Strut Cuffs for MK4

[skypics 89]

I have flown the model B and now have the MK4 with under camber STOL wing.

For many hours I had been bugged by the fact that there is severe turbulence just behind the leading wing struts at the wing juncture. If you place yarn tell tails in that area you will see that during climb the yarn is streamlined, but as you level off the yarn will actually turn 180 degrees and face forward.

I used 6061 T0 to form the cuff and attached it with common bathroom bath tub calking compound for a test, thinking I would switch to something better later. Turns out the calking has held up very well.

To fill the hollow inside the cuff I used air dry modeling clay. Again thinking temporary. It's still in there. 

There are a couple videos at You Tube skypics234, but they don't show it as well as I would like.

Here are some photos. I was using tape to hold them in this shot and later changed to the calk.

 

No off the shelf cuff available I tried various modifications .

Edited 3 Sep 2018 by skypics

[nlappos 413]

Any performance change?

[chopndrag 226]

I think he has a YouTube video but I'm in the same boat wondering about performance.

Any performance change?

[skypics 89]

Very slight performance change, but each small fairing helps and adds up. 

John M

[skypics 89]

I have cleaned up the strut cuff modification. I now see no turbulence behind the strut/wing junction. 

I referring to my under chambered STOL wing. You may not see the turbulence on the speed wing

Those with the STOL wing...

Next time out flying in level cruise look out at the fabric behind the strut/wing junction and see if you see the fabric buffeting. This is what attracted my attention to this area.

If you do see the buffeting, just place some yarn or string behind that area and notice what the telltales do in climb and in cruise.

John M

[skypics 89]

Any time you eliminate turbulence on an aircraft structure you get better performance. The only exception is VG that actually cause controlled turbulence.

I have always had a fantastic climb. Even more than all but one of my 6 wingmen. So I added 1/2 degree to my Whirlwind prop and the Avid speed increased from a typical 95-96 MPH TAS to 99-100 MPH TAS. Now I can still outclimb most, but can cruise better with them.

Climb was reduced about 200 FPM which leaves me with 800-900 FPM which is greater than most other aircraft on my field. 

Engine: Jabru 2200

Prop: Whirlwind 2 blade 66" prop

Aircraft: Avid MK4 tail drgger

[NorthIdahoAvidflyer 374]

I really like the sound of the Jab 2200 on the Avid. The Whirlwind is a great propeller too. Sounds like a great combo. 

Thanks for the info.

[skypics 89]

Vince:

The Jabiru is a very simple engine using much of the design of engines developed in the '40s.

My wingmen all have mostly Rotax 912 engines. Two other with Zenith 750s use the Honda Viking and UL engines. Both of those engines have no magnetos and MUST have battery power to keep running. In fact my friend with a 750 and UL engine had an engine failure while were in the High Sierras and totaled the aircraft. He bought another Zenith with a 912 engine and loves the way it performs.

I have see the 912 installed in an Avid and it is a VERY tight fit. The engine is very robust. The weak parts seem to be the gear box and the sprauge clutch. The engine runs forever, but they seem to be having the gear box or sprauge clutches worked on quite a bit. And the voltage regulators seem to fail a LOT.

The Jabiru is a mix of off the shelf parts which keeps the price down. As I recall they use Toyota pistons, A Kabota voltage regulator, Honda magnito coils, etc.

I am pleased with the performance of my Jabiru in my Avid MK4. Of course it would be nice to have the 100 HP of the Rotax, but the cost of the engine and the complexity, water pipes and water coolers and costly electronic ignition, make my coice of the jabiru the best choice for me.

John M

 

 

[dholly 718]

Nice post John. There are a lot of reasons to consider any K.I.S.S. engine option for aircraft. Fewer points of failure are a good thing and wringing every last hp is not the primary goal for every builder/flyer!

[Avid Randy 50]

John...Thanks for the posts.  Its Great to hear some more success with the Jabs.  I have a 582 sitting in the corner to use for my build but I would like to go with something different.  The Jab is at the top of the list.   So this is valuable info. Cheers Randy 

[skypics 89]

Randy:

The Jabiru engine, like others, have evolved over the years. The new generation 4 engines have many improvements over the gen three that I have. Namely, the gen four uses hollow push rids to feed oil from the hydraulic lifters to the rockers which is better than the  the solid ones I have with tubes to feed oil into the rocker box. They also have roller lifters.

Also, buying any replacement parts is much cheaper than the Rotax engines. In fact I had a top overhaul done on my Jabiru for $7,000 including shipping.

I  think a new Jabiru is $14,900 for the 85 HP 2200.  They also offer firewall forward kits. They also offer used engines taken on trade in. The 130 HP 3300 costs $18,500 which is cheaper than the Rotax 100 HP engine.

Finally, I am told by observers that my engine with the Whairlwind prop is one of the quietest on the field.

John M