Laminar Flow Wing and disturbed flow

[Guitarmaster]

As a point of reference, I have seen as much as 3 knots of decreased air speed while flying in rain.

The laminar flow airfoil is extremely sensitive to contamination. Not as much as a Varieze, but still.

You probably said an earlier post, but how much airspeed do you think you have lost?



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[jaylw314]

Late to the conversation, but I recall from the RV forums that paint jobs could range in weight from 15-35 lbs.  Since RV's generally have less surface area to cover, figure maybe as much as 20-50 lbs for a Mooney depending on the paint job?  The weight would also depend on the complexity and number of layers, so it seems theoretically possible there may have been a weight gain of 10-20 lbs.  That probably would not account for 5 knots of speed.

Just thinking out loud

[Bob_Belville]

1 hour ago, Guitarmaster said:

As a point of reference, I have seen as much as 3 knots of decreased air speed while flying in rain.

The laminar flow airfoil is extremely sensitive to contamination. Not as much as a Varieze, but still.

You probably said an earlier post, but how much airspeed do you think you have lost?



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Matt, at least 5 kts, probably more. What did you gain with the SabreCowl? Was your cowl/windshield original?

[Guitarmaster]

Matt, at least 5 kts, probably more. What did you gain with the SabreCowl? Was your cowl/windshield original?

Summertime flying gave me a gain of 4 knots. Winter time flying saw a gain of 6 knots.

I had the LASAR closure as the only mod to my airplane prior.

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[mike_elliott]

7 hours ago, Bob_Belville said:

At the same time that we added the new cowl we added bladder extensions. I am still in the habit of topping the tanks so I'm probably flying at a 75# higher weight

AH HAH!  Now you know why the E's are a couple knots faster than an F (until they have to refuel first, but I never had the Piloto Pee speed mod to get into one of those pissin matches with an E driver  

[M016576]

5 hours ago, Bob_Belville said:

Matt, at least 5 kts, probably more. What did you gain with the SabreCowl? Was your cowl/windshield original?

Just another thought.... when a Mooney adds TKS panels, it loses between 5-7 knots.  

I know it’s not the same thing- but as a comparison point- think about your new paint vs those panels...

the ridges would have to be significant and across the majority of the wing to create the same level of effect to the boundary layer that TKS panels do..... that’s 5-7 knots.

Edited November 9, 2018 by M016576

[McMooney]

I didn't even know you could get a clearcoat on a plane, def adding that when I get paint.

[Bob_Belville]

4 minutes ago, McMooney said:

I didn't even know you could get a clearcoat on a plane, def adding that when I get paint.

If you're using high end paint the painter may (try to) talk you out of it. There are several reasons it is not done. Weight, UV aging, delaminating, and more.

[McMooney]

Not overly concerned about weight, i can stand to eat less burgers anyway. 

A nice deep and wet automotive style looking plane would just make me smile from ear to ear. 

It's 2018 i'm sure there's a paint additive for UV.  Delaminating is just a sign of a bad paint job.

[M016576]

55 minutes ago, Bob_Belville said:

If you're using high end paint the painter may (try to) talk you out of it. There are several reasons it is not done. Weight, UV aging, delaminating, and more.

Here’s one final thought on the aerodynamics of your paint job can laminar flow....

The laminar boundary layer trips to turbulent very close to the leading edge- only a few inches aft, despite our low Reynolds number wing design (“laminar flow”).  Drawings would have you believe that the separation occurs half way down the wing cord or more- that is not accurate.  The natural trip from laminar to turbulent flow is approximated normally (albeit slightly forward if the natural point) with vortex generators... or, if you want an estimate of “where” it occurs on your wing according to the designing engineer- look for the first instance of “non-flush” rivets... the trip occurs forward on the cord of those rivets.

 What does this mean to you- Bob... 

well, taking another look at your paint scheme- it *looks* to me like the only areas that could possibly even be considered to trip the laminar flow to turbulent flow are only about 6-9 inches on either wing (that’s assuming these ridges only occur where the colors intersect and that assumes that the ridges are *so pronounced* that they would indeed trip the flow...so nominally, looking at at least .02” in variation).  That’s only 1 foot out 36 feet of decreased lift...and only at top speeds (any slower approach speed/higher angle of attack would see a substantially reduced effect of said ridges, as the boundary layer transition point from laminar to turbulent flow moves forward on the wing cord).

dont wreck your paint over this- please- it’s a BEAUTIFUL Mooney!

 

Edit-  I said the flow trips only a few inches aft on a mooney’s Wing: that may not be 100% accurate- I came across an article after writing this post by Bob Kromer that states he was able to keep laminar flow attached up to the main spar in parts of the wing (~30% of the chord) by polishing and heavily waxing the wing area up to those points.  It mentions that the inspection panels though caused a trip.  Any non flush rivets as well.  He claimed a 2-3 knot variation based on his tests using a specific type of paint to assess where anomalies in flow exist and attempting to correct them.  Based on the accuracy of the instrumentation in use at the time, I’m willing to bet it made a difference- but not sure of the actual magnitude.

Edited November 9, 2018 by M016576

[Bryan]

10 hours ago, Bob_Belville said:

...so I assume the new paint did not change the aircraft weight significantly.

I would think a new weight and balance would be required after major paint.

[ArtVandelay]

I would think a new weight and balance would be required after major paint.

You’d have to weigh it, I know mine wasn’t weighed when it was painted.

[Guest]

Wasn’t it the Cafe Foundation that found a several knot increase in speed on an E model just by waxing the airframe?   If wax alone can have that effect, other paint features such as ridges at paint colour changes, or orange peel finish, or dirt, dry paint etc. would have a great effect.  

Clarence

[jetdriven]

I’ve seen what Bob is talking about. You can knock the paint edges down by taping 3M 471 fineline tape about 1/16” away from the stripe edge on each side, or around a 1/8” total gap. Then wet sand by hand right down the stripe edge (between the parallel pieces of tape) with 1000-1500-2000 grit sandpaper. Then buff by hand in the same motion with Finesse-it II.  Then pull the tape off and machine buff the whole area a little. 

The fine line tape suspends the sandpaper enough where you only hit the edge that sticks up.  

It takes the edge right off.  

[Bob_Belville]

10 minutes ago, jetdriven said:

I’ve seen what Bob is talking about. You can knock the paint edges down by taping 3M 471 fineline tape about 1/16” away from the stripe edge on each side, or around a 1/8” total gap. Then wet sand by hand right down the stripe edge (between the parallel pieces of tape) with 1000-1500-2000 grit sandpaper. Then buff by hand in the same motion with Finesse-it II.  Then pull the tape off and machine buff the whole area a little. 

The fine line tape suspends the sandpaper enough where you only hit the edge that sticks up.  

It takes the edge right off.  

Byron, I've been waiting for you to chime in! I did a test a couple of days ago masking as you suggest and using 1200 wet sand paper. I only did about 12" but that was enough to make me feel it would work. 

[Bob_Belville]

1 hour ago, M20Doc said:

Wasn’t it the Cafe Foundation that found a several knot increase in speed on an E model just by waxing the airframe?   If wax alone can have that effect, other paint features such as ridges at paint colour changes, or orange peel finish, or dirt, dry paint etc. would have a great effect.  

Clarence

Clarence, that's the context that makes me want to remove the ridges, at least as far back as the flush rivets.

[McMooney]

Just a thought, could your plane be overly smooth at this point ?  maybe the rougher old paint was acting like a golf ball and causing the air to release at just the right points.

Edited November 9, 2018 by McMooney

[Lance Keve]

Hi Bob - 

All - I've had the privilege of flying in Bob's plane recently and we discussed the speed and paint issues mentioned here.  I'm no expert in the airflow or amount those stripe ridges may affect speed, however, when we were in level flight, the right aileron was deflected up an inch at the trailing edge while the left one was level.  Again, on the ground, both taxiing straight and then parked straight, the right one was up the same amount and the left was still level.  I would think that would have some affect on performance, regardless of ball being centered.

In Bob's picture, although, from the opposite side, you can see the aileron deflected up.

Bob, I think you took a picture from the other side showing that deflection and were going to have Lynn take a look.  If you have one taken from the fuselage side facing out and can post that, it's quite prevalent.  Any update on that front?

Regards,

Lance

[Bob_Belville]

1 hour ago, Lance Keve said:

Hi Bob - 

All - I've had the privilege of flying in Bob's plane recently and we discussed the speed and paint issues mentioned here.  I'm no expert in the airflow or amount those stripe ridges may affect speed, however, when we were in level flight, the right aileron was deflected up an inch at the trailing edge while the left one was level.  Again, on the ground, both taxiing straight and then parked straight, the right one was up the same amount and the left was still level.  I would think that would have some affect on performance, regardless of ball being centered.

In Bob's picture, although, from the opposite side, you can see the aileron deflected up.

Bob, I think you took a picture from the other side showing that deflection and were going to have Lynn take a look.  If you have one taken from the fuselage side facing out and can post that, it's quite prevalent.  Any update on that front?

Regards,

Lance

Thanks Lance, we are working on the aileron. With a straight edge across the yokes the right aileron is level and the left is down about 1/4". Lynn is going to check behind me and check with the rigging boards. He may need to tweak the adjustment even though we do not believe the painter could/would have messed with the adjustment. 

The rigging, particularly the subtle bending of the trailing edges of ailerons and rudder, is on going. As several here have noted this is no doubt a factor.

But I started this thread to get ideas from this august mob about the paint issue and possible fixes and I've received several useful ideas.  

[Bob_Belville]

1 hour ago, Freemasm said:

I've been following but trying to stay out of this since my initial response way back. Most paint systems that I'm aware of (there's many that I'm not) call for a dry film thickness of 2-4 mils. A second pass should still leave the total dry film thickness less than 1/100th of an inch. Everything adds up even in fractions of a percent; however, I still cannot get my head around your paint contributing to your rather large performance delta. it is probable you'll determine the final answer to be a combination of many contributors versus a single smoking gun.

I will state that the 1/16" you mention is almost unbelievable. If your measurements are accurate, it's time to put the aircraft on scales and get real W(&B) to see how much of a contributor it might be. You have a lot of people's interest if not attention. Best of luck, Sir.

See below. Now it feels like it's getting somewhere.

 

Thanks for that.

I do not know what the film thickness is for the Imron Matterhorn White AF-402 base color. The trim colors are all Imron metalics and are on top of the white. As I understand it, the offending ridges occur because the paint tends to wick up the edge of the masking tape. I know that Joe had a problem with the Ruby Red - Imron 748889. He said he would not let folks pick that color again. I love it but the metalic color looks different in some cases where some components were painted off the plane. So... some of the trim paint might be 2 or more coats thick. 

[Bob_Belville]

Interesting and coincidental comment by Brian Lloyd on the Mooney Pilot FB group a few minutes ago... (the question asked was about speed increase from the ram air MAP) but my take away is that while making sure that I'm getting rated power is important, minimizing drag is more important:

For a propeller driven aircraft speed varies by the cube-root of power. Yes drag varies by the square of speed but thrust decreases too, producing the cube-root factor. Power doesn’t buy you speed, aerodynamics do. That’s why smart people fly Mooneys.

[Yetti]

That blue swoop on top of the tail is installed backwards and catching the wind as it goes by.  If you install it the other way it will act like a wing and lift the tail and allow the plane to "catch the ride".  Should be good for a 3 knot increase.

[mike_elliott]

Back in the day, we would paint the decals on indy cars so we could feather the edges to prevent the boundary layer laminar flow disruption. Today, they are wrapped.

[Ragsf15e]

While I generally agree that drag is more important than power, I’ve done a lot of speed testing in my F and I know th ~.75” of manifold pressure I get from opening the ram air is worth a good 3 or 4 knots.  Are you sure your engine is running strong and all the compressions are good?  I had a barely noticeable exhaust valve wobble and I first noticed the speed loss.

 I have done speed tests at different weights and it definitely makes a noticeable difference, but really takes several hundred pounds of weight to make more than a knot or two.

Honestly though, probably a rigging issue since all the surfacs were just off, stripped, painted, balanced, put back on.  That’s the most likely.

Edited November 9, 2018 by Ragsf15e

[carusoam]

Extra point for M. E.!

Looks like getting the 5 kts back is going to take many small steps...

Each building on top of the next...

Precision rigging would be an interesting step... more engineering than artistry.

Adjusting levels of paint... that requires precision paint skills...

Best regards,

-a-