Cutting a perfectly good cowl

With the Skybolts and horizontal split line with hinges complete.. I moved on to fitting the two fairing pieces that come with the showplanes cowl. I trimmed the aft fairing to have 3/4″ flanges and then marked the center point. Additional trimming was needed to clear the front gear leg to get the aft fairing into position. 3 holes were drilled into the bottom of the fuselage for now to hold this in place. Later I will install rivets for screws to hold this into position.

Once you’re happy with this you place the lower cowl back on and drill a couple of holes on each side to lock in the position of the aft fairing to the cowling.

Aft fairing in place and cleco’ed to the lower cowling

Side view of the aft fairing.

Then with the aft fairing and lower cowling back on the bench. The forward faring was placed into position and trimmed as needed to get a good fit with the forward fairing. Once satisfied, 3 holes were drilled on each side to hold the forward and aft fairing together.

The aft fairing was put back into place on the fuse, and the forward fairing was trimmed to clear the engine mount tubes until the drill holes on each side lined up.

Front and Aft Fairing.
Front fairing fitting around the tube just below the donuts.

Once that was done, the lower cowl was put back into place to verify and adjust fit. The forward fairing needs to lay flat across the area of the cowling that has no honeycombed and be as centered as possible.

A decent fit to cowl.

I then had to add some fiberglass back to the slot that I had cut to get the lower cowl into position. I was careful to not cut too much when I did this originally, but I still cut a little too much. I sanded the edges at an angle to create a scarf joint with a fiberglass layup over the small area that needed to be added back. I put some peel ply on and let it cure overnight.

Adding material back to the nose gear slot.

The end result was pretty good.. There was a small gap in the rounded edge, which I later filled in with epoxy and let cure and sanded down.

I then sanded the inside of the cowl at the scarf joint that I added. You can see that below just about done.

Sanding the scarf joint.

I then put the lower cowl onto some cinder blocks so I could level it as needed. I used the laser level to make sure the intakes were level horizontally, which the vertical was aligned to the center line of the lower cowl.

The center line was marked with a sharpie as well as some blue painters tape. I also marked out the locations of 8 holes to drill into the forward fairing that will hold the cowl to that fairing with screws. I also mapped out 8 additional holes that mostly go through the aft fairing into the cowling.

The lower cowling was placed back on and the hole locations were match drilled to the forward fairing

Then the big moment came.. Using a diamond cutting wheel on a dremel tool to cut the lower cowling in half!

For whatever reason, despite having a plan… it seemed scary to me to cut this perfectly good cowling like this…

The cut will later be rejoined with another hinge pin like the horizontal sides. Skybolts could also be used here, but I opted for a hinge for a better look.

To prep for that hinge.. I marked out a 1.5″ wide rectangular area at the split point. I again used the diamond wheel on the dremel tool to just cut through the inner layer of fiberglass as shown below.

Cutting the inner fiberglass only. prepping for a flange buildup.

I then used a flat blade on a Dremel multi max oscillating tool to separate the honeycomb material from the inner layer of fiberglass. Below you can see me mostly done separating the fiberglass layer from the honeycomb.

Inner layer of Fiberglass removed.

I then used a chisel to carefully remove the honeycomb material. You have to use caution as the remaining fiberglass outer layer is very thin.

Using a chisel to remove honeycomb material.
Done removing honeycomb. Some additional sanding to be done still.

I then cut some strips of 9 oz. fiberglass to build up a flange where the structural material was just removed. I decided to use 6 layers of cloth for this. The end result was a thickness consistent with the other areas of the cowl that don’t have honeycomb.

6 layers of cloth in place
peel ply added.

Then after curing overnight you can see the results of the flange.

Next up will be re-combining what was just cut in half…

Cowling part 2

Indirect lighting from inside the cowl when drilling the skybolts can really misalign you. I started to also trace the inside of the hole with a fine point sharpie to get an even better view of exactly where the hole is and also use that to gauge walking the step bit in one direction or the other.

Even with that I’m ever so slightly off, although not by enough to matter.

Below is a look down the left side at the firewall with some skybolts installed. Here you can see my small light that I used taped in place to more directly illuminate the hole.

I then used a laser level to mark and trim the top cowling first. the 12″ Permagrit sanding block was used after getting close with a diamond cutoff wheel on the dremel tool.

I then used the top cut to mark out the bottom cowl cut location.

I trimmed close with the dremel tool and then used the sanding block to get it as close to perfect as possible.

Once that was done, I marked the location of the hinge pin covers from Aerosport. Location was based on the plans.

With the hinge pin covers located, I then got to installing the side hinges. I shifted the hinge downward so the eyelids won’t show. I used a AN257-P3 length hinge on the bottom cowl and a -P4 length hinge on the top cowl to get enough flange to maintain edge distances on both the hinge and the fiberglass.

Left side bottom drilled

The same was done on the right side.

Use of the laser level to get the rivets all aligned down the hinge.

One thing I did was to drill a couple of turns with a hand drill in each marked rivet location prior to using a higher speed air drill. This created a slight dimple so the high speed drill wouldn’t wander once I was drilling through. Very little pressure was used and just let the drill bit do the work.

A view of the right side done and hinge pin in place (not yet cut to length)

I then cut out the spot where the aerosport hinge pin cover template was to be placed. I got close and then used a file to get it to fit tightly in the cut hole.

I then followed the Aerosport directions and added tape and wax to the template along with putting a micro slurry on the inside of the area. I also wetted out 4 layers of 9 Oz fiberglass and placed it on the inside. Once cured overnight, I used a thin woodworking saw I had from a previous task of cutting casings to cut through the Aerosport template and separate the cowl halves. I will say that the very front part where the 2 cowl halves overlap gave me a lot of trouble as I bonded them together and it took a relatively long time to get them apart and left me very frustrated. There is not a lot of room to work either to try to use small files etc.. to chip away at the bond. So use caution when doing this with a Showplanes cowl.. It’s pretty easy to waste 3-4 hours hacking away to get them apart again..

The end result of the recessed area I built up in order to install nut plates for the cover.

The center area of the added fiberglass was cut out to allow the pin insertion into the hinge.

Finally the hinge pin cover was test fit into place. This is at such an angle that if the hinge pin were to start to wander forward with vibration, it would stop the pin from working its way further out as the pin would hit this metal cover. From all accounts that I’ve seen.. the pin doesn’t typically do that anyways, but it would only be allowed to move approx. 1″ forward worst case.

Cowling and skybolts

The Showplanes cowl is one of the last major fiberglass pieces to be worked on. The instructions basically tell you to follow Van’s instructions with a couple of exceptions. So I got to putting the upper and lower halves together and marking the upper cowling for trimming to meet the ratios needed to make a perfect circle for the spinner/prop as well as the air inlets on either side. Here you can see the mark made using a straightedge.

Staightedge used to mark uniformly across the top cowl.

Double checking that the radius is 7.5″ (15″ diameter)

Getting close now with some trimming

Using the prop tool to double check the prop area circle.

You then clamp things in place and drill holes in the flanges to hold this position.

I chose to just do a single cleco on either side of the inlets as well as one in the flange between the inlets and the prop area.

I then used a laser level to mark the center of the bottom cowling to create a cutout for the nose wheel gear leg.

Marking either side of the centerline based on Van’s dimensions for width.

I then used a dremel tool to cut the slot. I estimated the length to cut and then slowly increased 1″ deeper at a time until I was at the bare minimum to get the lower cowling into position. Below is the initial length cut.. I cut a few more inches deeper. My plan is to split this lower cowl into 2 halves to accommodate the 3 bladed prop and I’d like to keep as much original material in this area as possible.

I put the top cowling into position and used a laser level to get it level (after leveling the aircraft) The cowl tool has 3/32″ holes all along the surface to facilitate holding the cowling into place in a fixed location so it doesn’t move during trimming.

Lining up the center line both fore and aft.

With this “extended” hub prop, the cowling sits far enough forward that there is minimal trimming required at the firewall.

Both top and bottom in place

Then starts the task of trimming to the firewall. I used a light on the inside to mark the location of the skin. You then trim 15″ on either side of the center line along the top only. I used a cut off wheel on the dremel tool and left it 2-3mm short.. The 12″ Permagrit sanding block was used for the remainder.. Once this center section is trimmed, the cowl falls down and aft allowing you to get a more accurate trim line for the sides.

Trimming the top center section.
All trimmed

I utilized a couple of clecos in the rivet holes of the skybolts to hold the upper cowling in place so it won’t move. I’ll fill these holes in later.

Then starts the task of installing the skybolts and drilling holes in the cowling. I started with a method that I saw Mark use. Using cardboard to drill a hole, I used a small scrap of fiberglass trimmed from the cowl to mock its thickness, and the cleco adapters that came with the kit to mark where the center of the hole was. The cardboard was taped into place so it can be flipped up, the cowl put into place, and flipped back down to drill an accurate hole.

The cowl was then put into place and the hole was drilled.. However, It didn’t really seem to work that well for me. Probably the tolerance of the hole in the cardboard not being perfect.

The first Skybolt installed.

What I found was that the light inside the cowling wasn’t direct enough and caused some incorrect alignments when marking the hole and drilling. So I taped a small light into the bottom of the cleco adapter to shine directly on the cowl while it was in position. This was used to drill the remaining holes which were more accurate.

2 or 3 skybolts in place

I then worked one hole at a time from top center downwards towards the sides. Mark and drill the hole to 15/32″, insert the grommet and stud, rivet the receptacle in place, place the cowl back on and test fit the new Skybolt.

More to come..

Prop Woes

Some time back, prior to really needing to order a prop.. I had done a bunch of research on the options.. There’s the stock Hartzell 2 blade metal prop, and various other 2 and 3 bladed props from the likes of Hartzell, MT, and Whirlwind. I put together the following table based on the options I found.

BrandModelCostNumber bladesBlade lengthSpinner sizeWeight with spinnerMakeupTBO
MTPROP MTV12B/193-531385037643Composite6 yrs/1800hrs
MTMTV915900376or7854.9Composite6 yrs/2400hrs
HartzellPROP C2YR-1BFP/F8068D902528055.6metal6 yr/2400 hr
HartzellPROP C3Y1R-1N/N7605C SPINNER C-4582-P2035537867.5composite
Whirl Wind77HRT1210027743Composite6 yr/800 hr
Whirl Wind375HRT1435537555Composite6 yr/800 hr
Whirl Wind300-771250037742Composite6yr/800hr
RV-10 Prop options

While the stock 2 blade is widely regarded as the fastest prop, I felt the 3 blade was a better look for the Showplanes cowling I had already chosen. It also is much smoother and has a shorter blade length giving more clearance there.. Of course that doesn’t go without consequences largely in removing the lower cowling. The plan there is to split the lower cowl into two halves like several others have done. Just more fiberglass work.. 🙂

I saw several of the recent builders go with the Whirlwind 375HRT prop. I had spoken with them and was also planning on going in that direction. The main downside is the lower TBO times, but at about 100 hours per year.. I’d likely hit the same time limit prior to the hours limit, which is the same as the MT and Hartzell.

Fast forward to prop order time, which I had delayed a bit due to the 4-6 week lead times I was consistently given by Whirlwind. Guess what.. They no longer are selling that prop (support only) in favor of their newest 300-77 prop. Which is all fine and everything, but the weight reduction (of all things) was concerning.

Why concerning, you ask? Well weight and balance concerning.. The RV-10’s CG moves aft as you burn gas.. Ideally, you’d like to have your empty weight as close to the forward CG limit as possible so as to allow for max carrying capability and gas burn as you go longer distances, even if that means throwing in some ballast when solo. Having a prop that is over 10 lbs lighter out front combined with my Air Conditioning (mostly CG neutral, but ever so slightly aft), an O2 bottle, and a standard technology battery (read heavy) behind the baggage bulkhead, I was concerned.

It’s difficult to know exactly what my empty weight will be, but I grabbed 2-3 samples of what I could find and was provided by other builders. I did a bunch of playing with W&B by estimating the arm of the prop (approx at the hub) at 32.7″ from this picture:

I did that by measuring the scale with a ruler between 25″ and 50″ and then calculating the number of inches per 1/32″ and figuring out the distance to add to 25″ to get to a line drawn through the prop location.. I felt that was good enough for my comparison purposes..

I then found an example W&B spreadsheet online that had lots of weight scenarios listed for that given plane. I modified the spreadsheet to subtract out the weight of the prop that plane had on it and added back in the WhirlWind Prop weight. Here is the stock aircraft examples of empty and gross weight.

   AIRCRAFT EMPTY  
       
Location  ARMWEIGHT (LB)WEIGHT (KG)MOMENT
Left Main Wheel  124.31621282.2777197
Right Main Wheel  124.44608276.3675660
Nose Wheel  50.44328149.0916544
Main Fuel Tanks0gallons108.9  0
Pilot  114.58  0
Copilot  114.58  0
Passenger  151.26  0
Passenger  151.26  0
Baggage  173.5  0
Survival Gear  173.5  0
Remove stock prop  32.70 0
Add WW Prop  32.70 0
       
  Totals 1557708169400
       
  C of G Location 108.799  
  Forward Limit 107.80  
  Aft Limit 116.24  
       
   AIRCRAFT AT GROSS  
       
Location  ARMWEIGHT (LB)WEIGHT (KG)MOMENT
Left Main Wheel  124.31621282.2777197
Right Main Wheel  124.44608276.3675660
Nose Wheel  50.44328149.0916544
Main Fuel Tanks60gallons108.9360 39204
Pilot  114.5818680.0021312
Copilot  114.5817680.0020166
Passenger  151.2617680.0026622
Passenger  151.2617680.0026622
Baggage  173.559 10237
Survival Gear  173.510 1735
Remove stock prop  32.70 0
Add WW Prop  32.70 0
       
  Totals 27001028315297
       
  C of G Location 116.777  
  Forward Limit 107.80  
  Aft Limit 116.24  

So about 1″ aft of forward limit empty and slightly out of CG aft loaded up with full fuel and passenger weight somewhat equally spread around.

Now comes the effect of doing the prop swap:

   AIRCRAFT EMPTY  
       
Location  ARMWEIGHT (LB)WEIGHT (KG)MOMENT
Left Main Wheel  124.31621282.2777197
Right Main Wheel  124.44608276.3675660
Nose Wheel  50.44328149.0916544
Main Fuel Tanks0gallons108.9  0
Pilot  114.58  0
Copilot  114.58  0
Passenger  151.26  0
Passenger  151.26  0
Baggage  173.5  0
Survival Gear  173.5  0
Remove stock prop  32.7-55.6 -1818
Add WW Prop  32.742 1373
       
  Totals 1543708168956
       
  C of G Location 109.470  
  Forward Limit 107.80  
  Aft Limit 116.24  
       
   AIRCRAFT AT GROSS  
       
Location  ARMWEIGHT (LB)WEIGHT (KG)MOMENT
Left Main Wheel  124.31621282.2777197
Right Main Wheel  124.44608276.3675660
Nose Wheel  50.44328149.0916544
Main Fuel Tanks60gallons108.9360 39204
Pilot  114.5818680.0021312
Copilot  114.5817680.0020166
Passenger  151.2617680.0026622
Passenger  151.2617680.0026622
Baggage  173.559 10237
Survival Gear  173.510 1735
Remove stock prop  32.7-55.6 -1818
Add WW Prop  32.742 1373
       
  Totals 26861028314853
       
  C of G Location 117.202  
  Forward Limit 107.80  
  Aft Limit 116.24  

Note that empty, it moved the CG aft by 0.671″ and by 0.426″ in the gross case.

I, of course, ran a bunch of other configs including our Family profile (Assuming Declan fully grown) and it would all be okay.. but low on fuel you can actually get slightly aft of the limit with lighter prop. All would be fine with the weight of the standard prop. I won’t bore you with several tabs of spreadsheet data I played with here.. 🙂

All of this further reinforced the need to find a prop that was close to the stock 2 bladed prop for piece of mind. That left me with 2 choices.. Hartzell 3 blade (super expensive) or the MT MTV-9 prop..

There were a couple of other builders that have used this prop. Van’s typically recommends the MTV-12. One of the main reasons they used it was the max HP rating of the MTV-12 is around 300HP, and they had similar engines to me.. meaning cylinders ported and polished, cold air induction, and higher compression ratios.. MT suggests the MTV-9 for these applications as it can handle significantly more HP (thus the extra weight). In addition MT says that there is no issue using electronic ignition with this prop at my approx HP. (they didn’t support doing that with the MTV-12, even though I know others have done it). So all of that plus the weight that is very close to the Stock 2-blade and I decided to go with it. I placed an order for the MTV-9-B/198/52 with nickel leading edges in a matte black with white tips configuration. They also suggested this spinner, which I passed by Bryan at Showplanes, and he thought it would work fine.

P-810-5 Spinner.

The issue came with the 148mm prop flange to aft spinner dimension, which I was asked about and didn’t think it would be an issue. Especially seeing Bryan said it would be okay and I know other RV-10 builders that have used this prop.

Until it was an issue..

I bought the cowl installation tool from Flyboy accessories to place the cowl ahead of getting the prop. It is well designed and provides an adjustable 15″ wheel that serves to set the cowl back from the spinner assembly.

I set the spacing to 148mm minus about a 1/4″ gap. Placed the upper cowl in position.

And DoH! the cowling is so far forward it doesn’t even reach the firewall.

I contacted my MT guy and described the issue and to see if they might have any other spinner/hub setups that might work better for me. In the meantime I mulled over what to do.. I had heard of another builder using the Hartzell 3 bladed prop with an I-hub config which pushes the spacing out enough to fit the AC compressor without having to cut and bump out the cowling. I got some info on that as an option.. I also contemplated what would happen if I cancelled my MT order.. Either lose some/all deposit money or continue with the purchase and try to sell it..

MT got back to me with a spinner specification sheet. I used the info I got about the Hartzell I hub spacing and did some mock ups and measuring.. I ended up getting a range of prop flange to aft spinner measurements that matched the Hartzell I hub design at the minimum side and a measurement as far forward as possible with almost no cowling overlap onto the firewall at the max side. I filled out their form and asked if there was anything off the shelf that would work.. I came up with somewhere between 69.3mm and 82mm would work for me.

A couple of days later engineering came back with this .. A P-810-3 spinner with 75mm spacing. It’ll work!!! What a relief..

P-810-3 Spinner

I mocked this distance up and checked the spacing of the AC compressor up front with the lower cowl roughly in place.. There is plenty of room to not have to do any lower cowl modifications for the compressor. Win win!

I told MT that it would work. A change order was put in with no anticipated delays seeing I’m still far enough out in production. I’ve been able to continue forward with the cowling install now that the spacing is known.

I will say that I’ve encountered a bunch of negativity about MT and using their props along my research. While I agree that might become true with months of delays in the case of something catastrophic that needs to go back to Germany to get fixed. For most run of the mill things it shouldn’t ever be an issue (at least I hope not). The local guys are easy to work with and bend over backwards to make things right. I also have a certified MT service shop relatively nearby in CT that I could drive to if need be. Thus far, I’m impressed with MT.

Sorry for the long post… it’s been a few months in the making.