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Mods and new cruise speed

hlangebro

Well Known Member
Finally, I broke 170ktas with my 160hp 7A!!
The changes since last time was: vhf and vor antenna fairings, ailerons reflexed to match the flaps (-2deg), elevator trim rod fairings, aileron fences!
170ktas at 75% and 160ktas at 65%!
This is a 2-3ktas increase since last change.
So far I have a 12.7% drag reduction since phase 1. Flat plat drag has come done from 2.36sqft to 2.06sqft.
My cruise at 65% is almost the same as the cruise at 75% when I started modding!

Previous mod was primarily the wheel pants and tire seals.

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A couple of more ideas for you!

I sold my 160hp RV8A recently (based on a grass strip with seasonal terrain problems). I kept the plane very light with tight clearances and a number of speed mods. I typically flew at 200mph at 10,500 (2600rpm) when travelling. 205mph at 2700rpm. I think the biggest gains were from the following mods in order of benefit: 1. Cooling exit fairing 2. Shortened wingtips (only works for a very light plane). 3. Faired fuel drains and Xponder antenna (I combined these because I used the same process for all 3). 4. Flush tail strobe. 5. Deleted the external canopy handle.

The flush tail strobe was done by forming a 'window' out of lexan with the same shape as the bottom rudder fairing. I then cut away that part of the fairing and replaced it with the window. The LED strobe light was mounted inside.

The cooling exit fairing was copied from the ones seen on this forum, but made in 3 pieces. That made it a lot easier to form it. While on the subject of exit fairings,,, I made one for the RV6 I bought and a ramp inside to direct airflow and picked up 6mph.
 
The cooling exit fairing was copied from the ones seen on this forum, but made in 3 pieces. That made it a lot easier to form it. While on the subject of exit fairings,,, I made one for the RV6 I bought and a ramp inside to direct airflow and picked up 6mph.

Photos please..
 
Mods

Hello Hlangebro & Judoka5051
You look like "Van's mods" specialists.
Could you come back to us with much more details, photos and advices on mods you did on your plane already.
Two points are really interesting:
Cooling exit fairing. Hlangebfo, do you have chart of it (I fly RV6A with Weterman cross flow exhaust).
Flaps & ailerons -2 degres. Judoka, is there some effect on roll response. Is th ailerons are more sensitive?

Many thanks in advance
 
Yes, more pictures please!

Also, I noticed in the aileron fence picture that you seem to have done something over the pushrod attachment...
 
Here's a couple of pics ("I hope" says the caveman) this is a little different than what I did on the RV8A. The 6 exhaust is angled much more sharply down, so no center piece. The ramp that guides the air out of the cowl is likely as responsible as the 'flow straightener', but I'm too lazy to take the cowl of just to get a pic. Sorry about the dirty bottom... been doing a little acro.


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Silly Question,

You mention you broke 200mph, but the pictures indicate 140-150kias at 7500ft. Is your reference of 200mph the ground speed?

I mostly ask because I normally see 145-155mph indicated airspeed when I'm flying at 11,500ft with a 200mph ground speed and clam winds. I just want to see how my 8 is performing compared to everyone else. :D
 
Mods

Here's a couple of pics ("I hope" says the caveman) this is a little different than what I did on the RV8A. The 6 exhaust is angled much more sharply down, so no center piece. The ramp that guides the air out of the cowl is likely as responsible as the 'flow straightener', but I'm too lazy to take the cowl of just to get a pic. Sorry about the dirty bottom... been doing a little acro.


Lance: I'm very interested in the exit ramp, please post a pic when convenient. Don
 
Picked up 6mph!

I did a before and after within an hour, at the same altitude. So, while not 'scientific' it's close. The plane didn't have anything to direct the airflow out of the cowl (unlike the 8). Just the firewall and a sharp 90 degree turn at the bottom. So I added a ramp that is riveted to the top of the cooling exit and curves up to connect to the lower engine mount tubes. I've read many times in different place that it is important to get the exit air 'headed in the right direction' which makes sense. I added the exit fences at the same time, so I don't know which of the changes made the most difference, but I expect it's a synergistic effect.
 
I did a before and after within an hour, at the same altitude. So, while not 'scientific' it's close. The plane didn't have anything to direct the airflow out of the cowl (unlike the 8). Just the firewall and a sharp 90 degree turn at the bottom. So I added a ramp that is riveted to the top of the cooling exit and curves up to connect to the lower engine mount tubes. I've read many times in different place that it is important to get the exit air 'headed in the right direction' which makes sense. I added the exit fences at the same time, so I don't know which of the changes made the most difference, but I expect it's a synergistic effect.

Hello
As my English is not so fluent, I'd like to clearly understand what you did speaking about the "ramp". Could you send us a drawing (very simple).
About fences it's clear, but do you thing it could be better to close the bottom between the two fences, and to have two straight exhaust pipe blowing horizontally.
Have a nice Christmas
 
True Air Speed (TAS) is the performance measure

Indicated air speed, and ground speed should not be used when comparing aircraft performance. True air speed, calculated by your EFIS or (better) using a calculator, and based on Calibrated air speed (indicated good enough for government work), pressure altitude, and outside temperature is the best way to compare airplane speeds. (IMHO)

R.
 
Answers...

So, just back from vacation. As someone said here on the thread, NEVER use indicated airspeed to compare performance. It is utterly useless, unless you have enough data to compute the true airspeed. As you can see in my pics, the true airspeed is in ktas. I use knots as that is the standard since the mid 70's. Anyone who wants to use mph, feel free to do so, but that is not the standard unit of measure for aircraft. Enough about that.

Someone asked about the aileron fences. Since both the flaps and ailerons are slightly reflexed, the current spanloading now has a slight peak at the wingtip, as it is operating at a slightly higher aoa. In order to avoid vortices and extra drag, I am "blocking" the flow from the lower surface flowing to the upper surface, and thus minimizing drag. Ideally, I would at some point modify the wingtip with reflex as well.
As far as an exhaust fairing, I have one as well, not used for this test though. I will do some modifications to it, and then I will try it again. I have already a nice ramp on the inside of the cowling -exit, so it might not do that much for performance. I also have an adjustable cowl flap, which make more of a difference.
Next goal is to get to 173-175ktas, which will be very tough...

For all of you with 180-200hp in your RV-7A's, what is you true cruise speed at 8000-8500ft density altitude, at 65 and 75% power?
A buddy of mine has a clean 6, and a souped up engine. His cruise speed is 185ktas, but of course, he burns a bit more fuel and he has around 210hp I would guess...
 
mines a 7 not a 7A, but I cruise 178KTAS at 8.5gph 2440rpm around 8500'. normally though I cruise above 9500 to 12500 most trips. I see 178-180ktas burning 8.2gph 2380-2420rpm. If I want to burn more, I'll pushing the mixture to the ROP, where 9.5gph is about 182ktas.<---Not positive on these numbers since i don't spend much time there.
 
performance

What was your density altitude for those numbers? What engine do you have (hp)? I assume that these numbers are LOP?
 
Someone asked about the aileron fences. Since both the flaps and ailerons are slightly reflexed, the current spanloading now has a slight peak at the wingtip, as it is operating at a slightly higher aoa. In order to avoid vortices and extra drag, I am "blocking" the flow from the lower surface flowing to the upper surface, and thus minimizing drag. Ideally, I would at some point modify the wingtip with reflex as well.

Thanks. Were you able to isolate the effects of the aileron fence by themselves?
 
What was your density altitude for those numbers? What engine do you have (hp)? I assume that these numbers are LOP?

I went back and found couple pics of my panel with the numbers. DA varies. also note the MP gage reads high by 0.4 or something like that. and I have a airspeed error of ~2kts at this speed. I run a Lyco IO-360 parallel valve engine, 9:1 pistons, superior forward sump, AFP injection w/RamAir and Pmags, Vetterman 4pipe. swinging a WW200RV prop, Sam James long cowl w/plenum.

The cruise fuel burn in this pic is higher than i am used to seeing, can't recall why.




 
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Fences

No I wasn't able to isolate the effect of the fences. I added up a lot of things for this test (which I shouldn't have) so it is difficult to say. It should help, if nothing else, with better aileron effectiveness and lower induced drag. Induced drag is low at high speed, so the effect would be slightly higher climb, which I can not confirm yet.
 
Fairings

I tested (again...), my exhaust fairing. I spent some time cleaning and painting it and applied a heat shield, as last time there was burn marks from the exhaust. It didn't do anything for the speed at normal altitudes, however it will work better at 10,000 and up as it closes the exit more. This, on the other hand showed up by over pressurizing the cowl at 12,000 and the oil door popped open!!!! I need to fix that somehow. It was kinda funny how adjusting the cowl flap acted as a throttle for the door!
Otherwise, the speeds were the same as last time; 8500ft DA, 65, 70 and 75%, 160, 165 and 170ktas.
At 10,500ft/11265ft DA, at 65%, the speed was 164ktas, which is a good cruising altitude and speed for cross country!

Exhaust fairing and cowl flap

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Oil door popped open....scary when it happened...

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164ktas at 11245ft DA at 65%

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Here is the 2 piece fairing for the trim actuator rod

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This, on the other hand showed up by over pressurizing the cowl at 12,000 and the oil door popped open!!!! I need to fix that somehow. It was kinda funny how adjusting the cowl flap acted as a throttle for the door!

Increasing cowl pressure increases exit velocity, decreasing cooling drag. Pressure is good.
 
Cowl

Well, I have a pressure plenum, so all the high pressure is in there. When it exits into the cowl, you really want a pressure drop so the air gets sucked out. In my case, the high pressure from the plenum goes into the cowl volume, and since the cowl flap is reducing the exit, the pressure builds up, and yes, it increases exit velocity, however obviously, the inlets will supply as much pressure as they can, and what I need to do, is to seal the oil door and make sure the latch works properly.
 
Well, I have a pressure plenum, so all the high pressure is in there. When it exits into the cowl, you really want a pressure drop so the air gets sucked out.

Nope, no sucking.

There is a pressure drop across the fins, that drop being related to mass flow. You can read the mass in lbs per second directly from a Lycoming cooling chart.

The initial goal is maximum conversion of dynamic pressure to increased static pressure in the chamber above the fins. Pressure coefficients (a measure of that conversion) typically range from 0.6 to 0.85 (see NASA CR3405 or similar). Let's assume 0.75 for your James cowl (which may be a generous assumption). Standard day max dynamic pressure at 8500 altitude and 160 TAS will ballpark around 12.9 inches H2O. From the O-320 cooling chart, at 8500 an approximate 7" drop across the fins would result in 2 lbs per second mass flow, more than enough to keep CHT happy given winter OAT at altitude (say, 40~50F in SoCal?).

12.9 inches x .75 = 9.675"

9.675" - 7" = 2.675"

That 2.675" is positive pressure, the sort that would blow open a wimpy oil door. The atmosphere immediately outside your cowl exit is (usually) at freestream static pressure, so the 2.675" difference provides the force necessary to generate velocity through the exit opening.

Here's a key point. A cowl with a better pressure coefficient would have a higher static pressure in the upper cowl volume. For example, 0.85 would mean 10.965" in the upper cowl under the same flight conditions. The exit could be throttled so as to have the same 7" drop across the fins, thus the same 2 lbs per second mass flow and the same CHT. However, the lower cowl pressure would 3.965", resulting in higher exit velocity. Cooling drag = mass x loss of velocity, so the same mass and less velocity loss equals the same cooling with less drag.

... and what I need to do, is to seal the oil door and make sure the latch works properly.

Yes. From the beginning, my cowl was set up with an eye toward increasing pressure, so the oil door has 10 additional plies of 9 oz glass on the inner surface. Too bad I didn't have any honeycomb handy at the time, as it still bulges a little when cruising at 185 knots with the cowl exit pulled down to about 30 sq inches. That air leak is another drag source.

 
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Yes, I agree, you are correct.
My door was made quite stout with Carbon stiffeners. I checked it today and it is pretty stiff....
I might have to add some more stiffeners though.
Still a little disappointed that it didn't do anything for the performance at the lower altitudes, but higher up it will :)
 
Hakan would you please share some pictures and details of your cowl flap. Is it electrically or cable actuated?
 
What is the rationale for 'negative reflexed flaps and ailerons'. Wouldn't that tend to raise the nose? Why would that lower drag or increase speed?
 
flaps

Negative reflex is a strange term....as that implies that the flaps and ailerons would move trailing edge down....
Anyway, the reflex moves the nose up a tad and actually reduces wing shape drag. The big issue however is that there is always a point when shape drag of the fuselage will be higher then the shape drag of the wing. It is seems though, though my flight tests that even with a slight nose up of the fuselage ( and aircraft) the reduced drag of the wing wins. It could be that our RV's are flying a tad nose down to begin with...

Regarding the cowl flap, I will check to see if I have any pics. It is controlled by a vernier cable and closes beyond nominal and have cutouts for the exhaust pipes.
 
For all of you with 180-200hp in your RV-7A's, what is you true cruise speed at 8000-8500ft density altitude, at 65 and 75% power?

The best I could do at 8000 DA, 2600 rpm: 164 TAS.
This at: WOT (76% PWR), 9.8 GPH, 1605 lbs, 84.1" CG (some aft of center CG)

Airplane: 7A, ECI parallel valve 360, Vertical Induction, AFP injection, WW200RV prop, dual PMAGS, louevers on bottom of cowl, no antenna fairings.

With the lightweight prop, I tend towards aft CG, so not interested in reflexed ailerons/flaps, but would like to consider and learn more about: tire seals, antenna fairings, internal air ramp, and closing my louevers.
 
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Anyway, the reflex moves the nose up a tad and actually reduces wing shape drag. The big issue however is that there is always a point when shape drag of the fuselage will be higher then the shape drag of the wing. It is seems though, though my flight tests that even with a slight nose up of the fuselage ( and aircraft) the reduced drag of the wing wins. It could be that our RV's are flying a tad nose down to begin with...

Could it be that at the higher speeds, the wing is making much more lift than we need (particularly aft of CG) requiring more nose down trim causing drag. Reflexing the flaps/ailerons effectively shortens the effective chord (on the aft part of the wing) thereby reducing lift and moving the center of lift forward.

Bevan
 
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Reflexing doesn't shorten the chord, it changes your zero lift angle of attack, drag and pitching moment. You will also produce the same amount of lift, regardless of CG, however your pitching moment changes.
It is true that the faster you go, the less angle of attack the wing needs to maintain level flight. For aft CG, you elevator now moves trailing edge down a tad to compensate for pitching moment change, and this adds to the trim drag. I used to have the ground adjustable Whirlwind and it worked great, and with my 160hp I had a top cruise at 8500ft DA, 75%, 2700rpm of 164ktas. Since then I changed to the CS Hartzell and my CG became more manageable, plus I did many other drag reducing changes as well, to get the cruise to be 170ktas.
 
Did you make or buy your comm antenna fairings?

I doubt I have the artistic talent to make good ones.

Glenn
 
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