B

bendougy

Member
Just finished my RV8 after 14 years. I have a IO360 A1B6 with single mag and light speed ignition. Engine has 1500 hours on it. I am a aircraft engineer and went to painfull lengths to build it fast.ie no gaps at control horns/well fitted fairiings /clean lines / angles of incidence and rigging done with laser levels. Flys straight, trims in middle. My speeds are low. 5000 ft standard day25 inchs/2500 RPM 165 Knots /23 inches/2300 RPM 152 Knots/full power 171 Knots. I can blame full power speed on tired engine but at 25/2500 I am setting the power so the speed is airframe limited. My prop is a new hartzell constant speed with blended airfoil. Built a little heavy due to Christan inverted oil and large mountaint high oxy.530 KG basic wieght. Can anyone advise. Cant find any threads on it

Regards Ben
 
Need accurate\calibrated fuel flows.
Have you completely ran your numbers on your engine? Meaning have you plotted and coursed all manors of measurement? EGT's, CHT's, oil, fuel pressure, fuel flow, MP, OAT, timing, compressions,and so forth? The best guess for your problem is under your cowl.
 
Are these speeds IAS or TAS?

If they are TAS, how are you determining the TAS? Was IAS converted to TAS, or is the TAS based on GPS data from multiple legs? There are several reasons why the IAS could have large errors, which would make the calculated TAS wrong.

If you are using GPS data to get TAS, what method are you using?

What type of tachometer do you have? Any chance it reads too high?

Tell us more about the engine. New engine? Used engine? Overhauled engine? If so, who did the overhaul?
 
http://www.lazy8.net/rv8.html is the website of John Huft who has the fastest RV-8 that I have heard of. In Sport Air Racing League (SARL) sanctioned races he routinely beats me by 20+ mph. At his website he provides information on his RV-8 modifications. I have made a lot of modifications to my RV-6A which started out at 170.67 kts and the latest tested best speed configuration is 184.4 kts. I do all of my speed testing in three directions at 6,000 ft density altitude, with wide open throttle, maximum rpm, leaned to 1300 F (~100 F rich of peak) on Cylinder #4 (hotest), trimmed for hands off straight and level, TruTrak GPS track 360, 120 and 240 degrees, altrak altitude hold. Each leg is flown until five consecutive GPS ground speeds are recorded at 20 second intervals with no variation greater than 1 kt. The five speeds are averaged for each leg and the three resulting GPS track ground speed averages are plugged into the NTPS spread sheet which eliminates the wind effect mathematically and produces the true air speed. My manifold pressure is typically between 24" and 25", the rpm is 2720 to 2730, and the CHTs are in the low to mid 300 F range (except for cylinder #1 which runs cold less than 300 F), oil pressure is ~80 psi and the oil temp is in the 180 - 200 F range. This is with a 180 hp Lycoming O-360-A1A and a 72" dia Hartzell C/S prop with F7666 blades. I have recently installed a new 72" Hartzell blended airfoil prop with F7496 blades in a long range race configuration with tip tanks and homemade streamlined 3" span tips the new prop shows a 3 kt increase in speed over the old prop but I have not yet tested it in the the short range race configuration within which I achieved the 184.4 kt speed. I am nearing completion of the development of some semi-flat wing tips of 3/4" span after which I will run speed tests again in the 3" and 3/4" wing tip configuration. If the prop gain holds up I expect ~187.4 kts in the 3" tip configuration.

Bob Axsom

P.S. If you are flying multi-direction speed tests and using straight averages to calculate your speed ypu will at best get the correct speed but in most cases without the wind effect elimination of the NTPS spreadsheet you will get a lower than actual true air speed.
 
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bendougy said:
Built a little heavy due to Christan inverted oil and large mountaint high oxy.530 KG basic wieght.

Regards Ben
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Ben,
You may find it helpful to compare your aircraft weight with some of the speed demons aircraft. You didn't mention your test weight.
Don
 
Weight

How much does weight effect the speed? I was told that in cruise, a 100# heavier aircraft will fly 1 knot less than one that is 100#s less, all things being equal in these RV aircraft. I was very suprised to hear that, if in fact this is true. Does anybody know? And, if it's not true, what is the correct formula (at least rough formula). Dave
 
db8 said:
How much does weight effect the speed? I was told that in cruise, a 100# heavier aircraft will fly 1 knot less than one that is 100#s less, all things being equal in these RV aircraft. I was very suprised to hear that, if in fact this is true. Does anybody know? And, if it's not true, what is the correct formula (at least rough formula). Dave
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I'd say this is pretty close, at least at low altitude. I've tested top speed with and without a passenger and the difference was small. In the RV-8 the aft CG shift may actually decrease trim drag.
 
RV-8 Speed

Thanks Guys
My thoughts are that as I am limiting power by retarding power to 25 inches of MAP so the engine power is irrelevant as I know I am pulling 25/2500. I have calibrated the tacho against my ACES strobe gear and calibrated MAP gauge against another MAP gauge. Speed checks were done against my GPS flying a120 degree triangle. Can I have made any error with angle of incidence of wings or tail plane. If the error was 2 or 3 knots I would not worry but 15 Knots is a lot. . Have you heard of any error in the build that could cause this sort of speed loss? I have built it as per plans with no changes. Flys straight, trims in middle.
Regards Ben
 
Just to be sure Ben, as Kevin asked, are you talking TAS or IAS? I can't imagine you'd make enough drag with incidence changes to slow down that much.
 
db8 said:
How much does weight effect the speed? I was told that in cruise, a 100# heavier aircraft will fly 1 knot less than one that is 100#s less, all things being equal in these RV aircraft. I was very suprised to hear that, if in fact this is true. Does anybody know? And, if it's not true, what is the correct formula (at least rough formula). Dave
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At typical RV cruise conditions, the vast majority of the drag is profile drag, which is not affected by changes in weight. The induced drag is probably about 15% of the total drag. Thus the affect of weight changes on speed is fairly small.

The classical corrections for weight, applied to the most recent flight test data from my RV-8, suggest that a 100 lb weight increase, with no change in CG position, would give around one third of a kt speed decrease. If the CG moves aft when the weight increases (i.e. added a passenger or stuff in the aft baggage compartment) the amount of speed decrease would almost certainly be less than one third kt per 100 lb weight increase.
 
bendougy said:
Thanks Guys
My thoughts are that as I am limiting power by retarding power to 25 inches of MAP so the engine power is irrelevant as I know I am pulling 25/2500. I have calibrated the tacho against my ACES strobe gear and calibrated MAP gauge against another MAP gauge. Speed checks were done against my GPS flying a120 degree triangle. Can I have made any error with angle of incidence of wings or tail plane. If the error was 2 or 3 knots I would not worry but 15 Knots is a lot. . Have you heard of any error in the build that could cause this sort of speed loss? I have built it as per plans with no changes. Flys straight, trims in middle.
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I have a hard time imagining a small build error that could give a 15 kt speed decrease. It takes a huge change in drag to cause a 15 kt change in speed. My aircraft is far from perfect, and as near as I could tell my speed was within one kt of Van's perf claims when I had my old Hartzell prop (not a blended airfoil prop). The speed with my current MT prop is a bit slower, but I didn't get any really clean data before winter forced me to take the wheel pants off, so I'm not sure exactly how much speed I lost.

I wonder if your engine is down on power for some reason. It might be interesting to check the rate of climb at Vy at some known weight, as rate of climb is very strongly affected by amount of engine power. Report back with aircraft weight, pressure altitude, OAT and rate of climb. The results should give us a reasonable idea whether your engine is making the expected power.

Note - don't use the VSI to determine rate of climb, as VSIs are notoriously inaccurate. Use altimeter and stop watch.
 
RV-8 Speed

Thanks Kevin

Can you please explain, If i have a tired engine fitted to my plane and fly 23 inches 2300. I then remove and fit a powerful engine of the same type and refit same prop and fly 23 inches/2300RPM. Will i go faster? I know the answer is yes but as manifold Air pressure (MAP) is a reading of throttle butterfly positon and how hard the cylinders are sucking wont the rated output be the same. My prop is constant speed and will get corsen up to absorb horsepowerbut wont that added load reflect MAP

Confused Ben
(ie do I overhaul my gutless engine to increase speed at a given MAP)
 
bendougy said:
My thoughts are that as I am limiting power by retarding power to 25 inches of MAP so the engine power is irrelevant as I know I am pulling 25/2500.
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Ben,

Remember that just because you have 25 inches of manifold pressure does not mean your engine is producing the same amount of power as another engine also at 25 inches. The simplest example is if you pull the mixture to cut-off, you can get 25/2500 without producing any power at all. This is why multi-engine pilots cannot depend on engine instruments to indicate engine health. It would take a torque gage or something similiar to actually measure the power output.

Kevin's suggestion of climb testing is right on - this is a much better indication of engine power.

Pat
 
Ben,

I posted my reply before I saw your last post.

For clarification, the MP is a function of engine RPM and throttle position (and atmospheric conditions, etc), and NOT an indication of engine power or "how hard the engine is working."

Pat
 
RV-8Speed

Have just done a Vy climb from 2000 to 10000 Pressure hight at 35 celesus or 95 F full power 2-3000ft 48 sec./ 3-4000 48 sec./ 4-5000 54 sec./ 5-6000 60 sec/ 6-7000 70 sec./ 7-8000 68 sec./ 8-9000 74 sec./ 9-10000 77 sec.
Total of 8.20 from 2-10000ft. Wieght 793 KG Vy 95 Knots IAS

Regards Ben
 
Speed

Make sure your tach is accurate. One of the reasons the speed doesn't measure up to expectation is that the engine is not turning the rpm indicated on the tach. I realize that most of the RV crowd is using electronic tachs and they are probably very accurate. The mechanical tachs often read incorrectly, sometimes by quite a bit.
 
bendougy said:
Have just done a Vy climb from 2000 to 10000 Pressure hight at 35 celesus or 95 F full power 2-3000ft 48 sec./ 3-4000 48 sec./ 4-5000 54 sec./ 5-6000 60 sec/ 6-7000 70 sec./ 7-8000 68 sec./ 8-9000 74 sec./ 9-10000 77 sec.
Total of 8.20 from 2-10000ft. Wieght 793 KG Vy 95 Knots IAS

Regards Ben
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Hi Ben,

Just a cursory inspection of your data tells me you're getting only a little more than 1,000 fpm down in the thick air, and I'd expect more than that. I use 110 for Vy, so you may be giving up a bit at 95, but the curve is very flat, and climb rate doesn't change much between 100 - 120 KIAS. Still, only 1,000 fpm makes your power output seem pretty low - I'm sure someone else (Kevin) will do the math.

Paul
 
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bendougy said:
Have just done a Vy climb from 2000 to 10000 Pressure hight at 35 celesus or 95 F full power 2-3000ft 48 sec./ 3-4000 48 sec./ 4-5000 54 sec./ 5-6000 60 sec/ 6-7000 70 sec./ 7-8000 68 sec./ 8-9000 74 sec./ 9-10000 77 sec.
Total of 8.20 from 2-10000ft. Wieght 793 KG Vy 95 Knots IAS
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Ben,

If I extrapolate this data to sea level, it shows a raw rate of climb of about 1600 ft/mn. If I correct that for the warm temperature, I get about 1710 ft/mn (this correction is only for the effect of temperature on barometric altitude - it is not a correction for the effect of temperature on engine power).

It should take about 41 hp to maintain level flight at 95 kt at your test weight (based on the CAFE Foundation RV-6 data, with small corrections for smaller profile drag of the RV-8). It would take another 90 hp to climb 1710 ft/mn at your test weight, for a total power required of 131 hp. If we assume a propeller efficiency of 75%, the engine must develop about 175 hp to give this rate of climb. The Lycoming power charts suggest that a healthy IO-360-A1B6 should be able to make about 193 hp at sea level at 35 deg C.

This data strongly suggests that your engine is down on power. I'd focus on the engine before looking any closer at the airframe.

Tell us more about the engine. Was it new, overhauled or used? If it was overhauled, who did the work? Get someone else to double check the timing. What fuel flows are you getting at full power? How are you leaning the engine when you do your speed runs?
 
Low Power

I have an IO360A1B in my 8. A while back I noticed that my t/o performance just seemed sluggish compared to what it was.

A quick compression check showed I had low compression on #1 cylinder (80/57). The other three were at 76/77 range.

topped the front 2 cylinders (another story) and my climb and t/o were right back where they were.

Bottom line, sounds like your engine may be a little sick too.
 
Hi Ben,

For what its worth, our Lyc. IO-360-A3B6D was giving us compressions in the mid 70's on all cylinders, and full throttle at 1,000 PA was right where it should have been (28"/2700 rpms). We had the same problem as you and tried everything we could think of, but finally pulled the engine for an overhaul. It turns out our intake lobes on the camshaft had worn around .250" (acceptable is around .007"), thus lowering our 200 hp engine to about half that.

Long story short, the airplane with a freshly overhauled engine now climbs about 700 fpm more and cruises about 30 knots faster. As everyone has said, you may have some internal problems with your engine. You may want to look into how far your valves are opening, make sure your valve springs aren't cracked, etc....
 
RV8 speed

Mike_ExpressCT said:
Hi Ben,

For what its worth, our Lyc. IO-360-A3B6D was giving us compressions in the mid 70's on all cylinders, and full throttle at 1,000 PA was right where it should have been (28"/2700 rpms). We had the same problem as you and tried everything we could think of, but finally pulled the engine for an overhaul. It turns out our intake lobes on the camshaft had worn around .250" (acceptable is around .007"), thus lowering our 200 hp engine to about half that.

Long story short, the airplane with a freshly overhauled engine now climbs about 700 fpm more and cruises about 30 knots faster. As everyone has said, you may have some internal problems with your engine. You may want to look into how far your valves are opening, make sure your valve springs aren't cracked, etc....
Click to expand...

Mike
We have measured our valve movment. This is the valve movment measured on the valve spring retainer. inlet 1)0.417 2)0.4082 3)0.4305 4).4445 Exhaust 1)0.3882 2)0.5 3)0.3749 4)0.4149 Any idea how much they should move thus telling me how much wear I have
Ben
 
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