaft. That's at least one inch further inboard than a profile tank CAN be mounted, unless you dig out the outboard doubler and sink the inboard side of the tank further inboard... To get the same "centered" condition as on a built-up, upright or inverted mount fuselage, you might have to cut away the inboard doubler to clear the tank, too. Even then, there might be a difference in how it runs, usually blamed on the way fuel pools up inside the crankcase when profile mounted.

Hopkins' inboard sidewider engine/inboard tank layout models had a GREATER fuel head at the start of a flight, which gradually decreased as fuel burned off. An outboard/outboard layout has the opposite condition - largest positive fuel head at the start of a flight, decreasing all the way as fuel is used up. And, unless you carve up the outboard doubler, the out/out layout starts with the inboard wall about 1/2" further outboard than the shaft centerline. Hopkins' inboard/inboard layout's LEAST positive fuel head distance is about the same 1/2" to the high side.

BTW, I know that Bill H. probably didn't originate the inboard/inboard configuration, but since his nice flying foamies were mentioned in here, refer to them...

A proper uniflow tank setup can make enough difference that actual fuel head distances don't affect feed. (Fuel head distance is the height, measured along the direction of "gravity" from the fuel surface to the fuel jet - NVA outlet orifice - which we usually don't think much about.) The big secret of uniflow tanks may be that the "surface" open for venting is where vent air enters the tank shell volume, near the outboard wall. That then becomes the fuel surface reference involved in figuring "gravity head"...

The direction of the "gravity" force the fuel feels is due to the combination of simple gravity - 1 g, aimed at the center of the earth whatever the position of the model is - and the centrifugal force load, 3 g or so (generally accepted for stunt or sport stunt models) aimed horizontally outwards. (The axis for the CF radius at any given time is the vertical line through the center  of the flight path hemisphere.) So, the "effective gravity" aims out about three, and down one - at least in level flight. If the 1g and 3g estimates are exactly right at a given instant, that's aiming about 19.5° below horizontal, and out from the center, with a force of about 3.15 g. As the model flies at different heights and through different shapes, the tank, itself, is moved around, and the g from maneuvering loads adds in for finding the direction of "instantaneous g" and how many g it actually is at that instant.

The uniflow tube end is usually so near the fuel pickup "height" in all situations that g on the fuel itself means little. However, if the whole tank is too high or too low in/on the fuselage, that becomes another gravity head condition. Uniflow tanks are more sensitive to tank height than the old over/under open vented monsteres, but are so much nicer when they are right.
(edited because it posted itself before I was ready - oops...)