Desert Aircraft vibes

[thurmma]

Here is my two cents on the subject. I don't know much on the two stroke side, but when I was playing with small block Chevy's, the more radical the cam, the more the engine shook. Why is this you ask? When you leave the valve train open longer to get more fuel into and out of the cylinders, you do not have the opposing compression to offset what is happening on the other side of the block, so it would stand to reason that you have an engine that does not run as smooth as one that is building compression on one side while the other side is burning fuel.

With the two strokes, the same principal applies when you start playing with the port timing. The compression does not start building until the piston is further up the cylinder. You now have mass moving further than it does in an engine that doesn't generate as much power, but runs smoother because the compression is actually slowing the mass down sooner than it does in an engine that has port timing that is designed to get more fuel in the cylinder.

One other thing to consider is, the engines that are developing the monster power, both 4 stroke and 2 stroke, generally speaking have a higher compression ratio, which will create a more violent burning of the fuel thus developing more vibration.

If I am totally off my rocker on this subject, let me know, I have a thick skin and love to learn where I am misinformed

 

[Truckracer]

thurmma, I think you are pretty much on tract with your line of thinking, especially the compression ratio thing. DA doesn't list the compression ratio of their engines, at least I have not seen it in print. But my bet is their newer engines are higher performance engines than previous models and higher compression is one of the key ingredients for their increased power. That sure would help account for their improved acceleration.

While it isn't quite the same as a model engine, in the case of high performance automotive or motorcycle racing engines, higher compression always changes the seat of the pants feel one gets from the engine. In extremes, sitting behind or above some engines allows one to feel with force every time a cylinder fires and especially at lower RPM. By comparison stock lower compression engines are quite smooth. I have to believe this carries over to our model engines with higher combustion impulse induced vibration.

 

[jamesrxx951]

Im not sure I agree with the compression being an issue with vibration. For example most modern 4 stroke automobile engines have 10:1-11:1 or maybe even slightly higher compression ratios and the engines are very very smooth. My motorcycle I had was 12:1 (I think) and it was pretty smooth. When I raised the compression on my 2 stroke jet ski the vibrations never changed. On my other jetski the compression was low but it required a balance shaft. My belief is vibration is caused by light crankshafts that may not help to isolate the vibrations.

 

[49dimes]

I edited my post because I have a bad habit of getting to technical but engines as related to this hobby is a passion of mine. I enjoy seeing other "gear heads " contribute and that is all I try to solicit when posting. Not trying to be master anything just trying to type my views. If I am wrong then I am wrong and it does not bother me to be wrong when corrected by right. I don't think however I was to far off base with cylinder /piston/rod mass weight ratios and timing /compression. A tachometer on my plane is telling me that. Multiple cylinder firing orders on 4 or more cylinders is a huge factor on engine vibration. Our twin cylinder 2 cycles lack harmonic isolators to help subdue the knock force other than beefing up the crank cases.

 

[Torquerolljoe]

I tend to generally agree with the timing/porting theory. Having turned wrenches professionally for 20 years, when you advance the timing on a four stoke, the engine will run less smooth. Retard the ignition timing and it would get smooth. This goes hand in hand with the size of the cam used. On two strokes, your ignition timing can be adjusted as well. The cam in two strokes roughly crosses over to porting. just my 2 cents.

 

[49dimes]

Well one thing is certain to me.....The DA 120 (and I imagine the 70 as well, have not run it yet) run like no other DA I have ever owned (original 100 and 50). And I say this under the best of terms. Not Fanboying but the 120 made the cost most worth-wile. The Pro Flow canisters aint to shabby either. I feel like the 120 and Pro Flows is an awesome match. My 104 is just over 28lbs and it is "foamy like".

 

[Truckracer]

Im not sure I agree with the compression being an issue with vibration. For example most modern 4 stroke automobile engines have 10:1-11:1 or maybe even slightly higher compression ratios and the engines are very very smooth. My motorcycle I had was 12:1 (I think) and it was pretty smooth. When I raised the compression on my 2 stroke jet ski the vibrations never changed. On my other jetski the compression was low but it required a balance shaft. My belief is vibration is caused by light crankshafts that may not help to isolate the vibrations.

I think it is the whole package that makes for the increased vibration. Rotational and reciprocating weights not quite balanced out by the crankshaft combined with more efficient cylinder charge filling and an advanced timing plus a higher compression ratio all contribute to the problem. I've always wondered how far a designer can go with a cantilever crank and still come anywhere close to balancing the engine. A two counterweight, 3 main bearing crank would seem much better from the smoothness and balance point of view but there is an overall weight penalty. As long as these are aircraft engines and most users and designers are so weight conscious, we may just have to live with the vibes.

 

[Pistolera]

I can't speak to why one engine vibrates more than another, but I will relate my experience with 2 DA120's in 2 completely different planes.

My first 120 went into my 3DHS 103 (mod'ed to 105") Extra 330SC. It ran very smooth on 2 or 3 blades and of course had gobs of power. Later I got a Beast and bought another DA120 for it. I was surprised that it really shook the crap outta that plane, especially at low rpms. You could barely focus on the tail feathers as it shook the whole rear of the fuse so bad.

Thinking I had a rough running engine, I swapped the DA's in both planes. Low and behold, the Extra ran just as smooth with the other engine and the Beast shook just as bad!!! I was surprised to see this result, but it answered my question about my "shakey" 120.

 

[Terryscustom]

I can't speak to why one engine vibrates more than another, but I will relate my experience with 2 DA120's in 2 completely different planes.

My first 120 went into my 3DHS 103 (mod'ed to 105") Extra 330SC. It ran very smooth on 2 or 3 blades and of course had gobs of power. Later I got a Beast and bought another DA120 for it. I was surprised that it really shook the crap outta that plane, especially at low rpms. You could barely focus on the tail feathers as it shook the whole rear of the fuse so bad.

Thinking I had a rough running engine, I swapped the DA's in both planes. Low and behold, the Extra ran just as smooth with the other engine and the Beast shook just as bad!!! I was surprised to see this result, but it answered my question about my "shakey" 120.

Good test, I've wondered about that. I have had mine in three planes now and in the new 3DHS 108 it seems smoother than a slightly smaller ARF. Either way, the power is addicting.

 

[Pistolera]

You should try one on pipes....HOLY MOLY!!!! It would rip a Biela 26x10 3-blade that was quiet on canisters!!!