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How big a problem is to much engine

Old 04-19-2010, 11:44 PM
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rltrahan
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Default How big a problem is to much engine

I have an Easytiger TBF avenger which the sight recommends a 26cc gas. I live in Colorado Springs and our altitude is at 6500 ASL which most engines will run about 20% less efficient than at sea level so bigger is always good and sometimes a must. I came across a good deal and purchased a 3W CS 75cc gas engine that weighs 98 ounces with Pitts muffler and ignition. Don’t know max weight yet for meeting the CG on the Avenger but I’m sure that the 98 ounces will make the plane quit nose heavy. The plane has an 81" wingspan weighing around 14 pounds. How will this affect the flight performance if I have to actually add weight to the tail increasing the planes over all weight? Power wouldn’t be an issue as much as the wing area having enough lift for the weight I would think. I’ve already sent this question to Curtis over at easytiger and hope to hear from him tomorrow but I thought I would post it here for any open suggestions.

Link to easytiger TBF Avenger ;

http://easytigermodels.com/index.php...products_id=18
Old 04-20-2010, 01:56 AM
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Sandmann_AU
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Default RE: How big a problem is to much engine

How much of a problem is too much engine? That really depends on how much too much engine you have, and what sort of plane it's going to go into. You can generally go "one size up" without much of an issue as long as it'll physically fit into the airframe, but going from a 26cc gasser to a 75cc is a whole 'nother kettle of fish! You're talking about an engine that's 3 times the recommended size there which will have a bunch of implications. Here's a few...

Weight. You've already mentioned this as refers to CG but if goes further than that. The extra lead you're going to have to add to the tail to get the plane balanced is really going to add a lot of weight to the plane. This in turn will push up the wing loading (ratio of lift generated by the wings to weight of the plane) quite a bit. You're considering putting it onto a warbird and these planes already have wing loadings that're right up near the edge of the envelope to get good manoeuvrability. I think this would make the plane very prone to tip stalls and be fairly un-flyable.

Ground clearance. As soon as you go up an engine size, you also have to go up a prop size to absorb the extra power or the bigger engine will just spin itself to pieces. This is why Spitfires, Corsairs etc "grew extra blades" with later versions as the engines became more powerful. Generally going up "one size" isn't a big problem so long as you're careful with your landings, however I think you'll find that the prop the 75cc needs will be so big you won't even get the plane level on its wheels before you're digging holes in the ground. You can reduce your overall prop size by adding blades (eg: going to a 3, 4, or more blade prop) but you then lose efficiency from the extra air friction & weight of those extra blades. You could always dig a trench up the middle of your airstrip for the prop to fit into but that's a little unwieldy.

Fuel Supply. A larger engine is going to be a thirstier engine, however there's only so much tank you can fit into a model. Often ARFs come with tanks (and mounting points for them) that are already on the small side for the recommended engine and you end up choosing between carving out balsa (eg: structure) to fit a bigger (and heavier - see wing loading) tank, or only having flight times of a few minutes.Your engine has three times the volume of the recommended one and will require a tank that's about three times the size of that plane's standard tank to get about the same flight time... that's going to be hard (or even impossible) to fit into the fuselage.

Airframe Design. Our models are designed with a given engine range in mind and it's assumed you'll use one of the recommended prop sizes for that engine which will deliver a known amount of power and vibration to the airframe. The designers allow for this in their design and use structures that will absorb that power & vibration for the expected lifespan of the plane, using no more material than they need to in the trade-off of strength & weight. This is why no-one designs a model to tolerate a 300% increase in power or vibration - it would simply be too heavy to fly and too costly to build & sell to the intended market.

Now... I'm all for overpowered planes - all bar one of mine have bigger engines than recommended, in fact I once shoe-horned a twin cylinder 160 4 stroke into a biplane intended for a 90 4 stroke! A little extra power's a useful thing and there's always a throttle if you're going too fast, however in this case I think you're exceeding the envelope of sanity and getting yourself some really expensive trouble with this combination. I suggest you get yourself an engine to match the plane you want, or buy a bigger plane to match the engine you have.
Old 04-20-2010, 03:19 AM
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bogbeagle
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Default RE: How big a problem is to much engine

In addition to all of Sandman's comments ... there's gyroscopic precession from that great big prop; this could make your model less-than-pleasant when it comes to performing aeros and maybe a little tricky if you need to fly a go-around.

You might also find that either, A ... the model is tricky to land, 'cos it's carrying too much power at idle, or B ... there is a vast amount of drag from the prop at idle; again, this could be a surprise for you. The actual effects will depend upon the prop which you choose and the idle rpm which you select.

Gosh, we are negative, aren't we?


On the plus side, if you get over those problems, you will have a model which will do those thundering fly-pasts and effortless, huge loops, which are typical of the full-sized warbird.

TBH, I'd have thought that a 14lb model would fly very well on a 1.50 -sized four-stroke glow engine. Add a bit for your altitude and you are looking at a 1.80 -sized engine ... maybe 2 horse-power.

Be interested to hear how it works out for you, though.
Old 04-20-2010, 05:21 AM
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rltrahan
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Default RE: How big a problem is to much engine



Thanks Guys for the info and I wouldn’t consider any of it negative for sure. Like allot of the stuff sandmann was mentioning at first I was figuring for effects. Deep down inside I wanted to see if it’s possible because the 3W 75cc is such a sweet engine. I do have an evo 26GX but again at my altitude I’m looking at the power and performance from that engine more like a Zenoah 23 with the loss at this altitude. As far as clearance goes on the Avenger I have a 3 blade Biela 22 x 12 that will clear and is a good midrange for that 75cc engine. I’ve checked fit and clearance of every aspect and it can work, its just the weight that is really the big factoring issue. Although to me it is a bit odd that this big bird fly’s with a 26GX or 150 4 stroke as those engines do seem a little small for a giant scale plane this size to begin with. But easytiger has been doing this for awhile and they know what works for their planes. I think I did want to see if anyone out there had done something this significant as far as sizing up an engine like this in a giant scale. I think what will satisfy me and be within a good range for still performing well in this bird is to get something like an Evolution 35GX which will give me the power I want.

Thanks guys for the advice and I will post back what I go with and how it works.
Old 04-20-2010, 06:05 AM
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Default RE: How big a problem is to much engine

I run big fourstrokes in small planes they swing big props and behave well apart from an initial pitch down if you throttle up unthinking.Do you know how much your battery pack weighs and does the plane balance if you stick the pack down the back?
Old 04-20-2010, 07:15 AM
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rltrahan
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Default RE: How big a problem is to much engine

Yea that’s not a bad Idea, I have a 6V 1700 mAH nicad @ 10 ounces for the servos and a 4.8 4000 mah nimh for the ignition @ 9.2 ounces which both packs together are between 19-20 ounces that I can use to counter weight the back. Don’t know how much difference it will be to make up for the CG as Ive just started the plane. Its a quick build arf so it shouldn’t take long to figure out if that will work. It is a giant scale Avenger though so it would also require some long connections to run all the way up front, especially the ignition. My major concern was that it will turn the intended flying weight of a 16 pound plane into a 19 - 20 pound plane. Ive done YS 110's in 40 and 50 scale planes but never anything this big before. The Avenger does have more wing area than most scale warbirds but will it create enough lift after all the extra weight is the question.
Old 04-20-2010, 07:33 AM
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jester_s1
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Default RE: How big a problem is to much engine

As mentioned above, tip stalling will be your biggest worry. I'd expect torque effects on takeoff to be a worry too, especially if you want to takeoff in a scale like manner. Of course with that plant, you could probably go straight to vertical in about 6 feet! There is also the increased loading on the airframe. I remember from somewhere that an aerobatics plane will take around 10 G's on the spar in the hardest maneuvers. So if you add 2 pounds total, that's 20 extra pounds of stress that the wing and attachment area have to take. Few planes are that overbuilt.
Old 04-20-2010, 07:47 AM
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Default RE: How big a problem is to much engine


ORIGINAL: Sandmann_AU


Weight. You've already mentioned this as refers to CG but if goes further than that. The extra lead you're going to have to add to the tail to get the plane balanced is really going to add a lot of weight to the plane. This in turn will push up the wing loading (ratio of lift generated by the wings to weight of the plane) quite a bit. You're considering putting it onto a warbird and these planes already have wing loadings that're right up near the edge of the envelope to get good manoeuvrability. I think this would make the plane very prone to tip stalls and be fairly un-flyable.
Ditto, ditto, ditto!

Simply put, regardles of how high your location is, that engine is too big for that airframe.
Old 04-20-2010, 07:57 AM
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rltrahan
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Default RE: How big a problem is to much engine

Thanks Jester, Minnflyer

That’s what I was worried about and ultimately what I wanted to find out.
Anybody got a new Evo 35GX to trade?
Old 04-20-2010, 10:32 AM
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Default RE: How big a problem is to much engine

Hi!
Have several...How many do you want?

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