Drag reduction as a function of vibration

[av-mech]

Friends,

One of the things I offer in my business is Dynamic Prop Balancing.  I have been doing this for about 10 years.  This works very well on tractor type aircraft but has always been a bit of a disappointment on pushers.  I have been talking this over with people much smarter than I.  We are aware of course that the prop ends up getting really dirty air and this plays a very large part in making the engine run smooth.

It recently occured to me that we should be able to qualify drag reduction/airflow improvemnet around the fuselage and cowling by comparing inflight vibration signatures.  I have flown my Vari with vibration equipment installed on the engine, which is what led to me trying to figure out why it was smooth on the ground but had more vibration in the air. I believe that cleaning up the airflow should produce a reduction in vibration signature. 

Anybody have a thought on this??

[Bill James]

Two thoughts Arnold,
A cockpit prop vibe indication would be an excellent display to have. Several folks have said that a simple version could be assembled and installed quite reasonably. Like most things on an airplane  
Two, the title of your thread reminds me of an “out there” story, which I will gladly try to accurately embellish and repropagate  
A Lancair guy stated that his second cousin’s buddy got 22 mph more out of his Lancair after installing a dynamic(?) balance flywheel on his plane.
Of course, I defended the side of impassioned reason saying, hey, that can’ be.
He insisted it was true and possible and explained the phenomenon something like this.  
The propeller on an internal combustion engine appears to turn smoothly. However, there are multitudes of minute stops and starts during the four stroke cycle of multiple opposing compression and exhaust strokes.  
At this point I was right with him on the setup, expecting a really gratifying helping of physics lore, which would leave me stunned and impressed for the rest of the day. Something you could hang your hat on. Maybe pick up a knot or two. But he continued.
So…there are seemingly invisible starts and stops that actually significantly affect the propeller. His twist on it, so to speak, was that when the prop accelerates, it flexes from the air resistance, which increases the AOA, which increases the drag. And drag is the enemy. Then the prop decelerates abruptly, then accelerates again, and the drag producing sequence starts up again.
He defended that if you could just get the prop to run smoothly, it would naturally produce less drag, produces more thrust, and you get more speed. Nothin to it.
Add that to the oft stated pronouncement that the most efficient propeller ever made had only one blade, and you have several feet keeping the old ball rolling.
I believe that these two issues are slightly different, with yours related to balance, and mine with drag. Oh! you do mention drag!
I wonder what would happen if you….
Never mind. Food for thought.    …two thoughts  
Have a great weekend!

[saybeengineering]

It would be nice to get the perspective of this LongEZ owner advertised on DynaVibe of before and after of this procedure....

http://www.youtube.com/watch?v=iIZ0fkAuoIU&feature=player_embedded#!

Regards,
Afif

[av-mech]

That guy started out at a .88" ips, that's super high and any balancing would have help that rotating mass.  Bare in mind that your not just balancing the prop, your actually looking at the whole rotating mass.  Prop, spinner, spinner backplate/bulkhead, flywheel, ring gear and crankshaft.  No flywheel for continantal engines.  They ended the balance at .06 which is not bad but I always work to get my balances under .030"ips, especially on Lycomings because they are easier to do than Continentals.

Now what we have found with pushers is that the dirty air coming into the prop really effects the balance job.  I can do my Vari on the ground and get a reading in the .020" ips range, take it up flying and it will jump to .100" ips or so.  This is not a common problem on tractor props.  Why is it a problem for pushers?  The dirty air causes more pronounced torque fluctuations in the crankshaft and this is felt as vibration.  BTW I use the MicroVib II equipment which also performs advanced spectrum analysis functions, something the Dynavib does not do.

Trivia question:  How is torque transmitted from the crankshaft to the engine case?  I have the answer, do you?   

[dhanson44]

Do you have wheel pants? Brian Alley sells carbon fiber pants preasure recovery style.They will help you handeling and speed. Sam James sells them too they are both knock offs of Gary Hertzlers wheel pants... Fast.
Dave hanson

[av-mech]

Dave,

I am in possesion of the CSA molds for the Varieze pants and they look like they will do just fine,  as soon as I can get them made!

[dorr]

Bill,

I tried one of those dynamic rings on my starter ring - seemed smoother, but if I got 2 knots I'd be surprised.

I'll have to ask my balancer guy, Fackler, if he'll sense in the air and tell me if my clean airplane gives me a differential like you get.

Beagle

[flyingwaldo]

The fellow in Watsonville that does this service uses 2 sensors to measure in two different planes.  Isn't this the preferred method?

[Bill James]

OK Arnold-
i have been patiently waiting, unable to sleep since your challenge. so...
>>Trivia question:  How is torque transmitted from the crankshaft to the engine case?
Thanks
Bill James    

[av-mech]

Bill,

Ok, someone finally ask's....

On the power stroke, as the piston is being pushed downward the connecting rod has angular displacement between the wrist pin and the crankshaft journal.  This connecting rod offset makes the piston itself "lean" on one side of the cylinder wall.  Remember that the piston is being forced down by combustion gas,  Depending on the position of the crankshaft,  you get a little torque at the beginning of the downward travel and as the connecting rod becomes further off center more torque is applied to the cylinder wall via the piston, it all happens in 90 deg of rotation as the 91 st degree starts to unload the leverage the connecting rod had on the piston.  So since the torque is transmitted through the connecting rod to the piston which leans on the cylinder wall and that cylinder is bolted to the engine case which in turn is connected to the engine mount and then airframe.  You will of course note that the crankshaft has no mechanical connection that can provide leverage from the crankshaft itself.  the crankshaft rides entierly on a film of oil.

It's a little tough to visualize, I will try to post a pictoral representation with force diagram that will help.

Ok, time for another trivia question>>  What type of wing construction makes the main wing spar act as a sinasoidal wave when loaded??