Vertical Coil Nelson Heads

 

The Nelson plug is designed to seal at the beveled lower edge of the plug body isolating the threads from the combustion chamber.  This concept has been widely applied to modern high performance schnurle ported model engines.  Not only is performance improved but the plugs have provided excellent durability.  For the past 10 years I have been fitting hemispherical Nelson heads to both schnurle-ported engines and to loop scavenged. If the engine is fitted with a button head and a clamp ring only the button is replaced. For engines with one-piece heads completely new heads are made.  Any head modifications can be completely reversed at ay time by reinstalling the original parts, which are returned to the owner.  This preserves the original engine configuration for collector purposes. Glow plugs are not provided with the reworked engine. 

Internally the hemispherical heads are contoured to match piston baffles and domed piston crowns.  For older loop-scavenged Nostalgia engines the heads are designed with a compression ratio for optimum performance on moderate levels of nitromethane and rpm levels.  Consideration is given to the inherent structural strength of the engine in fitting the head.  For example an OS Max III is set up with a higher compression ratio for operation at higher rpm than the more fragile Torpedo Green Head.

Heads are available both with and without fins to match original engine configuration and application. In general fins are not required for free flight where the engine runs are short and cooling is excellent. For other applications such as  some control line events, fins are certainly appropriate. Shown is an ST G15 with fins only on the exhaust side to match the original. The McCoy 60 is fitted with a smooth head that is internally contoured to match the domed head of piston. The OS 25, a modern schnurle engine, is an excellent free flight engine if you can find one.  This engine was converted from an R/C version to free flight and set-up with a high compression ratio for high rpm operation.

Some of NOS engines for which I have verified designs are:

    Torp Green Head 09, 15, 19, 201, 23, 29, 29S, 29R, 35, and 35CS

    OS Max III 09,15, 19, 29, 29X and 35; OS Max 15 I and II, and OS 19

    Fox 15, 19, 25, 29X, 35 Black Head Combat Special, 40, and 59

    Johnson 29, 29R, 32, 35, 35SS and 35CS

    Veco 19, 29 and 35

    Enya 19, 29, and 35

    Super Tigre C35

    OK Cub 049

    McCoy 60

    Dooling 29

       


Some vintage and modern engine which have been fitted with Nelson heads are:

    Rossi 15* and Nova Rossi 21

    OS 25 Ducted Fan, 40, 46, 60

    ST G15, G21 29 and 35

    Cox TD 020, TD 09*, and TD 15*

    Cox Conquest*

    Mecoa 32 R/C

    GMS 32 R/C

    Pico 15

    K&B 3.25, 3.5, 4.9, 6.25, 7.5 and various Series 61, 64, 65, and 66 engines


* Stock glow head is usually modified for Nelson plug


Design of heads for different engines is accomplished using a combination of CAD and Excel analysis.  For rather simple designs the combustion chamber geometry is established using an Excel spreadsheet.  The geometry is then drawn in 2D using AutoCAD.  For more complex geometries the combustion chamber is modeled in 3D to accurately determine chamber volume and resulting compression ratio.  Adjusting the head depth in the cylinder is used to obtain the desired deck clearance and compression ratio.  This is necessary to accommodate geometry variations between engines of the same design.  Further minor adjustments are often made on the test stand to achieve desired performance.

3D CAD Model of Johnson 35 Combustion Chamber

Cutting Hemi Combustion Chamber