Holland Hornet

Welcome to the CEF Mick!! Take a look at this sale on Ebay https://www.ebay.com/itm/394025988920?hash=item5bbdc77338:g:mM4AAOSwgiZiU3l9

I really appreciate the help and support. I was able to pick up a machined glow head to accept the Nelson plug. Is there supposed to be a gasket between the cylinder head and original glow head. I know that the Cox engines had a copper one (I think copper). If the Hornet is supposed to have one, I can’t find one. I looked on Mecoa, they list one but out of stock. Hope this makes sense.

A couple of points-Doug Galbreath must be knocking on the door of 90 years old now-so its not surprising that he's stopped most if not all of his specialist accessories manufacture. The Galbreath head-of which I have two (and no they're not for sale!) takes Nelson plugs. The two heads I purchased from Doug about a decade ago came with two 0.004" brass shims-but I note that standard Hornet replacement heads in original factory packaging (part no 2260) do not come with any head gasket.....

I would imagine any adaptor that takes a 1/4-32 standard glowplug would result in the normal loss of performance seen with similar Cox adaptors-but one for a turbo plug (if it exists) would be comparable to the Galbreath one.

ChrisM
'ffkiwi'

ffkiwi wrote:I would imagine any adaptor that takes a 1/4-32 standard glowplug would result in the normal loss of performance seen with similar Cox adaptors-but one for a turbo plug (if it exists) would be comparable to the Galbreath one.

A little while back, someone documented their experiments by carefully measuring the combustion volume of a Cox head, then modified a spent Cox head by tapping it for a glow plug, shaving the head seat until he had the same volume as a Cox head.

By doing this, he was able to show quantitatively by rpm that he could obtain performance equivalent to a Cox head using a glow plug.

Most of the current adapter heads drop compression, so unless one goes to higher nitro content, they lose rpm.

If one has an aircraft that doesn't require a higher performance engine, then the loss in rpm may be acceptable. Back in the late 1940s to early 1950s, the .049's were not as powerful as later. By carefully matching available HP, was able to still have successful flight. Also, I think a key to successful flight has to do with selecting an optimal prop that performs well with the loss in compression. A 1/4th hp Brown Jr. ignition was able to power an 8 foot wingspan free flight cabin airplane with ease, using much larger diameter props (16", 18"). A .15 with same hp would not succeed spinning its smaller 8" prop.

However, those with a "need for speed" of course would be sorely disappointed.

GallopingGhostler wrote:

ffkiwi wrote:I would imagine any adaptor that takes a 1/4-32 standard glowplug would result in the normal loss of performance seen with similar Cox adaptors-but one for a turbo plug (if it exists) would be comparable to the Galbreath one.

A little while back, someone documented their experiments by carefully measuring the combustion volume of a Cox head, then modified a spent Cox head by tapping it for a glow plug, shaving the head seat until he had the same volume as a Cox head.

By doing this, he was able to show quantitatively by rpm that he could obtain performance equivalent to a Cox head using a glow plug.

Most of the current adapter heads drop compression, so unless one goes to higher nitro content, they lose rpm.

If one has an aircraft that doesn't require a higher performance engine, then the loss in rpm may be acceptable. Back in the late 1940s to early 1950s, the .049's were not as powerful as later. By carefully matching available HP, was able to still have successful flight. Also, I think a key to successful flight has to do with selecting an optimal prop that performs well with the loss in compression. A 1/4th hp Brown Jr. ignition was able to power an 8 foot  wingspan free flight cabin airplane with ease, using much larger diameter props (16", 18"). A .15 with same hp would not succeed spinning its smaller 8" prop.

However, those with a "need for speed" of course would be sorely disappointed.

I make the point that by shaving the head-apart from increasing the compression by reducing the volume, you are also altering the head shape-and you now have two variables affecting the result. Head shape is important-not just compression ratio....a shaved Cox head by default introduces a squish band into the equation-a squish band virtually absent in the unmodified unit....

The way to do of course -if you have the gear-is to blind tap it-leaving just a hole the original element cavity diameter-then there is no 'excess volume' to be concerned about-other than that surrounding the element-and you control that by your choice of 1/4-32 plug. Equally-if you're sneaky like me-you use a flat element Globee plug and again the 'excess volume' issue becomes irrelevant. [and it works fine in a through tapped hole as well...] You do of course in that particular event still face an altered combustion chamber SHAPE -as opposed to volume-but that might prove beneficial-and if you have the gear there is nothing stopping you making up your own head shape profiles-trumpet, conical, squish, double bubble.....and experimenting. I even have- tucked away some where-a Rossi #2 head carefully machined and tapped for a 1/4-32 plug in a blind hole...dating from my F1C days 30 years ago...I might well-having acquired the machine tools and other tools to do so....experiment with other burned out heads to see what might be achieved....Cipolla 1.5 PF7 heads, Fox 09/10, MP Jet 061, Gilbert 7 and 11 heads....when I get suitably motivated.

ChrisM
'ffkiwi'

If I may chime in here with what I know and have experience with too, the standard glow plugs used in converted COX heads  reduce compression ratio not just because their filament cavity volume may be larger than in a stock COX head, but, the unsealed space between the threads of the plug and the modified, tapped COX adaptor head also adds to the compression volume, given that the gasket of the plug sits on the plug shoulder, and the volume of the combustion chamber is increased by the inter-thread volume...hence the advantage of the Nelson plugs, and turbo plugs that have the tapered tip that seals right on the plug tip and cuts off the inter-thread volume...

The red postage stamp backplate COX 290 reedie, upgraded with a Kamtechnik turbo head  on my Quickie 100 racer unloads above 28k in the air (60/20/20 fuel, 4.5 x 3.5 APC prop), (also defying all theories of the reed floating and thus limiting the reedie engine speed at around 20k) a speed never to achieve even with a high-comp TeeDee head, let alone a normal glow plug adaptor...https://www.coxengineforum.com/t13833-quickie-100-rc-flight-video?highlight=Quickie+100

[quote="balogh"]If I may chime in here with what I know and have experience with too, the standard glow plugs used in converted COX heads  reduce compression ratio not just because their filament cavity volume may be larger than in a stock COX head, but, the unsealed space between the threads of the plug and the modified, tapped COX adaptor head also adds to the compression volume, given that the gasket of the plug sits on the plug shoulder, and the volume of the combustion chamber is increased by the inter-thread volume...hence the advantage of the Nelson plugs, and turbo plugs that have the tapered tip that seals right on the plug tip and cuts off the inter-thread volume...

ok yes I must say that it is possible to have an adapter better than an original standard cox glow plug... that is Not hard To beat i made a couple adapter by the past m'y self and found a way To seal the internal tread wit selant gasket maker it worked just fine Currently I can do any kind of model I want

ffkiwi wrote:I make the point that by shaving the head-apart from increasing the compression by reducing the volume, you are also altering the head shape-and you now have two variables affecting the result. Head shape is important-not just compression ratio....a shaved Cox head by default introduces a squish band into the equation-a squish band virtually absent in the unmodified unit....

Chris, the man who did explained his methodology very thoroughly plus backed it up with photos and videos, detailed explanations.

He used a water droplet methodology to ensure he obtained the same volume, which included the volume cavity of the glow plug, so that he got the same.

Altering chamber shape was only slight. He didn't have to shave off much, just enough of the bottom edge of the glow head. Was this slightly altered combustion volume shape so significant to affect RPM numbers? Apparently not. I don't have a PDF printout of his thread, nor remember the exact location of what forum he posted it in, otherwise I'd reproduce here.

There was science in his methodology. And, regarding combustion shape theory, Cox heads as well as others have used different shapes, cone, hemispherical, modified cone, and etc. There are other variables involved such as the pattern the gasses take as the exhaust is expelled and the anew populates the combustion chamber. We only have the explanations, but no science to truly back up that what engine OEM experimenters did. (Marketing: Pretty packaging, it works! it works! sell heads, sell heads! I know this sounds simplistic.       )

Would I do this for a proto-racer? May be, may be not. Would it be worth doing for fun flying, as a replacement alternative to harder to obtain glow heads? Of course.

May be head shape isn't quite as big of a deal as we are lead to believe ....

On large size engines the shape of the combustion chamber has a higher impact than in small engines. The shape impacts the time within which the fuel burns out in the chamber, and thus the time the heat into the Carnot cycle is introduced. The quicker the heat intro into the cycle, the higher the Carnot cycle efficiency. (This is one of the basic rules of thermodynamics in internal combustion engines.).

The flame front advances from the glow filament and continues its way towards the depth of the chamber until all the fuel molecules have burnt out. The faster the flame reaches the last segment in the chamber to ignite the fuel there, the shorter the burn time of the charge, and higher the thermal efficiency of the engine. This will translate into more power at the end.

Shaving the COX head bottom for higher compression ratio will widen the shoulder ("squish band" ) on its bottom with a inside diameter smaller than the cylinder, and this irregular space under the head squish band, more difficult for the flame front to reach, will elongate the burn time of the charge, reducing the  thermal efficiency of the Carnot cycle, and the output of the engine at the end..

So this shoulder, or squish band, if excessively wide, may really destroy the engine power.

COX heads  initially came with trumpet shape combustion chambers (some of my 1st production run COX Tee Dee 049 and 051 engines have this trumpet head) , that later were replaced with the dome shape heads that probably proved better than the trumpet heads.

The above is more or less theoretical with small engines where the combustion chamber volume is very small therefore the time for a full burn of the charge is very short, no matter what the shape is. But for larger engines the shape does have an impact.

There also another factor to take into account. The experimenter used the same number of head shims as one would on a regular Cox head, so the narrowed squish area toward the edges would not be as great as if the head sat shimless. If I ever find that article, will post a link to it. I found it rather enlightening. It was may be 8 or 10 years ago, perhaps more.

When Stuka Stunt and Brotherhood of the Ring forums folded, a wealth of information disappeared down the rabbit hole, not to mention also historic but privately maintained Sterling Models websites.

You have enough responses already, but I would like to add that the turbo plugs are 1mm smaller in diameter than the Nelson type. That makes a difference on a very small motor. You may find someone who can tap out your existing head if there is enough meat on it. It goes into a blind hole with a 60 degree taper and a 3/16" hole. The tap is 8mm x 0.75 pitch. Be careful of too much compression or high revs, as years ago I heard the Hornets would break when pushed hard. I had good luck on the .049 heads with a 45 degree angle combustion chamber coming from the tip of that 3/16" diameter through hole, which is what the Merlin plug had looked like. One motor I have was 7500 rpm better than the stock head that I had been using on the CS.

May be head shape isn't quite as big of a deal as we are lead to believe .... [/quote]

Yes-it is-though not necessarily at 049 level. To give a real world example-since the late 80s/early 90s-the so called 'double bubble' head has been THE head for .15 size FAI combat engines-and if yours didn't have that you got one made that did. FAI combat fuel is 10% nitro (different from FAI speed and FAI FF fuel which was then 80:20 (and still is-but with synthetic oils allowed now as well as castor.)-I had quite a lot of blown Rossi and Conquest heads that I got machined for Nelson plugs and a double bubble chamber by Ed needham in the UK-who offered this service to CL fliers...and anyone else who wanted it. On FAI 80:20 fuel they were hopeless....now performance wise-and rpm wise-there is not a lot of difference between FAI 80:20 and 10% nitro in most engines-and the rev bands on the respective props were approximately similar-high 20s to low 30s -say 28,000-31,000 for both F2D and F1C-yet a head that worked extremely well for combat engines was useless on an F1C engine operating in the same rpm band... [what seems to work for F1C is a sharp cone, not a double bubble-but it's not intuitively obvious...]

Had the double bubble heads worked it would have been quite advantageous, and its much quicker to change a plug than a button-and more importantly your head clearance doesn't change every time you change a button-and need checking and/or re-adjusting with shims.

ChrisM
'ffkiwi'

ffkiwi wrote:Had the double bubble heads worked it would have been quite advantageous, and its much quicker to change a plug than a button-and more importantly your head clearance doesn't change every time you change a button-and need checking and/or re-adjusting with shims.

Chris, I think you may have a point in engineering called modeling. Perhaps that when we go smaller, the impact of combustion shape isn't quite as dramatic as it is when with larger displacements.

Question to the scientific minded:

Perhaps the particular plug insert using a special head, which is surface mounted versus "well mounted" (in a cavity recess) is more sensitive to combustion shape for the micro combustion chambers of under 1 cc (.061) displacement half-A engines? I think the size of the standard recessed cavity of the glow element may negate or diffuse the effectiveness of the combustion chamber as it is covers a greater percent of the volume, versus what Chris has pointed out, for larger engines has less impact.
** End of question **

Also as you point out, sometimes I think that big "celebrity" names and marketing wins out over engineering. Otherwise, the more durable A.C. Gilbert ready-to-fly airplanes would have dominated and we'd all be collecting A.C. Gilbert engines. (Through these CEF discussions, we have found that the Gilbert RTF's really do have unsung merit in their durability, versus the more fragile Cox and Wen Mac RTF's (except the PT-19).)
(Regarding collecting Gilbert engines, I am being facetious, me and my dry humor.)  

I understand where you as well as others are coming from. The forum brings us all together with different perspectives to bring out the truth in matters, little known or overlooked facts, as well as display the better side of human nature.



Oops! And, I forgot .... to the forum, @Mikhart8675 !