O-470 in an RV-8

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Bert, Sep 16, 2019.

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  1. Sep 16, 2019 #1

    Bert

    Bert

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    I'm not sure if this is the correct forum for this conversation but it seemed to fit.

    I am considering buying an RV-8 kit and I was hoping to get some input from people more knowledgeable than myself on the pros and cons of putting an O-470 engine in it rather than the designed-for 320 or 360. I know that the 470 is heavier by about 100 lb and delivers a bit more HP at 230 but is this a huge issue? I have heard of -470's, -520's and -540's being used in an RV-8 but most of the information I can find is a bit dated. If advisable, where would one find things like cowlings (I assume the stock one will not fit)?

    Thanks

    Bert
     
  2. Sep 16, 2019 #2

    BoKu

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    The first thing to understand is that the Continental O-470 is designed to be supported from below at four mounting points in a horizontal plane near the underside of the engine, with two under the aft cylinders and two under the forward cylinders. The Lycomings, on the other hand, are designed to be supported at four mounting points in a vertical plane at the back of the engine, with two near the top of the accessory case and two near the bottom. So you'd be on your own to design and engineer an engine mounting frame that is completely different from what Vans provides.

    As for the cowling, that's the easy part; it's just a handful of fiberglass chopping and channeling.

    If you search the Vans Air Force forums, you see some discussion on the topic. I remember about five years ago when someone mounted an O-470 on an RV-8 by attaching a Beech Bonanza FWF to the RV-8 firewall. The pictures were not particularly flattering, and I haven't heard of the airplane since then.

    Bottom line, I'd file this one under a fairly strong "not recommended." Unless you have experience designing and fabricating custom motor mounts, the oddball engine will add years to your build with few redeeming qualities. If you want a six cylinder engine for its smoothness and extra power you're much better off using a Lycoming 520 or 540. There are several builders of such "Super Eights" who would be glad to advise you on the pros and cons.
     
    Last edited: Sep 16, 2019
  3. Sep 16, 2019 #3

    Bert

    Bert

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    I am familiar with the difference between the 470's bed type mount (I have one) and the 360's conical mount. I don't have the kit yet so I have no idea what the dimensions of the mount points are, though, so I don't know if the mount I have will fit in an RV-8 or not.

    I had heard that the stock cowl can be modified to fit but I haven't found any specifics on it yet.

    I read the accounts on the Vans Air Force forum but they are the ones that are a bit dated. I haven't read anything recent about the outcome of any of these applications so I know next to nothing about them.
     
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  4. Sep 16, 2019 #4

    Toobuilder

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    Speaking as one who has both a 540 Rocket and an RV-8 I believe this is a poor choice. The flying qualities of the -8 are significantly degraded with added weight, and especially higher polar inertia like you will have with a 470 on the nose. Even if you can keep balast out of the tail (unlikely), just moving stuff aft is going to put you in a whole new airplane WRT spin recovery and general flying qualities.

    I think it can be done safely with enough work, but it's going to be a one off pig of an airplane in the end.
     
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  5. Sep 16, 2019 #5

    Dana

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    What are you trying to achieve? The RV-8 has great performance with the engine it was designed for. Adding more power will improve your rate of climb but have much less effect on speed. And as Toolbuilder said, the added weight is not a good thing.
     
  6. Sep 16, 2019 #6

    Bert

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    I had heard that the behavior of the plane changed but I don't know by how much. I don't plan on doing any serious aerobatics so I don't know how much it would effect me, so I decided to post here to find out from those who have experienced it.
     
  7. Sep 16, 2019 #7

    Bert

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    I'm not trying to "soup up" an RV-8 to get more/better performance out of it. My reason for considering it is quite simple: I have an O-470 and CS prop in my garage and I am negotiating the purchase of an RV-8 kit. If I can mount that engine/prop combo on an RV-8 easily without jeopardizing the plane or myself, that would make things much easier for me. If not, I will go the O-360 route.
     
  8. Sep 17, 2019 #8

    Rockiedog2

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    what Toobuilder said
    We see this all the time...guys adding big weight FWF and then gotta add weight in the tail and then the wing loading goes way up etc etc etc and we end up with a piece of crap airplane. Toobuilder got it right...one off pig.
    I wouldn't get near that thing...it'll pee on your foot.
     
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  9. Sep 17, 2019 #9

    Toobuilder

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    I think that's false economy. Three things are inescapable with your plan -all bad:
    1. You will at least double the build time;
    2. The resulting airplane will fly poorly (compared to a "light" RV-8);
    3. The resulting airplane will have substantially reduced value in the market if you need to sell.

    My advice: Sell the 470 and prop and put the money toward a Lyc 360.
     
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  10. Sep 17, 2019 #10

    TFF

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    Everyone I know who flys an RV8 adds some ballasts to the cargo to ease how nose heavy it is with just one person. One friend said he prefers his 7 over his old 8 because of ballast loading. Fill seats and go fly instead of configuration changes with the 8. Add another 100 in front is not going to be friendly.
     
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  11. Sep 17, 2019 #11

    Bert

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    I guess I am going to be in the market for an IO-360 and prop. Anyone wanna buy a freshly rebuilt O-470 and prop?
     
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  12. Sep 17, 2019 #12

    Rockiedog2

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    Bert
    I been down this road on my 8.
    The IO360 is ok but not so much as it would seem. The O360 parallel valve while 20 or so less horsepower is also 30 or so # lighter. And cheaper. I went from the IO to the parallel valve O360 on my Acrosport and the weight loss made an amazing difference. I never would have figured it would have been so noticeable. If you do the power/weight calc on the 2 engines I think you will find that in a practical sense the IO mostly just hauls itself around. There are other benefits as well; low pressure fuel system vs high(big $) lower compression(I used to run mogas premuim in mine); smaller lighter oil cooler(7 row vs 11 row), when you get into it you'll find other weight and cost benefits. I learned it by doing...I'll take the 180 anyday.
     
    Last edited: Sep 17, 2019
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  13. Sep 17, 2019 #13

    Bert

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    Thanks for the insight. I hadn't thought of it that way. It will be a while before I have to make a final decision but I will definitely bear this in mind.

    Bert
     
  14. Sep 17, 2019 #14

    TFF

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    I know if a fixed pitch wood prop RV8. The guy loves it. He has had Pitts and other stuff.
     
  15. Sep 17, 2019 #15

    BJC

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    Agree with all of the above. Wrt weight, I will add that, when doing serious aerobatics, the difference in my standard fuel load of 8 gallons verses 12 or 15 gallons was very significant in the S-1S. Light and nice handling is a winning combination for fun flying.

    BJC
     
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  16. Sep 17, 2019 #16

    Rockiedog2

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    Yessir. Big difference.
    The most graphic example I have seen is my Legal Eagle. 5G fuel capacity/237.5# EW/45 horsepower/450# typical operating weight. The difference in 2G FOB and 5G is astounding. Obviously but underappreciated by many of us; our homebuilts are relatively light and have relatively small wing area so weight and horsepower variations are a bigger percentage as opposed to say a Champ or 172 etc and have a much more noticeable effect.
    I've said this before so pardon the repetition and this is obvious too but many haven't figured it out yet. Say we are able to lighten a part by 3 ounces. It's common for some of us to consider that as not worthwhile. But if the part weighed 14 ounces we've reduced the weight by 20% which is huge. Do that all over the plane and we'll have an outstanding airplane. I learned to look at the ounces and percentages and it changes the whole perspective. For a percentage/ounce guy, adding 100 **POUNDS** anywhere on the plane particularly either end is horrifying.
     
    Last edited: Sep 17, 2019
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  17. Sep 17, 2019 #17

    Rockiedog2

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    I like simple and light like that but in the case of my 8 I reluctantly accepted the weight/complexity of a constant speed prop. Reason is my strip is short and obstacles and even tho the RV's are good in a strip like mine the high pitched fixed prop has enough idle thrust to be too much compromise for me. And the CS acts like a speed brake. With the speed range the RV's are capable of IMO they deserve a CS in spite of the downside
     
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  18. Sep 17, 2019 #18

    TFF

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    I agree. It’s kind of weird.
     
  19. Sep 17, 2019 #19

    BJC

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    Another highly experienced builder who is fanatical about weight is Ted Setzer of Stoddard-Hamilton / Glasair. Ted built a light weight, highly modified, Sportsman, and saved every ounce, literally, possible, including drilling lightening holes in phenolic pulleys.


    BJC
     
  20. Sep 17, 2019 #20

    pictsidhe

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    Mitsubishi controlled the weight on every part at least 0.1% of empty weight of one aircraft. The result was the Zero.
     

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