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Bob Grimstead
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Posted Thursday, April 29, 2010 @ 09:45 AM  

Our Fourniers continually breathe hot air from the lower cowling.

This means we never need to select carburetor heat, but it also means we're always being robbed of some power.

I flew for a while with temperature gauges in the lower cowlings and on the carb air filters, and discovered that the intake air temperature is generally 20-40°C higher than ambient.

For a long time I've thought I might get a significant performace increase if I had a ram air intake. So I made one up. For long-term use this would need an air filter and a selectable carb heat system, but for a one-off experiment I thought I would do fine without.

Here it is.

The tubing is lightweight aluminium, 28mm internal diameter, to fit the carb's 28mm venturi.
It's a length of old vacuum cleaner pipe.

At the weekend I'll let you know the reuslts.

Yours, Bob

jb92563
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Posted Thursday, April 29, 2010 @ 10:44 AM  

I have always wondered about the under cowl air temp going into the carb.

You should get ~100 rpm more revs by using the cooler air.

On the ground I see about 150 rpm drop on my 2000cc VW in the Grob 109 when switching to carb heat.

Actually ramming more air into the carb vs sucking it might also lean the mixture.

Keep a close eye on the engine temps.

Your experiment results should prove very interesting.

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Jorgen
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Posted Thursday, April 29, 2010 @ 04:25 PM  

Wow Bob,
you actually drilled that hole in a Zenith carb- that is not produced anymore? You got some cojones, Hombre!

I've been thinking along the same lines, in a small engine cold air to the carb might do a significant difference, it will be interesting to hear about your findings. By the way, did you try out the Mikuni?

May the 4's be with you/ Jörgen

Bob Grimstead
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Posted Thursday, April 29, 2010 @ 08:34 PM  

Hi Jorgen,

Like you, I'm very nervous about fiddling with an irreplacable carb.

However... When I last saw him, Ken Ballington had some of these air filter housings/plenum chambers. He may have some carburettors too. Be warned, he charges like a wounded elephant, while saying (of everything in his posession) "These things are as rare as hen's teeth, but I can let you have this one for..."

I bought an air filter housing (cost - £25!), and it is that into which I carved the hole.

I still have the original.

As to the Mikuni, the old 1400cc Rectimo came out yesterday for a very well deserved retirement and overhaul.

The 1750 Aeropower goes in as soon as I can do it, and the Mikuni carb will go on shortly afterwards.

All will be revealed in a full post.

Yours, Bob

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Posted Friday, April 30, 2010 @ 02:14 PM  

OK Bob,
"stay tuned folks---" as they say on TV, it'll be exciting to hear about this and thanks for pushing ahead Bob.

Incidently, does anyone know which tractors had the Zenith carb? There is a fairly large "vintage tractor" community out there so it might pay off for Fournieteers to socialize with tractor freaks and maybe frequent their swap meets.

May the 4's be with you/ Jörgen

Bob Grimstead
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Posted Friday, April 30, 2010 @ 10:55 PM  

Hi Jorgen,

There was a list of applications for the Zenith 28RXZ carb in one of the carburettor maintenance documents Eugenio recently translated. I don't remember seeing a tractor among them.

I would guess, considering the tiny size of the thing, it was more likely to have been a ride-on mower kind of thing, although I don't remember any mention of that.

Maybe Googling would help?

Ken Ballington's at:
http://www.ukaircraftparts.com/Category.asp?Category=Fournier%ACRF%AC4&session=ObfnaLCvDyciO2Guie2FkiWb14

Yours, Bob

Bob Grimstead
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Posted Saturday, May 22, 2010 @ 11:04 PM  

Hi Guys,

I said I'd let you have the info "at the weekend."
Luckily I didn't say which weekend!
I've been flat-out with this re-engining... baffles, cowls etc. More on that in another post when I have time.
Meanwhile, here are comparative figures, although I must say I am surprised that the initial, original climb rate was so poor.
I'd always reckoned HDO climbed at around 600-620fpm at aerobatic weight, but maybe the engine was just getting tired.

Test Flight 23rd November 2008
Heliptera 52 x 34 (blue) propeller
Take-off weight 377 kg (17 litres of fuel aboard)
OAT 25°C, 1006 HPa, altimeter set to 1013
Wind 230/10 Serpentine Runway 23
Thermic and windy. Climbing cross wind at 60 kt IAS.
Standard warm-air lower cowling induction

0 minutes 500 feet
1 minute 960 feet
2 minutes 1500 feet
3 minutes 2010 feet
4 minutes 2540 feet
5 minutes 2940 feet

Total altitude gain 2440 feet
Average climb rate = 488 fpm

Test Flight 25th April 2010
Heliptera 52 x 34 (blue) propeller
Take-off weight 377 kg (17 litres of fuel aboard)
OAT 25°C, QNH 1013 HPa, altimeter set to 1013
Wind 230/5 Serpentine Runway 23
Minimal turbulance. Climbing cross wind at 60 kt IAS.
Ram-air inlet.

0 minutes 300 feet
1 minute 850 feet
2 minutes 1420 feet
3 minutes 1960 feet
4 minutes 2530 feet
5 minutes 3030 feet

Total altitude gain 2730 feet
Average = 546 fpm
=12 per cent increase

Don't expect to get an immediate 12% climb rate increase if you do this because, as you can see, the initial and subsequent climbs were nearly eighteen months apart.
Nevertheless, it would seem that there may be some merit in fitting a direct, cold air inlet with a selectable carburetor heat system.
I might try this with the new engine when its Mikuni carb is fitted and sorted, but since I'm leaving for Enagalnd next week, that's likely to be nearly a year away.

Yours, Bob

Donald
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Posted Sunday, May 23, 2010 @ 04:19 AM  

An interesting experiment, Bob. My RF3 had a carb heat system fitted when I bought her but not a cold air inlet. The carb breathed either from below the engine as normal or through a cuff over one of the rear exhaust stubs. It was installed by previous owners after the engine was upped to 1400cc and they felt there was a tendency for carb ice to form at around 2900RPM. The modification, though, was rubbish. The cuff was too short and as far as I could see did nothing other than corrode the exhaust stub, and the flap valve box they put on the Zenith only served to strain the float chamber/carb body joint and leak fuel. I ditched the system 12 years ago when I replaced the exhaust pipes at great expense from EIS Aircraft and I've never known carb ice form, not even during ground runs in damp conditions with the cowlings off.

But I'm surprised by your climb rates. Looking back at some of my historic figures for the RF3 I have average rates of climb over 5 minutes of 660ft/min - 670 ft/min. I even have one over 700ft/min, but that was probably a weather induced aberration or sloppy note taking. My take-off weight was 350Kg, though, so maybe your figures are excellent for 377Kg. I lack the ability to calculate that.

Re. 'tractors', I understood our little Zenith was fitted to agricultural implements, which I took to be things like rotovators or mowers. Don't know where I got that idea, though.

Bob Grimstead
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Posted Monday, May 24, 2010 @ 08:22 AM  

Hi again Donald,

If asked, I would have said HDO climbed at 600 fpm at aerobatic weights (me plus 15-17lit fuel) in 25°C temperatures or better, but remember, those were figures I got when I bought it, six years ago.

The poor old thing is now 42 years old, and has accumulated 1250 hours, more or less, with the last 250 at full throttle, so it's only to be expected that it is not as powerful as it once was.

HDO now has a new engine (see thread) and should go really well when I get it flying.

Yours, Bob

Bob Grimstead
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Posted Monday, February 27, 2012 @ 04:41 AM  

Hi Guys,

As you will probably remember, I replaced my blue Fournier's original Rectimo 1400 with a new Aeropower 1750, but that exhibited serious problems (I will address these in the relevant post).

So for a year, the old Rectimo has been back in HDO.

I was concerned about the validity of those earlier climb figures with the ram air intake, and I soon hope to re-fit the Aeropower engine, so yesterday I gave the old ram-air intake a go again.

The two comparative flights were less than an hour apart, with near-identical conditions:
Pressure 1013 HPa, temp 24C, high stratocumulus overcast, wind 220/5-8.

First flight with standard cowlings and intake and 16 litres of fuel, climbing at 60kt (70mph, 110kph)
Take-off rpm 2750
climb rpm 2850 at 60kt
Height climbed in five minutes 300-2830ft = 506 fpm

Second flight with direct cold-air intake and 17 litres of fuel, climbing at 60kt (70mph, 110kph)
Take-off rpm 2800
climb rpm 2900 at 60kt
Height climbed in five minutes 300-3200ft = 580 fpm

I make that a 14.6% increase in climb rate.
So with an extreme need for performance and ignoring the consequent complete lack of carburettor heat, a direct cold-air intake might occasionally be useful.

Yours, Bob

--------------------
Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

jb92563
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Posted Monday, February 27, 2012 @ 11:05 AM  

Wow thats a pretty impressive difference for a relatively simple change.

Does the tube have any filter in it to keep dust etc out of the carb?

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Bob Grimstead
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Posted Monday, February 27, 2012 @ 11:34 AM  

Nope!

As I said, I'm replacing the engine.

However, if I were using this set-up long-term, I would cable-tie the foot of an old pop-sock/stocking/tights/pantie-hose over the front.

Incidentally, you will see that the true aerobatic contest and air race aces do the same thing with their four-inch intakes. Ram air, no filters, nothing to inhibit the flow of huge gobs of air into the engine.
With my set-up, any rock would have to be smaller than about 10mm to get past the butterfly etc anyway.

Yours, Bob

--------------------
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Posted Tuesday, February 28, 2012 @ 10:13 AM  

My Zlin as no air filter and the throttle butterfly is right at the front of the intake manifold just behind the prop and so it is not feasible to add a filter. You see some pictures of Zlins that look like they have a filter but it is a very course mesh and would stop bricks and seagulls but let humming birds, insects and dust pass right through.

I fly out of a very active grass airfield that can be very dusty in late summer. I have over 500 hours flying off this field and have had to pull heads off of the old Walter engine periodically to work on leaking exhaust valves. If it is any consolation the bores and piston skirts showed absolutely no sign of scoring as a result of dust intake. I am careful not to taxi behind someone else or get in their prop wash during their run up. I have given up worrying about it. I think that because the nose is so far above and ahead of the wheels that you don't generate any dust to get sucked into the engine during taxi and once you are airborne the dust issue is minimal. Insects seem to be no issue either. I see a few squashed bugs on the throttle butterfly after a flight but no sign of them inside the engine (octane boosters perhaps?) Personally I would be much more concerned about sucking the sock or panty hose into the engine, resulting in a sudden silence very soon there after, than I am about damage caused by dust.

Speaking of Octane fuels. 100 Low Lead Avgas has 4 times the Tetra Ethyl Lead that 80 Octane Avgas used to have. So I think that Bob's 25% Avgas 75% Mogas blend should simulate old 80 Octane Avgas pretty well. Some lead can't hurt valve guide lubrication and detonation prevention at all so I think the blend is better than straight Mogas. From talking to the antique airplane guys that hang around Van Sant it seems that old low compression engines (6:1) like the Walter and many radial engines don't tolerate high levels of TEL as found in 100LL very well. The lower combustion temperature apparently causes deposits on the valve stems that results in sticky valves and burned exhaust valves. I used to get about 100~150 hours to a set of exhaust valves in the Walter. I have now started running TCP in the fuel in both the Zlin and the Vivat on the advice of many of the 220 Hp Continental radial powered Stearman owners (6:1 CR) who have found the same issues with exhaust valves. I'll know in a few years whether it is worthwhile!

Best regards, Dave

Bob Grimstead
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Posted Tuesday, February 28, 2012 @ 11:14 AM  

Hi Dave,

Thank you very much for all that interesting info. I didn't know that about the magnificent Zlin's intake having no filter. That makes me feel a little better about my unfiltered set-up, even though it is only just over one foot off the ground! But Perth is all sand, and where it's not sand it's gravel & dust (you've seen the Australian movies) so I'll only do this with the old engine (for a while, anyway)

Also if what you say about the lead levels in 100LL is true, with my 8.2 to 1 compression engines I should probably go back to the fifty/fifty Mogas/Avgas mix I used to use.

What is very, very interesting is the practice aeros sortie I just flew today (1400cc engine, uinfiltered 'ram' intake). As usual, I dived from 800 to 500 feet (my legal minimum here) for the first loop, and flew my 2012, 15-figure, four-minute sequence (admittedly all easy manoeuvres because I'm starting low). All through the sequence I monitor my height, although I'm not obsessive about it. Positioning is more important (especially with the on-crowd crosswind I had today) because I can see from the proximity of the trees when I'm getting too low. All my brain was processing was that I was not becoming unsafely low, although I did notice with surprise some numbers above 1,000 feet from time to time.
But when I finished I was at 1,500 feet!

That's a whole 1,000 feet higher than I started, after about 3 min 45 secs, or over 250 feet per minute climb rate while flying aerobatics! I could see no signs of convective activity (5/8 stratocumulus at 4,500 feet), the temp was 24C (mid 70s F). OK, so I was quite pleased with my flying, and I managed to complete the sequence without the engine coughing and spluttering for a change, so I would not have expected to lose any height, and even a small gain might have been expected (say, to 800 or 900 feet ).

http://youtu.be/4EGDxM2HtDE

Could it be that I'm getting some kind of ram effect from a randomly sawn off eight-inch length of vacuum cleaner tubing? Tomorrow I'll have to make a high speed run at full throttle and 1,500 feet to compare it with previous performance figures.

Don't all go rushing out and cutting holes in your cowlings and carbs yet, but I might, just might, be on to something here.

Yours, Bob

--------------------
Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

jb92563
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Posted Tuesday, February 28, 2012 @ 12:28 PM  

Thats interesting about the Zlin, but keeping the sand out couldn't hurt if one can find a decent filter of some sort that is not too restrictive.

The bad thing about the 100LL is that you get more build up of deposits than the auto fuel and when its on the valves, its a pain.

Since its a VW and was designed to run in the 60's on some lead then I think Bob has the right idea of mixing some 100LL in with the auto gas.

Bob, you must be getting more power from the ram effect.

The engine needs a certain air/fuel ratio and if its slightly rich and you are forcing more air in courtesy of the ram tube it makes sense that the engine could be producing more power.

Imagine what a tapered tube could do for you.

I have a mixture control for my carb so I suppose there is some potential for additional HP with a ram tube.

With the low carb position I would want some filtering though (Flying off a dirt runway) or be prepared to do more frequent top end overhauls.

While I was working with some aluminum flashing material making a heat shield for my fuel lines it also occured to me how simple it would be to make an aluminum cone for the carb air supply. The cone could even go around my carb filter since its all oriented in the right direction.

[Edit by jb92563 on Tuesday, February 28, 2012 @ 12:29 PM]

[Edit by jb92563 on Tuesday, February 28, 2012 @ 12:30 PM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
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http://www.touringmotorgliders.org

Collin
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Posted Tuesday, February 28, 2012 @ 09:33 PM  

Hello,

You can see that Revmaster has a ram air but filters the air when the carb heat is on.

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Posted Wednesday, February 29, 2012 @ 10:54 AM  

That Revmaster intake is really a compact package. The only downside to this idea is that you won't get any ram air manifold pressure increase from airspeed or prop wash but you will get a self cleaning air filter! The ram air pressure will cause there to be some outward flow through the filter when the pressure in the intake is higher than the pressure in the lower cowling. To get the benefit of ram air you would have to come up with a way of pulling or rotating a sleeve over the outside of the filter to close it off entirely - or devise a diverter valve like a carb heat system. If you were pulling warm air from the cowling like you do currently through the filter for take off and landing and taxi you are no worse off than you are now. If you need a performance boost on take off because of a short field, hot day or no head wind you might elect to run the ram air on the take off roll. Once airborne, unless you elect to fly through a Sahara sand storm, there is probably very little risk if dust damage to your engine running without an air filter.

As another point of reference on air filters. My dad has a 1930 Model A Ford. Back in the day air filters could not be fitted to right hand drive Model A's because the steering box is right up against the carb intake. That car lived its life on dirt roads in South Africa and has survived for 80 years (along with many others) admittedly the clearances were a lot larger back in the day. It doesn't have an oil filter or a pressurized oil system either. Just scoops on the connecting rods that lift oil up into the camshaft gallery where it dribbles back down to the main bearings by gravity.

Best regards,

Dave

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Posted Wednesday, February 29, 2012 @ 03:45 PM  

Thanks Fournieteers for a really interesting discussion and Bob for neverending quest for Fournioptimization!

I accept that maybe we don't need to be overly afraid of bringing unfiltered air into the carb, possibly even less so from the lower cowling. After all, that air flew there between the cooling fins around the cylinders, consequently undergoes some turbulent flow which could at least in theory discard at least some dust and particles due to the centrifugal forces (compare with that vacuum cleaner; Dyson).

It does strike me that forcing more air into the carb due to the Ram-Air effect could perhaps cause a leaner, hotter fuel burn. Could that cause problems?

May the 4's be with you/ Jörgen

Bob Grimstead
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Posted Sunday, March 11, 2012 @ 03:39 AM  

OK Guys,

Here we are again with more info.

First, this post was wrongly headed. My mod was not (initially) a ram-air intake, but merely a cold air intake.

Second, I very, very strongly recommend that only Fournicators in Arizona, Nevada, South Australia, Western Australia and northern Africa (or other extremely dry places) try any of this, because with this system there is no safeguard against carburettor icing.

And be warned, warm air holds more moisture than cold air, so you will always get worse icing on a hot day. You get more frequent icing on cold days, but more ice is accumulated on a hot day.

The humidity here in Western Australia yesterday (and indeed, all last week) was below 25% and often below 20%, so icing might be less of a problem. Also I operate pretty well all the time a) overhead the airfirled and b) with the throttle always fully open -- both of which minimise but do not eliminate the risks of carb ice.

Back to the mod.
As Jorgen suggested, it was obvious from the exhaust pipes' colour that the engine was running a little too lean with this direct cold-air inlet, so out came the NGK AFX lambda sensor.
After eight or ten ground runs I established that the main jet diameter I need for this set-up is 1.40mm, which is very close to the 1.42mm optimum for my 1750cc engine.

A five-minute timed climb in a much hotter OAT gave me a climb from 400 feet to 3200 feet, similar to previously, but with 22 litres of fuel rather than 17 litres, and in a furnace-like OAT of 36C (that's in the high nineties Fahrenheit, and the previous figures were in a temperate 24C) so I am very pleased with these results.

At the end of this climb, the exhaust interiors were a perfect colour.

For this climb, I secured the pipe into place with RTV silicone sealant and also sealed the carb's original sideways intake, to try and ensure that it really was a ram-air inlet.

Details: It's a randomly-cut 25.8cm length of thin-walled vacuum cleaner tube 3.1cm OD (3.0cm ID).
For our American friends, that's 10 1/8 inches of 1 3/16 inch tubing.
It's pleasing and neat that a grey plastic Kodak film pot lid pops into place on its forward end to keep out the dirt, dust and gravel when I'm not flying.

In the immortal words of Forrest Gump, "and that's all I have to say about that."

Yours, Bob

[Edit by Bob Grimstead on Sunday, March 11, 2012 @ 03:44 AM]

--------------------
Flying and displaying Fournier RF4Ds VH-HDO and G-AWGN, building replica RF6B G-RFGB and custodian of RF6B prototype F-BPXV

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