Ram air supercharge

[John Carrodus]

Q.  Would a forward facing enlarged venturi scoop increase power?

[Dave Hull]

Not a noticeable amount. If you want more power try using a larger choke area inside the venturi.

[frank williams]

A forward facing venturi though might give a more uniform and consistent airflow into the venturi.  It always seemed a difficult proposition to have the air intake at ninety degrees to the assumed direction of the freestream.  Another case for rear-rotor  intakes for stunt motors; hide the intake in the dead air of the fuselage!

[John Carrodus]

Frank- did the ships funnel intake make any difference on that red model? Tell me more please, That s the kind of thing Ive been thinking of giving a crack.

[Brett Buck]

A forward facing venturi though might give a more uniform and consistent airflow into the venturi.  It always seemed a difficult proposition to have the air intake at ninety degrees to the assumed direction of the freestream.  Another case for rear-rotor  intakes for stunt motors; hide the intake in the dead air of the fuselage!

  If nothing else, making the front intake longer at least straightens it out before it gets to the important part. This was a flaw with the first venturis from the RO-Jett, they were so short and shaped like a trumpet that is seemed very sensitive to the angle of attack. 1/8" longer, and a pantyhose diffuser ,and a spigot, nearly perfect.

         Brett

[frank williams]

John, the most noticeable difference is that its a lot harder to choke but its a good conversation starter.    Brett's observation about tall venturi's is right.  Panty hose as a last resort.

[Air Ministry .]

Would be to erratic for aerobatics . Essentially you using it to increase atmospheric pressure ( at the intake ) .



Where the black bit under the top orange bit seals against the hood underside .



Though the tooned length induction ( Ram Chargeing ) has comparable enhancing capeabilities .



eat your heart out , mercedes banz .

[Air Ministry .]

 

 

https://rcbookcase.com/categories.php?publisher_id=6&sessionid=cf2arhdrkel6s83g5e7fpak314

around 78 , here , the Metkemeyer Bros , did a " F M V Story " on the development of their Team race Engine .
With a diagrame of the air flow set up . A plenum Chamber to the intake . ( sealed carb box effectively ) .

Clarkeson in his Control Line Column also drew what he thought they used , Theyed replied it was his idea .

ALSO , the 1964 ish SUPERNOVA , Their blue & silver model that ' line broke ' had a distinct snorkle to a ' cold air box ' ,

Just visable here is one similar . just behind the canopy .

just behind the canopy .

===========================================================================================

This is a smart ass attept at COOLING the Crank Case to nrease charge density / efficency . Or reduce case heating & inefficency ! .


also with a carb fresh air feed . Since opened out And blocked rear as F I fitted , and you can get to tecnical .



So , Team Race ( or Rat . ) a ' cool air box ' at the carb could tecnically enhance efficiancy . Overfed a chamber with minimalist clearance to crankcase will  still function , with feed near intake so its not heated .

So , the Harworth Place Blue & Silver plane was decades ahead of its time ,

You essentially want cold air intake , air box , ducting outlests , and the oil cooler in the seat fairing . 




and seperating the cooling / heated air , from the R V crankcase chamber cooling / intake .

for F2B Id think the Ram air to Air Box would screw up the 4-2 trip , maybe . Tho as Brett Says , LOAD is a big part of the internal engine ; switch ' function . If its doing it good .

The FOLKERTS with a RPR  Hirtenberger with dropped liner  , ran split dual intake .Like That.   Cowl off / on was 1/4 turn on the needle , Gave a superb run in the wind , so you never know ,
Often things you havnt allowed for or didnt consider , make themselves known , when you start to get a bit tecnical , with aerodynamics . Which Ram Charge Cooloing IS . 

[Lauri Malila]

..

[katana]

'Ram Air' was tried by most of the auto and bike OEMs and developed a lot by the bike factories in the 90's. Even with their technology and money, when tested independently on a 600 sized bike, the best 'boost' pressure that could be recorded was about 0.5psi @ 170mph (figures subject to aging memory LOL) Fast moving air has lower pressure - its how a venturi works - air speeds up through constriction and lower pressure sucks fuel out of spray bar. A bigger venturi opening has opposite effect hence why pressurised fuel is required if you go 'big'. As said if the 'pressurised' air can be fed to a plenum / still airbox from which the engine breathes the performance could be enhanced but difficult to measure reliably.

[Brett Buck]

'Ram Air' was tried by most of the auto and bike OEMs and developed a lot by the bike factories in the 90's. Even with their technology and money, when tested independently on a 600 sized bike, the best 'boost' pressure that could be recorded was about 0.5psi @ 170mph (figures subject to aging memory LOL) Fast moving air has lower pressure - its how a venturi works - air speeds up through constriction and lower pressure sucks fuel out of spray bar. A bigger venturi opening has opposite effect hence why pressurised fuel is required if you go 'big'. As said if the 'pressurised' air can be fed to a plenum / still airbox from which the engine breathes the performance could be enhanced but difficult to measure reliably.

The ram air pressure is trivial to calculate, so no experiment is required. It’s the dynamic pressure q. At 170 mph it’s indeed .496 PSI. At stunt speeds it’s ~.05 PSI. The pressure or pressure variation from maneuvering is probably negligible. I think the issue is more with the relative wind changing the flow direction inside the venturi. That’s why venturi length and diffusers matter.

Brett

[Air Ministry .]

Yep ,

Had snotty runs on a G 51 , the intake stock is pretty level with a 3 inch spinner , I figure the sudden turn on inside squares upset the thing - with the onrush of fresh air .

BUT ,

The Tank Vent on a certain contraption ( uniflow ) varied 1/4 turn on the setting , from aiming straight forward - to straight in with the end trimmed 45 Deg , say 3/4 in from fuse. wall .
The Straight forward aim on the vent was way better & closer air to ground . The extra 1/4 rich was needed with the angle cut end . which brings us to . . ,.
The nasty Rich into wind - Lean Down wind - LETS HIDE the Vent in the Cowl - trip - trick . etc .

So , with the Nasty nose intake  ( Split at the fins )  and YES the ' choke ' is near totally inaccesable . So tecniques need to be applied . with the rear intake a Q.D. Cowls nearly a must .
A Full fuel line or NO START , R V or F I , ( essentially - Blow into the vent to prime line , and DONT flood it ) 

NOW ,
If the ' Pressure Variation '- can cause the lean downwind - rich into wind annoyance , Does a Semi Sealed intake tract on a rear intake set up actually give an advantageous opperation . Something Did .

So , We will put the Picture On . Here I pressume its just with the 21/40 . A 9 mm ( rat race ) intake & a 11 X 4 prop , and it yammers through Any wind . Even Upwind stuff . But needs 10 or 12 ounce tank !

The HP 40 RPR ( Rear intake Pylon ) with the dropped sleeve - Std HP F2B sleeve timing . Std HP Disc , gets a Three ' break ' run 4-2-howl to hold against ocean breezes . And switches Back down instantly .
The cowl outlets ( the Brass Tubes ) are to steer the nose out proportional to wind speed. Theyre the ' cooling air outlets ' the Lower nose intakes cylinder cooling - seperated aft briefly , too .
The upper half of the split nose intke , feeds past the front of the motor , up around the case to the intake area . Nothing particularly sophisticated or planned . But it did end up ( by luck mainly )
giving a strong steady power response . Particularly in nasty air .




Kinda Inspired by a picture of John Havels Folkerts in a ancient American Modeler Mag , and John Hary's P S S , back in 76 . The big ugly one with the RB 75 dosnt shut down / switch back - redilly yet . ands too big.

Being a Slope Soarer WIND is part of the Equation .
Havels looked a clean lean machine , in the PAMPA artical he mentions WIND too ( Theres a thread on L E Sweep , tthese're all semi relivant , there . too .  )



Ran a Std'ish size nylon intake with the needle flat across it . Rear Intake F2B engines arnt particularly unusual . Over time at least . MVVS 5.6 , later 5.9 , a few commie ones etc etc .

The point being , perhaps ( IF its how its working ) the rear of the engine compartment is a plenum chamber . Differant kettle of fish to a front intake .
BUT ,
seeing the differance from upwind to downwind on ' merely ' a tank vent reposition , there has to be a similar pressure variation , if minimal , at the cowl opening , one would think .
Do electric contraptions give sufficent download to find motor load upwind Vs downwind .

( And how do you get a RB 75 to stop clawing away for 50 yards out of loops etc when its blowing & loaded up . Ton of grunt but not to delicate in its characteristics  . )

As an aside , coats & things flapping about and unobstructed splay port Bonneville heads , can get ' wots going on ere ' when they mix . Not to mention flooded & miss fire .
A finger near a open running intake , gets sucked , cold , and things . Feeding a intake puts that pressure variation elsewhere . Hence a ' tuned length intake ' - Ram feed ' ' Plenum ' etc .

But the tank vent & stagger / leanness into & out of winds perhaps the simplist conundrum . 

[Air Ministry .]

H = E + PV


Bert Willke
Ph.D. in Physics, Massachusetts Institute of Technology (Graduated 1963)Author has 11.5K answers and 1.8M answer views2y
Air static pressure has the dimensions of energy density, and you can think of it as potential energy density with the capability of doing work expanding into a lower pressure environment.

Conversely, the kinetic energy density of wind is also called velocity pressure or stagnation pressure; the latter because it equals the increase in static pressure caused when the flow hits straight into a wall and stagnates. The lost velocity pressure becomes static pressure.

Bernoulli’s principle asserts that the sum of velocity pressure and static pressure; called total pressure, is conserved along streamlines of a flow. This holds to the extent compressibility*; viscous losses; heat transfer; etc., can be neglected. As you see from the above, you can think of Bernoulli’s principle as a form of energy conservation: kinetic energy density plus potential energy density is conserved along streamlines when the remaining energies can be neglected.

*You can bring in compressibility by replacing static pressure with enthalpy.


H   =   enthalpy
E   =   internal energy
P   =   pressure
V   =   volume

[Brett Buck]

H = E + PV


Bert Willke
Ph.D. in Physics, Massachusetts Institute of Technology (Graduated 1963)Author has 11.5K answers and 1.8M answer views2y
Air static pressure has the dimensions of energy density, and you can think of it as potential energy density with the capability of doing work expanding into a lower pressure environment.

Conversely, the kinetic energy density of wind is also called velocity pressure or stagnation pressure; the latter because it equals the increase in static pressure caused when the flow hits straight into a wall and stagnates. The lost velocity pressure becomes static pressure.

Bernoulli’s principle asserts that the sum of velocity pressure and static pressure; called total pressure, is conserved along streamlines of a flow. This holds to the extent compressibility*; viscous losses; heat transfer; etc., can be neglected. As you see from the above, you can think of Bernoulli’s principle as a form of energy conservation: kinetic energy density plus potential energy density is conserved along streamlines when the remaining energies can be neglected.

*You can bring in compressibility by replacing static pressure with enthalpy.


H   =   enthalpy
E   =   internal energy
P   =   pressure
V   =   volume

I am not sure what you are trying to describe
dynamic pressure q=1/2 rho v^2, who = density of air, 0.0023 slugs/cubic foot

170 mph = 249 feet/second

q=1/2(.0023)(249)²
=71.94 lb/square foot = .496 lb/square inch. 1%, not nothing, but not much of a "supercharger" although useful.

Stunt speeds (56 mph) = q=1/2(.0023)(82)² = 7.733 lb/square foot = 0.054 lb/square inch = 3/10% increase in supplied pressure = negligible.

   Brett

   

[Steve Helmick]

Note that the Rossi .15 (and clones like the Cox Conquest and Taipan 2.5 rear exhaust and maybe some Nelson engines) have a rather forward sloped venturi. I read that the fuel/air flow into the crank bore was significantly improved and thus, power increased. But that is with rpm ranges that are about 3 to 4 times that of a typical stunt application.  Steve

[Dave Hull]

Steve--where did you read that info? And how did they run a comparative test?

D

[Brett Buck]

Note that the Rossi .15 (and clones like the Cox Conquest and Taipan 2.5 rear exhaust and maybe some Nelson engines) have a rather forward sloped venturi. I read that the fuel/air flow into the crank bore was significantly improved and thus, power increased. But that is with rpm ranges that are about 3 to 4 times that of a typical stunt application.  Steve

    It has less of a corner to turn when going from down the venturi to through the crankshaft, presumably that was the effect they were shooting for. A rear-exhaust engine has even less of it (zero).

    Brett

[Lauri Malila]

..

[Brett Buck]

I guess you mean rear intake? If yes, the closest to straight line and shortest path without discontinuations would be Zimmermann valve. But in our use, it is better for big end cooling & lubrication, to use a reversed rotary valve. Like what you can see in my pictures above. In most fuselages, it is also better for ideal positioning of fuel tank.
Theoretically Zimmermann would be interesting to try, but I think it would require a needle bearing for conrod big end. I don't think I'll bother with that.
But as for ideal engine layout for our use, we should look closely at the F2C engine development. just some tweaks in scavenging and metallurgy. L

  Yes, just missopke.

   Brett

[Air Ministry .]

1 mph = 1.46667 fps
thus;
50 mph = 73.333 fps
X 12;
880 in per sec. cording to' computor . Which I wouldnt trust for a second . But if the Computors right , heaven forbid , that'd be 880 Cu In per sec from a 1 in square cowl intake . if you get my drift .

I think it'd be better to redo / check the math .

Assumed 50 mph .

==================
50 mph   = 4400.00 ft/min -> x 12 , / 60 getsit . in inches !

[Brett Buck]

1 mph = 1.46667 fps
thus;
50 mph = 73.333 fps
X 12;
880 in per sec. cording to' computor . Which I wouldnt trust for a second . But if the Computors right , heaven forbid , that'd be 880 Cu In per sec from a 1 in square cowl intake . if you get my drift .

I think it'd be better to redo / check the math .

Assumed 50 mph .

==================
50 mph   = 4400.00 ft/min -> x 12 , / 60 getsit . in inches !

   I am not sure what you are trying to prove here - it appears to be the flow through a hole, sort of. That is not what the engine is going to suck in, it will suck in air based on the pressure present at the inlet.   It would be pretty easy to make a cowl that has the effect of reducing the pressure.

     The pressure at 82 FPS/56 mph is as described earlier and is the absolute maximum pressure you could achieve if everything was perfect. A real situation is liable to be less than that. At stunt speeds, it's negligible, at Indycar speeds, small but helpful.

    Brett

[Lauri Malila]

..but I guess the air speed behind propeller is more that the 56 mph..? L

[katana]

..but I guess the air speed behind propeller is more that the 56 mph..? L

I think not as its cancelled out by forward momentum . . . . 56mph is the corrected airspeed over engine / flying surfaces.

[Brett Buck]

..but I guess the air speed behind propeller is more that the 56 mph..? L

   An inch from the root?  Is the slowly turning very high pitch section right outside the spinner helping or hurting in level flight?  I don't know.

    Brett

[John Carrodus]

Lots of fascinating discussion here. I gather scooping and directing a greater bulk of air down the intake generated from forward movement of the model- has very little positive effect on producing more HP overall?

Another question.
              If at 50mph the extra air pressure produced is less than useful, why do (some) motors wind up heading into wind on windy days ?( as at our stunt comp last weekend)

[Brett Buck]

Lots of fascinating discussion here. I gather scooping and directing a greater bulk of air down the intake generated from forward movement of the model- has very little positive effect on producing more HP overall?

Another question.
              If at 50mph the extra air pressure produced is less than useful, why do (some) motors wind up heading into wind on windy days ?( as at our stunt comp last weekend)

   That one is easy, and much more relevant - coming around into the wind, the airspeed goes up (for a while) and this "unloads" the engine, that is, it's easier to turn the prop at that RPM. This is a necessary feature of any decent stunt power system, that's what regulates the speed as you maneuver.  That's a far more powerful effect that the relatively feeble pressure changes.

   Additionally, why you aren't going to get a lot of extra power,, the net feed pressure at the engine is also very small, and even a very small ram pressure isn't negligible. but there are plenty of cases where the feed pressure goes down. but the engine "goes rich" anyway, from the unloading effect.

     Brett

[Steve Helmick]

While I don't claim to be up to date with F2A technology, it is my understanding that the fastest current F2A engines are front intake, and with the more sloped forward venturi concept something like the Rossi .15's. As Brett wrote, that improves the flow into the crankshaft.

Rear intake engines are fine, but the mechanical drag of whatever design (i.e., drum, disc, or something else) generally isn't a good thing. For Stunt use, it's not a problem. For F2C, it appears that reduction in crankcase volume is more important than mechanical drag, but it's also pretty clear that the F2C guys design, machine and fit their rear intake system  so carefully that the mechanical drag apparently is less significant than the reduction in crankcase volume.  Steve

[Chuck Matheny]

If you have a "ram air" system that is producing significant boost ...you are either forced to launch super blubbery rich...or else you have an inflight mixture control valve via remote control.

[Lauri Malila]

..

[Steve Helmick]

My last .29 speed model did that to a significant degree (rich & blubbery before release), until I closed off the air intake hole in the cowling with a handy BandAid. It would have been ok, except for excessive gap between the spinner backplate and upper shell. The reduced ram effect was effective, and allowed leaner launches, and thus more chance of getting it into the air.  Steve

[Steve Helmick]

   That one is easy, and much more relevant - coming around into the wind, the airspeed goes up (for a while) and this "unloads" the engine, that is, it's easier to turn the prop at that RPM. This is a necessary feature of any decent stunt power system, that's what regulates the speed as you maneuver.  That's a far more powerful effect that the relatively feeble pressure changes.

   Additionally, why you aren't going to get a lot of extra power,, the net feed pressure at the engine is also very small, and even a very small ram pressure isn't negligible. but there are plenty of cases where the feed pressure goes down. but the engine "goes rich" anyway, from the unloading effect.

     Brett

Not sure about that. The upwind/downwind speed change is one of the reasons I like muffler pressure. I reckon the extra pressure into the uniflow vent makes it go richer on the upwind leg and leaner on the downwind...as I recall. Some folks turn the uniflow air inlet tube straight down, and say that works. But I haven't tried that myself.

I tried pipe pressure on the .46VF AAC, but had problems with fuel siphoning out of the tank and into the pipe. If that wasn't bad enough, it under-ran. Otherwise, it ran fine. Potentially, the siphoning could have been fixed with a check valve, but IMO that's unnessecary complication that would probably bite me. The only reason I tried it was because the pipe already had a pressure tap in it.  Steve

[Brett Buck]

Not sure about that. The upwind/downwind speed change is one of the reasons I like muffler pressure. I reckon the extra pressure into the uniflow vent makes it go richer on the upwind leg and leaner on the downwind...as I recall. Some folks turn the uniflow air inlet tube straight down, and say that works. But I haven't tried that myself.

I tried pipe pressure on the .46VF AAC, but had problems with fuel siphoning out of the tank and into the pipe. If that wasn't bad enough, it under-ran. Otherwise, it ran fine. Potentially, the siphoning could have been fixed with a check valve, but IMO that's unnessecary complication that would probably bite me. The only reason I tried it was because the pipe already had a pressure tap in it.  Steve

   I think I said that...

the net feed pressure at the engine is also very small, and even a very small ram pressure isn't negligible

but the more important point:

 

coming around into the wind, the airspeed goes up (for a while) and this "unloads" the engine,

   which is a far more powerful effect than the ram air changing the fuel delivery pressure. Flying level in the wind, you have both effects operating in the same direction at once, plus, the feed pressure also goes up due to the groundspeed/inertial speed slowing, because of hydrostatic pressure, also in the same direction. In other cases, like taking off, the feed pressure is higher on the ground than in level flight, but the engine unloads tremendously, and (apparently) "goes rich". There are lots of examples like that.

       Brett

[Chuck Matheny]

I guess the 2 reasons for front intake in F2A are "Less complicated=Less risk" and shaft cooling.
Chuck, while I'm not sure if the RAM would cause the behaviour you describe, a solution for blubbery rich ground setting would be to move fuel tank towards inside of circle (left) in fuselage. L

https://www.physicsforums.com/threads/explain-to-me-how-it-is-possible-to-go-above-100-volumetric-efficiency-on-an-ice.152096/#:~:text=It%E2%80%99s%20possible%20to%20obtain%20greater%20than%20100%25%20volumetric,tuned%20intake%20runners%20were%20the%20vertical%20velocity%20stacks.

This article is about how to exceed 100% volumetric efficiency [a supercharged condition]  fails to recognize is that once you design a 2 stroke engine to exceed it's normal output with more aggressive porting and  timing AND with a tuned pipe ...IT HAS BECOME A SUPERCHARGED ENGINE that has disqualified itself from the question of whether or not RAM AIR ALONE CAN PRODUCE A SUPERCHARGED CONDITION\
There IS a difference between being OPTIMIZED for natural aspiration and being truly supercharged..

[John Carrodus]

Hiya Chuck
Yes I know that a PROPERLY TUNED pipe works like a supercharger for 2 strokes. I read books about that and built systems that worked when I was a 2 stroke motorbike nutta back on the 70s.
My interest at present is the effect of induction on a 2 stroke nitro model plane motor , not the exhaust side. But yes, I hear you and agree 110%.

[Lauri Malila]

Yes John, but remember that a properly tuned exhaust also kind of supercharges the intake end; Resonance is not only between cylinder and exhaust pipe, but it reaches all the way to air/fuel intake, making the air suction and scavenging more efficient. However, I have no idea how much it works like that with typical stunt timings. L

[Chuck Matheny]

Hiya Chuck
Yes I know that a PROPERLY TUNED pipe works like a supercharger for 2 strokes. I read books about that and built systems that worked when I was a 2 stroke motorbike nutta back on the 70s.
My interest at present is the effect of induction on a 2 stroke nitro model plane motor , not the exhaust side. But yes, I hear you and agree 110%.

John....you could use a hair dryer as your wind source if you wanted to experiment with air intake ideas on a test stand .
Someone has already mentioned that the prop blast at the hub is not much.....so a person could try making an airscoop that extends 4 inches [or so] to get a bigger gulp of air.
If you see a gain..but the added "boost" is obliterating plugs....then you want to add enough  head shims to temporarily cancel your gain.
 This is just to protect the engine until you subtract just the right amount of shims [via subsequent test runs]  to see peak performance and  glow plugs that aren't totally obliterated...just "consumed".

[Brett Buck]

...IT HAS BECOME A SUPERCHARGED ENGINE that has disqualified itself from the question of whether or not RAM AIR ALONE CAN PRODUCE A SUPERCHARGED CONDITION\
There IS a difference between being OPTIMIZED for natural aspiration and being truly supercharged..

    I am not sure what your distinctions are here, you can have multiple things going on at once (and in your example case, 4-5 different things) and there is no question at all that raising the pressure at the intake could, as a general proposition, force more air into the engine, which will raise the power available. That doesn't invalidate the other effects or make it "disqualified".

    If you really want to be picky, "supercharging" generally means that you have a compressor driven from the engine shaft that forces more air in, a turbocharger has a compressor driven by a turbine in the exhaust. "Ram Air" means exactly that, it uses the dynamic pressure from forward motion to compress the air. Tuned pipes uses pressure waves to increase the combustion pressure at particular frequencies or range of frequencies.

   I think the only real finding is that the ram air effect is very feeble at our speeds compared to the usually applications of supercharging, turbocharging and tuned exhausts. Ram air can obviously compress air, that's how a ramjet works, but most ramjets only work when accelerated to very high speed some other way first.

   Brett
     

[Chuck Matheny]

Brett...I'm reacting to what the thread title means to me.
For the sake of brevity I'm calling the  'supercharging effect" any method that raises the passage of air through the engine [volumetric efficiency] above 100%.

For decades hobbyists played with the air horns and crankshaft porting  of the Fox Combat Special to get some gains but I don't think anyone knew if they were getting a "supercharge effect" or just simply raising the VE closer to the magic 100%.
Not until Henry Nelson redesigned the .36 with a much larger diameter crankshaft air passage did we see the biggest gain of all...but who knows what percent of VE these engines run at...?

Would Henry Nelson know...?

[Brett Buck]

Brett...I'm reacting to what the thread title means to me.
For the sake of brevity I'm calling the  'supercharging effect" any method that raises the passage of air through the engine [volumetric efficiency] above 100%.

For decades hobbyists played with the air horns and crankshaft porting  of the Fox Combat Special to get some gains but I don't think anyone knew if they were getting a "supercharge effect" or just simply raising the VE closer to the magic 100%.
Not until Henry Nelson redesigned the .36 with a much larger diameter crankshaft air passage did we see the biggest gain of all...but who knows what percent of VE these engines run at...?

Would Henry Nelson know...?

  Making the passages larger reduces the flow restriction, same with (supposedly) the heavily-tilted intake axis. That's different from raising the pressure to force more air in.

       Brett

[Chuck Matheny]

  Making the passages larger reduces the flow restriction, same with (supposedly) the heavily-tilted intake axis. That's different from raising the pressure to force more air in.

       Brett

In the end....IF a naturally aspirated engine can achieve greater than 100% VE...then it is in effect "supercharged".
I gave the Nelson combat engine [open / untuned exhaust] as an example of about how far I've ever seen intake side improvements taken with great results...but I have no idea if these engines surpass the magic 100% VE "supercharged" mark.
Once again I refer back to the title of this thread.

[John Carrodus]

Chuck
         Many thanks for your ideas. I'm not for one moment challenging your comment about zapping glo plugs, but why does that happen with lower compression and more air?
Cheers to everyone's sharing

[Chuck Matheny]

Chuck
         Many thanks for your ideas. I'm not for one moment challenging your comment about zapping glo plugs, but why does that happen with lower compression and more air?
Cheers to everyone's sharing

Obliterating glow plugs to the point where the element is broken off and farted out the exhaust is dangerous for the engine because the wire element can get wedged between the side of the piston and the liner.
Badly blown plugs indicate overcompression and overload from too much prop.
Prop overload exacerbates [and mimics?] overcompression....especially once the plane has gotten up to speed and is full supercharge mode and you wish you had some way to shut it down ASAP.
When you are "power tuning" a racing engine [especially if it is piped]you want to tune it like you would a diesel...starting out UNDERCOMPRESSED to the point where it wont even start and then subtracting shims until you see the max rpm no longer improve.
If you are doing this on a test stand you want to use a test prop that is a little smaller than the prop you intend to run in the air and keep the "test runs" brief.
Another sign of overcompression is the combustion chamber and top of piston look "pock marked" and you have  black sooty exhaust that you sample while the engine is running with your hand or with a white rag.
Dark brown is what we want to see in the oil spray..not black.
if a high perf engine is ran overcompressed too long it shortens the life of the connecting rod and invites crankpin failure.
I used to have Dave Shadel of Performance Specialties on "speed dial" for new Nelson engine parts until he was able to hammer these points about power tuning home with me.
Some of this advice applies to "sport" or "stunt"engines that put out  less power...but these engines much less stressed and are factory set for the intended prop and fuel.
Here's a piped Nelson 40 FAI "zero nitro" engine [no throttle, just a fuel line pinch emergency shut off] going into full scream once the model gets a big enough gulp of air [@100 mph..?] to get the model up to 200 mph.
This is audio of 600 foot RC laps...not  C/L laps....

[John Carrodus]

Chuck - Bloody Hell that thing screams. !!
How long do they last?
Thanks also for your explanation re plug destruction.

[frank williams]

I.m guessing that was 30K?

[Chuck Matheny]

Chuck - Bloody Hell that thing screams. !!
How long do they last?
Thanks also for your explanation re plug destruction.

I can't really say how long the FAI Pylon engines usually last [in the hands of a more skilled operator than me] because I've never been part of that class of competition...I've never even seen FAI Pylon Racing in person.
I'll wager a guess that you can expect to get dozens of runs  but probably fewer than 50 before you lose the perfect pressure seals that a high timed / piped engine requires. High exhaust timing lessens the amount of time an engine has to build compression, so a perfect pressure seal is more critical...and this seal has to be perfect  from the air intake to the outlet of the tuned pipe.
American style 40 sized pylon is / was  dominated  by Nelson engines, but they run side exhaust mufflers, less radical porting and timing, so they are much simpler to tune...but are still capable of 200mph in those super clean pylon planes.
On a test stand when you see the Nalson FAI 40 engine go into full boost...it looks like NIAGRA FALLS when you look into the huge venturi and see how much fuel is getting pumped in there thanks to the considerable pressure built in the pipe .

 

[Chuck Matheny]

I.m guessing that was 30K?

According to a friend with an "app" who looks at my videos it was just shy of 30,000 on the final pass. !
The prop that worked for me was from  Steve "Eliminator Props" Wilk that comes out of the mold as a 7 x 7 and then it's up to you to take just enough mass [and / or diameter] from the blades to make the engine happy and not too stressed out.

[Lauri Malila]

High exhaust timing lessens the amount of time an engine has to build compression, so a perfect pressure seal is more critical...and this seal has to be perfect  from the air intake to the outlet of the tuned pipe.

Fun fact; if you calculate the cylinder and piston dilatations at running temperature, you'll end up with something like 0,1mm (1/254") piston-cylinder clearance.
The Nelson-era F3d engines were fast but much more fragile; one lean run could destroy not only the piston but also the drop-in cylinder sleeve. With modern MB:s and such, with monoblock cylinder the cylinder usually survives and you can just fit a new piston. Also the overall stability and hear transfer of a monoblock AAC is much better, something that we should look for in stunt, too. L

[Chuck Matheny]

Fun fact; if you calculate the cylinder and piston dilatations at running temperature, you'll end up with something like 0,1mm (1/254") piston-cylinder clearance.
The Nelson-era F3d engines were fast but much more fragile; one lean run could dertroy not only the piston but also the drop-in cylinder sleeve. With modern MB:s and such, with monoblock cylinder the cylinder usually survives and you can just fit a new piston. Also the overall stability and hear transfer of a monoblock AAC is much better, something that we should look for in stunt, too. L

I didn't know that was a major feature  of the MB ..!
I  am always amazed when a Machinist can look at something that already looks "maxxed out" like the Nelson engine and say to himself "I can do better than that".
What a major  "breakthrough" improved reliability and forgiveness would be [not to mention better speed].
The "drop in" cylinder sleeve of the Nelson should be a tight fit...but the high frequency vibration makes all joined parts "sing" and eventually chafe each other until they wear each other out
IIRC...the MB also has an impressive forward facing air scoop cast into the case.

[Lauri Malila]

I don't do pylon but know quite well the MB people. Really don't know what the single major feature is, all the structural differences are just common sense. You go stability and innovation first and not just look at what technologies are available and don't play with leftovers. And only make things that make most sense in either structural or thermodynamical point of view.
That is also the driving force in my work; it was all started by Robbie figuring out the ideal layout and then me slowly perfecting it.
My only problem is that as I don't care of mass production, my things tend to become quite complex, at least if you look it from outside.
L

[Chuck Matheny]

WOW....very intricate work...!
Whatever you got going on with that piston...it looks like your aim is to increase  pumping efficiency and you have managed to keep the piston as light as possible.