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RV-6 on a diet

penguin

Well Known Member
My RV-6 weighed 1127lb when I bought it – rather too much but there were some compelling reasons why that was not a deal breaker such as an angle valve motor. When it was built 20 years ago the choices made were entirely sensible, but the world has moved on. It is now time for a diet. The aim is for an empty weight below 1050lb and to shift the empty cg rearwards, currently it is at 69.2” aft.
This project will take several weeks, some of the parts are still in transit!
I have made a prediction for the weight that can be saved, current estimate is 1035.4lb empty weight. Yesterday I started by removing many of the engine accessories and weighing them, so I know most of the weights in the attached table are correct. Going to be interesting to see what the eventual result is. I'm not smart enough to format a table here so have attached a picture of what I am planning.
Headline items that save the most weight
Prop, Hartzell removed Whirlwind fitted (-23lb)
Battery, PC680 removed Varley Lithium fitted (-10lb)
Cowling, old uncored version removed new cored cowl fitted (-15lb)
Sump & inlet. Lycoming items removed SkyDynamics Maxisump fitted (-9lb)
Alternator, regular front mount alt removed Monkworkz fitted with light SkyDynamics starter ring (-9lb)
Changes in the panel, including new transponder (-12lb)
Lighting changes, replacing incandescent with LED (-7lb)
Pete
 

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RV-6 diet

Thanks for posting Penguin. Look fwd to hearing about your results. Couple years ago I decided the the best "bang for the buck" was the accumulated 30 lb off of my own rear end :).

That done, just replaced battery. Engine component upgrades at overhaul, and panel soon.

How do you tell if your cowling is cored? Have you considered making a mold and doing carbon fiber?
 
Good plan.
One thing to check is if your starting weight and CG location is accurate. Is the 1127 lbs based on weighing when you bought it a year ago or is that the original weight when it first flew? It is amazing how much weight is added just due to time passing. I did a few things recently to save 20 lbs and when I reweighed it I only lowed it by 10 lbs. when I tried to figure out why, I looked back through logs and found a lb here and there over past few years added up.
 
How do you tell if your cowling is cored? Have you considered making a mold and doing carbon fiber?

A cored cowl has a 1/4" layer of honey comb between the inner & outer skins. Anything made after around 1999 is cored. Far too much work for too little gain to mould my own, carbon would not save much, and I work for a composite company. I could probably lose 10lb, but I need to get more than that out of the aircraft!.

@Philip, yes, weight always goes up!

Pete
 
yes, weight always goes up!

Not always, as gravity will happily demonstrate... ;)

Seriously now, I bought my ship some 3+ years ago, and though it wasn't that heavy for a O-360/3 bladed MT CSP config, had her on a slight diet:
- removed the tail beacon and associated power supply, good for almost 3kg
- glassed the panel with minimal equipment for my requirements, e.g. dual G5 and GNX375, no back-up instruments. Also removed the whiskey compass. Saved about 1kg on the panel
- replaced the tailwheel (Lightweight from Flyboy) and the springs (Lynx), saved 300+ g.
- biggest save yet (probably not applicable in the US...), exchanged the muffler and shaved almost 5kg at once
- SDS engine covers, lighter for sure, but don't remember the figure

On the other side, there were a couple of fatty items added, such as Supertracks for the sliding canopy, a fifth point for the harnesses, and some other gimmicks.

Actual weight is 485kg (1070lbs), but this a simple machine, no AP, no heated pitot, etc.

Still pondering if going for the battery... not too sure since mine is located in the alt box between the PIC and PAX pedals. That would be a nice one, but still 100% convinced about the reliability issue, inside the cockpit.
Another one would be one of those SDS flywheels, they sure look pretty hi tech :)
 
From my recent experience I would recommend weighing your craft with very accurate scales for yourself if you are a buyer and not the builder. Long story short. Bought mine around 2006 and recorded weight was 1098. Over the years I have made many changes which should have resulted in a lighter aircraft. Just weighed it a month ago. 1130. Asked the builder about it. His weight was before the first flight with no paint.
 
One further observation in my humble opinion. Most of us could shed around 30 lbs and live longer to enjoy flying
 
I could loose 10-15 pounds off my RV-7.
And I do nothing to change my plane.
Not a New Years Resolution (dont do that), but losing body fat gives me more motivation when I also think I will reduce my plane's gross weight.
 
Airplanes Get Heavier

Mm hmm, airplanes get heavier due to meteorite dust, if nothing else. 1037 lbs. for an RV-6 with a constant speed prop seems pretty good to me. It's surprising how much weight builds up in stuff like leftover wiring when electrical equipment is removed. If somebody does a quickie sand and paint job instead of stripping and painting, it can add as much as 20 lbs. A friend is doing an annual on a Bellanca with 3 paint jobs on it. I wonder what all that weighs? LED lights can often save a bit of weight on their own, especially if you can remove power packs for old timey strobes, plus you can often remove even more weight if you do a load analysis and discover that the reduced electrical land allows smaller, lighter wire. LED landing lights can make a decent improvement. If you do this, be sure you end up with the right value circuit breaker or fuse, BTW. Sheetmetal and fiberglass repairs can add significant weight when they're overdone. Bolts and screws that are too long, using bolts where screws or clevis pins are OK and using full depth nuts where shear nuts are OK and adding extra washers build up weight eventually. I use flanged nuts for low stress fasteners when I can. If you can remove vacuum-driven instruments and the pump you can get several pounds off, plus clearing a pad for a backup alternator or just cleaning up the engine room. Upholstery is heavy. So is sound insulation. Choose (or remove!) those things wisely, Grasshopper.
 
When I bought my RV-6A, it was supposed to be 1114 lbs. Once home, I weighted it and it came out to 1136 lbs (!). I was not happy.

I also put it on a diet, first removing a 24 lbs battery and installing one that is better and 10 lbs lighter. Then, mods on instruments panel, replacement of overweight electrical wiring, leds instead of traditional strobes and lights, removal of unused accessories such as the governor adapter on the engine…

I also added weight: landing light, heated pitot and aux battery for avionics.

It now sits at 1119 lbs which is still higher than anticipated weight when I bought it.
 
One thing I noticed is that removing weight in front greatly affect how much you can put in the baggage compartment. This should be considered as it is possible to have a lighter aircraft that is also less practical.
 
Yes!

One thing I noticed is that removing weight in front greatly affect how much you can put in the baggage compartment. This should be considered as it is possible to have a lighter aircraft that is also less practical.
Michel, so glad you brought that up. I was going to mention that when removing weight you need to pay attention the CG movement. I left that out for brevity and also because I figured builders and owners are smart enough to track CG movement when adding or removing stuff. But yes, you can absolutely lighten yourself into a corner if you're not paying attention. On our airplane, nearly all the weight saving things I did moved the CG aft. What we have now is an early slow build RV-6 tip up with the relatively light O-320, one magneto and a Plasma III ignition, lightweight Lycoming (B&C knockoff) starter, slightly lightish early Plane Power alternator, PC680 battery on the aft side of the firewall, a nearly weightless Catto airscrew, a minimal VFR instrument board with a G5, mini iPad and engine instruments, and an Aviation Products tailwheel with the lightest wheel I could find. Result is an airplane that wants to go just a bit out the aft CG limit with no fuel, 170 lbs in both seats and 60 lbs in the baggage box. I don't know how I'd ever get more weight off the back end and I've had to abandon fantasies about replacing the .016" skinned rudder with the more robust .020" skinned rudder or the EarthEx battery unless I add a plate of iron on the front of the propeller. If I did that I could still see a net reduction of a pound or two or three, but neither of those things are planned. Our Empty Weight is 1040 lbs and I 'spect that's about as svelte as Lola the RV-6 is likely to be for the foreseeable future.
 
This is where I am at so far. For those concerned that the CG will go aft - that's one of the objectives of this project! My CG is too far forwards and needs to shift aft. I realise this is not a usual problem for the "average" RV-6 that has an O-320 and wood prop, but I have an angle valve IO-360 and heavy Hartzell!

What changed Weight saving in lbs
Propeller 23.25
Governor 00.90
Alternator 05.75
Ignition 03.70
Battery 11.00
Battery wiring 02.50
Strobe pwr supply 02.20
Transponder 05.00

Total so far 54.30

Now working on the cowling and some electrical updates in the cockpit. Cowling might take a while...
Also waiting for some engine parts to arrive, can continue without but would be good to include.
 
CG Shift - attached is a picture of the CG shift with fuel burn for one & two 180lb pilots with no baggage.
This is for an aircraft weighing 1050lb empty with an empty CG of 70.4".
Max fuel for aerobatics is around 24 gals (US) solo.
With 2 pilots there is no problem with the CG
With 2 pilots and 100lb of baggage & max fuel (140lb over max weight) CG is inside the aft limit but goes outside the aft limit with 25 gals remaining
With 2 pilots the maximum baggage is 75lb to remain inside aft CG limit with only 5 gallons of gas.
This spreadsheet was written before the 50lb increase in RV-6 max weight!
 

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Well, this has taken much longer than I was anticipating!!
One major aim was to make the cowling much more straight forward to remove and fit. These changes have been very time consuming but I hope are coming to a conclusion. I have decided on a snorkel to reduce the drag of a ram inlet, and to make the air inlet part of the baffling so no cowl interface. The injector end of the snorkel is done but took three weeks of making changes almost every day. It looks rather ugly but that was the only way to clear the B&C starter and mate with the FM200. The knock-on was the cowl wouldn't fit, so I end up cutting away a large chunk. There are some photos of the snorkel development attached. It took a while to get the contour of the FM200 joint sorted. I will add some of the cowling later.

Due to some unique English rules this project will have to be done in 2 or 3 bites. The aim is to go flying this year with some of the changes but leave most of the engine parts to early next year.
 

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Cowl

The cowl started as a standard RV-6 cowl, with an angle valve motor it is too narrow and the top cowl is too low. The first change was to make some small holes above the front rocker covers in the top cowl to get it to fit. The flange at the front had to be removed as the WW spinner wanted that space! Once the firewall attachment was done with ¼ turn fasteners and the sides with hinge, and the cowl matched to the Whirlwind 74HRT prop, I could start making the snorkel fit inside. After flattening the bottom of the snorkel slightly it was clear more major action was required. I had already cut off the front half of the scoop and moulded a piece from the front half of an IO RV-7 cowl coal shovel exit ramp. I ended up cutting quite a large hole in the front.

To enable some kind of clearance with a reasonably fair line was going to be tricky as the position of fuel inlet and mixture control of the FM200 meant a sizeable bump has been required. I taped some layers of 1/8” foam on top of the snorkel and cowl to give around 3/8 clearance and then moulded a skin of glass over the top. I could then bond on the part moulded from the RV-7 cowl and cut away most of the rest of the lower cowl. That formed the basis of the lower contour fleshed out with microballoon. A blister over the fuel inlet was created with foam and balloons – looked like a chocolate cake! Once the shape was reasonable some layers of glass were laid over the top. The balloons were ground out of the inside and more glass laid in. Next is the rear wedge.

More pictures soon
 

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A few more pictures of the cowl progress
 

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That brings back memories! I did that mod back in 2002 with my original non-honeycomb cowl. I also have on the list of things to do to replace that heavy cowl with a new one. Plus the epoxy ones don't move as much so the paint stays much nicer.

Question: (maybe too late now, but) why didn't you start with a -7 cowl for an angle-valver? You shouldn't have to do any fiberglass mods that way. The -6 and -7 share the same firewall dimensions.

P0000896.JPG P0000897.JPG

P0000905.JPG PICT3505.jpg
 
Well, this has taken much longer than I was anticipating!!
One major aim was to make the cowling much more straight forward to remove and fit. These changes have been very time consuming but I hope are coming to a conclusion. I have decided on a snorkel to reduce the drag of a ram inlet, and to make the air inlet part of the baffling so no cowl interface. The injector end of the snorkel is done but took three weeks of making changes almost every day. It looks rather ugly but that was the only way to clear the B&C starter and mate with the FM200. The knock-on was the cowl wouldn't fit, so I end up cutting away a large chunk. There are some photos of the snorkel development attached. It took a while to get the contour of the FM200 joint sorted. I will add some of the cowling later.

Due to some unique English rules this project will have to be done in 2 or 3 bites. The aim is to go flying this year with some of the changes but leave most of the engine parts to early next year.

I hate to be the bearer of bad news, but I think you should be prepared for some engine operational issues because of the shape of your modified snorkel. The 200 series fuel servo is very sensitive regarding clean inlet airflow.
We put quite a bit of effort into finally get a very good performing snorkel shape when we did the development work with the EXP 119 engine in the RV 14. That very sharp turn that your snorkel shape takes as it changes to go aft into the fuel servo is likely going to cause problems.
 
Yep. A well done pitot style inlet is virtually "free" from an aero standpoint, and is just about the best you can get for ram recovery. Dave Anders told me this a long time ago and because I'm just that stubborn, I cut the snorkle off my Rocket and did a very nice flowing inlet in the cooling duct. The airplane lost MP and slowed down just like Dave predicted. Looks like I'm going back to the snorkle.

Just dont throw that one you cut off away - you might be grafting it back on after your next few flights!
 
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Thanks for all of your thoughts
Question: (maybe too late now, but) why didn't you start with a -7 cowl for an angle-valver? You shouldn't have to do any fiberglass mods that way. The -6 and -7 share the same firewall dimensions.

When starting out on a project like this there are always a few goals, mine were
- Save weight (goal is 70lb lighter) allows sensible fuel load at aero weight and some baggage when my wife is along.
- Save drag & go faster or further
- Repair the cowl and make it much easier to remove and refit

The cowl (an old non-honeycomb item that had been adjusted to accommodate the angle valve motor) was rather tired and every year at the annual need some repair. Plus it was not at all easy to remove and refit. I was offered a just delivered RV-6 lower cowl for a very good price. Remember that shipping anything from Van’s to England is really expensive, when shipping a cowling the price of the shipping approaches that of the part. So that made the decision to change the cowl as part of this process. At the time I wasn’t aware that the 7 IO cowl is wider than the 6 cowl. Once I had the cowl the die was cast.
I hate to be the bearer of bad news, but I think you should be prepared for some engine operational issues because of the shape of your modified snorkel. The 200 series fuel servo is very sensitive regarding clean inlet airflow. …
That very sharp turn that your snorkel shape takes as it changes to go aft into the fuel servo is likely going to cause problems.
I have already found that out! The original installation had an AFP extended inlet with a diverter valve. When I bought the aircraft I was instructed to select alternate air on the ground and ram air in flight. At low throttle settings, such as just after start up, the engine really struggled to run and needed constant large throttle movements to stop it quitting. Yellow arrow in picture below shows where the air enters in Alt Air.
Eventually I figured out that the FM 200 was getting very turbulent air. Selecting ram air on the ground made the engine run much more smoothly. The radius on the left hand lip of the snorkel is as large as I could make it (after reading some Dan Horton comments). It is substantially reshaped from the original profile. I hope it is large enough but am aware of the risk. Exactly what to do if it does not run smoothly is tomorrow’s problem!!

Talking to the original builder the cowl was intended to be the first iteration of an ongoing development, except he was having too much fun flying to move on to stage two! The scoop I cut off won’t really fit – needs to be higher up – I might have to rip off the scoop from an Extra cowl.
 

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Well, this has taken longer than I was hoping!
The SkyDynamics cowl arrived eventually and has pushed the fuel servo forward enough to require another cowl rework. There is no way the snorkel is going to fit in its current form so I have gone with a hole in the front of the cowl. Before making the hole I took a mould off the cowl so hope to get the snorkel to fit over the winter (another winter project...). Cowl is now in the paint shop and the plenum/baffling is on the way to being complete...
Might get to fly this year ...

Original cowl bump with cut outs for fuel servo controls with SD intake
https://vansairforce.net/community/attachment.php?attachmentid=44149&stc=1&d=1687172970

Cowl bumped again with card and filler https://vansairforce.net/community/attachment.php?attachmentid=44150&stc=1&d=1687173030board

Laid up in glass with a layer of carbon - more for looks than anything else
https://vansairforce.net/community/attachment.php?attachmentid=44151&stc=1&d=1687173093

Hole in the front for the intake
https://vansairforce.net/community/attachment.php?attachmentid=44152&stc=1&d=1687173183

And profiled to give (I hope) reasonable engine handling
https://vansairforce.net/community/attachment.php?attachmentid=44153&stc=1&d=1687173192
 

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The cowling is now out of the paint shop and looking good!

Installed it looks equally good - the stripes can wait ...

With the cowl installed it was time to get out the scales - the really good news is the number is 81lb less that the last weighing (was 1127lb, is now 1046lb), CG is at 70.5"!! I am really pleased.
After a little tweaking of the plenum & baffling it will be time to fly and find out if all the work has been worthwhile.
 

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