RV-7 cowling exit bump

[Bavafa]

Originally Posted by DanH

Excellent advice. Seems I meet quite a few builders who have never established an accurate baseline speed (or calibrated the ASI, although that's another issue). As a result, they really don't know if their airplane is fast or slow, and worse, they never really know if a mod made a difference.

Guilty as charged. Although I have done it a number of times for some other testing, but I have not got a kept a good record nor believe my tests were set very accurately.

Would matching density altitude and weight (varied by amount of fuel) be a good way to establish max speed? Of course then you would go WOT and max RPM and try to set the FF or EGTs to the same number as before and after?

[GLPalinkas]

Great discussion guys. CradAndy, Keep us posted on your changes because, apparently, a lot of us are considering them.

[rv7charlie]

Don't ignore the possibility that simply adding the exit ramp ('bump' sounds external to me) will likely improve cooling, but will also likely increase drag, as well. You'll be moving more air through the cowl, therefore, high probability of more drag. (Next step: cowl flap to reduce exit area at speed...)

Also,
There's a NACA/NASA paper floating around that deals with inlet lip shape. IIRC, it basically says that if the diffuser uses internal diffusion and the inlet/outlet are sized correctly, the lip can be fairly sharp & that's the lowest drag configuration. If external diffusion is used, the lip should be more blunt and rounded, to avoid separation & turbulence on the outside of the lip.

Every domino can touch every other domino....

Charlie

[DanH]

Originally Posted by Bavafa

Guilty as charged. Although I have done it a number of times for some other testing, but I have not got a kept a good record nor believe my tests were set very accurately.

Would matching density altitude and weight (varied by amount of fuel) be a good way to establish max speed? Of course then you would go WOT and max RPM and try to set the FF or EGTs to the same number as before and after?

Certainly standardized as much as possible improves accuracy, so yep, all of the above.

You can fly four legs and input different combinations of three into the NTPS spreadsheet as a cross check. If the answers are always close, you didn't have a major variable or error on one leg. It all helps.

[crabandy]

Ok, I've got the Doug Gray instructions for determining TAS from GPS track and speeds. I may need a little help with the spreadsheet, my laptop doesn't have Excel and haven't been able to find a link to work yet. I'm hoping to record these numbers this weekend with and without the outlet ramp.

I'm also debating if it's also a good time to establish pressures within the cowling , perhaps give me an idea of how well my baffling is sealing. I'm thinking of the yardstick manometer with tubing ran through the heater inlet and on tube in the center of the upper "plenum" and one somewhere in the lower cowling. Thanks for the ideas.

[Bavafa]

Originally Posted by crabandy

Ok, I've got the Doug Gray instructions for determining TAS from GPS track and speeds. I may need a little help with the spreadsheet, my laptop doesn't have Excel and haven't been able to find a link to work yet. I'm hoping to record these numbers this weekend with and without the outlet ramp.

I'm also debating if it's also a good time to establish pressures within the cowling , perhaps give me an idea of how well my baffling is sealing. I'm thinking of the yardstick manometer with tubing ran through the heater inlet and on tube in the center of the upper "plenum" and one somewhere in the lower cowling. Thanks for the ideas.

One of the members here (can'r remember his name) create a free app for iphone and it makes this calcuation very easy. It is called gps2tas and I wonder if it is still available on itune.

On other notes, I am going to try this mod very soon as it is getting time for another oil change. Will report back when I get some thing to show for it.

[Tom Martin]

I did between 75 and 100 of these tests and used them to check results of modifications. I used four runs rather then three and then was able to use four separate three leg calculations to verify any error of the data. I always used 6000' PA and the same rpm and mp. After two to three years of this kind of testing I discovered that there is a considerable difference in engine power based on OAT. I guess I should have known this but it meant that all my data was essentially useless unless I just compared those days with the same OAT.
I am not sure if using Density altitude rather then Pressure altitude would have made a difference. I think that density altitude would be better as at least the "air density" should be the same but I am not sure that it would take into account the HP difference in temperature. Perhaps when my EFFIS states 75% power the calculations have a temperature correction but I would doubt that. Years ago one of the rocket guys made a little on line calculator for comparing HP with temperature, it was significant enough to make use of the above method unusable for multiple day/season changes. That is why I went to comparing my speeds based on a chase plane. That takes all the variables out and you can actually see your progress based on the other aircraft using predetermined power settings. Of course this only works if the chase plane is NOT making any changes. We discovered that having a passenger in the back could also change the data, thus every test flight should be a the same weight and C of G.

[Bavafa]

For these reasons, I thought to use density altitude as oppose to pressure altitude.

Also, if we are merely trying to get a comparison of before and after speed, then do we really need to do the four-leg GPS to determine the true speed? Any reason not to use what the EIFS reports as TAS if density altitude is the same? What other factor, beside density altitude is used to draw the TAS? I ask this while I realize each EIFS manufacturer probably uses different formula to calculate their TAS.

[crabandy]

Originally Posted by Tom Martin

I did between 75 and 100 of these tests and used them to check results of modifications. I used four runs rather then three and then was able to use four separate three leg calculations to verify any error of the data. I always used 6000' PA and the same rpm and mp. After two to three years of this kind of testing I discovered that there is a considerable difference in engine power based on OAT. I guess I should have known this but it meant that all my data was essentially useless unless I just compared those days with the same OAT.
I am not sure if using Density altitude rather then Pressure altitude would have made a difference. I think that density altitude would be better as at least the "air density" should be the same but I am not sure that it would take into account the HP difference in temperature. Perhaps when my EFFIS states 75% power the calculations have a temperature correction but I would doubt that. Years ago one of the rocket guys made a little on line calculator for comparing HP with temperature, it was significant enough to make use of the above method unusable for multiple day/season changes. That is why I went to comparing my speeds based on a chase plane. That takes all the variables out and you can actually see your progress based on the other aircraft using predetermined power settings. Of course this only works if the chase plane is NOT making any changes. We discovered that having a passenger in the back could also change the data, thus every test flight should be a the same weight and C of G.

Tom,
That's a lot of testing! I assumed the test were all using Density Altitude.

I originally made the cowling bump/ramp only to see what it would do for my CHT's, I was going to see if it did anything to my IAS and leave it at that. I've never done the 3 or four leg TAS calculation because of all the variables involved, I assumed my results would probably end up inconclusive.
My homemade pitot and static test passed with flying colors. I've flown with numerous aircraft and found my IAS/TAS displayed on the EFIS to be within 1-2 knts of aircraft that had performed the 3 leg TAS calculation and have current Altimeter Static checks.

Thanks for the tips, probably easiest to do every test solo starting with full tanks for consistency. I will try to incorporate another aircraft when I can, but sometimes hard to fit schedules. I'm going to give the 4 leg NTPS thing a couple shots and see if it shows something definitive between the cowling ramp and no ramp.

Do you by chance have any photos of your foam seal behind the spinner? I saw a post where you glassed in the cowling behind the spinner but the pics disappeared that might've shown how you finished sealing it off. DanH's seal looks great, but because of my landoll ring I'm going to have to do a little glass work to make it work. The more options to look at and copy off of the better!

Thanks,
Andy

[DanH]

Originally Posted by crabandy

I'm also debating if it's also a good time to establish pressures within the cowling , perhaps give me an idea of how well my baffling is sealing. I'm thinking of the yardstick manometer with tubing ran through the heater inlet and on tube in the center of the upper "plenum" and one somewhere in the lower cowling. Thanks for the ideas.

This is the standard piccolo tube installation we used in previous data gathering. Information is more global when everybody uses exactly the same setup.

There are two piccolo tubes in the upper cowl and two in the lower. Each pair is tee'd together. The goal is an overall average pressure for each space.





It's easy to drill all the little holes if you make a drill jig: