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  #21  
Old 09-10-2006, 08:51 PM
Mark7986 Mark7986 is offline
 
Join Date: Jul 2006
Location: Fairmont MN
Posts: 41
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Vans told me it takes 22-28 HP withour gear leg and wheel fairing. My counter weights are also slightly above the stabilizer.

Mark
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  #22  
Old 09-11-2006, 05:14 PM
gmcjetpilot's Avatar
gmcjetpilot gmcjetpilot is offline
 
Join Date: Jan 2005
Posts: 4,285
Default Close but no cigar

Quote:
Originally Posted by Bob Axsom
In a fixed pitch prop situation, with a common design airplane with known performance characteristics, well built and rigged, at a set RPM with an accurate tach how can it be anything but a bad prop?

Bob Axsom
Dear Bob, the prop has to work with the airframe and engine, they are all interrelated. It may not be a prop issue but an engine or even airframe issue. Not all airframes and engines are the same. I understand your point, most RV's get pretty consistent performance from one plane to another. However you, "Mr. Speedy" , should know with all the work you have done on your bird, that small seemingly unnoticeable changes (unnoticeable to an outside uninitiated observer) can make a big difference. There are ways to milk speed out of any stock RV. If he reduces drag he will increase speed and of course RPM, with out a prop change. I'm going to keep my speed secrets, secret for now, since I might be racing you some day.
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Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767

2020 Dues Paid

Last edited by gmcjetpilot : 09-11-2006 at 07:07 PM.
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  #23  
Old 09-12-2006, 12:06 AM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Well Shucks, I thought I had it

Sure sounds like an over pitched prop at arms length but I'm sure that several things could be improved to get more speed in the lift/weight/thrust/drag trade-off. Flying in cruise with down elevator in Mark's latest post got my attention. That could be rigging, CG or thrust line related.

George, I know from previous posts you raced before and it would be great to have you in the Air Venture Cup Race in the future. It is pure racing with no gimicks and both the quantity and quality of people and airplanes are very good. 407 nautical miles at wide open throttle and high RPM and the short recovery leg up to Fond Du Lac requires some fuel planning but it's a good race.

Bob Axsom
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  #24  
Old 09-12-2006, 12:47 PM
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gmcjetpilot gmcjetpilot is offline
 
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Posts: 4,285
Default Thanks Bob, you probably are right

Quote:
Originally Posted by Bob Axsom
Sure sounds like an over pitched prop at arms length but I'm sure that several things could be improved to get more speed in the lift/weight/thrust/drag trade-off. Flying in cruise with down elevator in Mark's latest post got my attention. That could be rigging, CG or thrust line related.

George, I know from previous posts you raced before and it would be great to have you in the Air Venture Cup Race in the future. It is pure racing with no gimicks and both the quantity and quality of people and airplanes are very good. 407 nautical miles at wide open throttle and high RPM and the short recovery leg up to Fond Du Lac requires some fuel planning but it's a good race.

Bob Axsom
Hey Bob you are right, it sure sounds like over pitched prop, but there is something else going on. At least that is what I think. A little of this and little of that, but probably mostly the prop?

Thanks for the invite, love to join you at the race. I am working on it. When I make the show I will be sure to say hi.

BTW I just saw this on jamesaircraft Yahoo group. Another gent did some lower baffle work that was very interesting. Check it out in the photo section. I'll try to link below:

http://us.a2.yahoofs.com/groups/g_15...wvzBFBiTLn_YSM
http://us.a2.yahoofs.com/groups/g_15...wvzBFB2ejHUFxt
http://us.a2.yahoofs.com/groups/g_15...wvzBFBnLNDzx25
http://us.a2.yahoofs.com/groups/g_15...wvzBFBqMIIBJb1
http://us.a2.yahoofs.com/groups/g_15...wvzBFB4hiZ94N3
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Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767

2020 Dues Paid

Last edited by gmcjetpilot : 09-12-2006 at 12:50 PM.
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  #25  
Old 09-12-2006, 04:16 PM
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Kevin Horton Kevin Horton is offline
 
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Location: Ottawa, Canada
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Default

Quote:
Originally Posted by gmcjetpilot
Hey Bob you are right, it sure sounds like over pitched prop, but there is something else going on. At least that is what I think. A little of this and little of that, but probably mostly the prop?
But, apparently he has the pitch that is recommended by Sensenich (78"), and Stein said that on his 150 hp O-320 powered aircraft, that it would turn up to almost 2600 rpm.

So, it must be either an engine that isn't making the expected power, or an airframe with much, much more drag than normal, or a bad tach, or bad airspeed info, or a prop that isn't at the pitch that is advertised (i.e. the actual pitch does not match the part number).

Mark - you say you get 145 kt at 8,000 ft. Is that IAS or TAS? If it is TAS, please detail how you determined the TAS.

You checked the tach, and reported it was good. Just to be sure, check it again, using yet another prop tach. Do the check at several rpms. I'm not an expert on prop tachs, but is it possible to use one in flight? If not, given that you can only get 2100 rpm statically, you aren't able to check the tach at the rpm of interest (2450 indicated rpm). You might have a tach that reads correctly at lower rpm, but reads too at higher rpm. Try to borrow a different tach.

It would be awfully strange if the prop pitch didn't match the part number, but stranger things have happened. Find another RVer with a 78" pitch Sensenich. Get a digital level, or some other similar reference. Turn the props so they are horizontal. Use the digital level to measure the pitch at the end of both ends of each prop. The aircraft pitch attitudes will probably differ, so for each prop, average the angles you measure on the two ends of that prop to remove the effect of the pitch attitude (you'll need to get creative on the math here). Repeat these comparative measurements at a few other stations measured in from the prop blade tips.

Assuming this is a power problem - confirm you are getting full throttle. Confirm no intake obstructions. Do one flight without the air filter installed. Do a compression check.

Good luck. Keep us posted.
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  #26  
Old 09-12-2006, 08:12 PM
Mark7986 Mark7986 is offline
 
Join Date: Jul 2006
Location: Fairmont MN
Posts: 41
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Here the update. I got looking at the airframe over the weekend and noticed the fairing between the gear leg fairing and the fuselage was hanging down. I pushed them up tight to the fuselage and taped them there. Test flight at 8000 feet, 23.5 MP, 2450RPM--true A/S was 158Kts. using garmin 430 to calulate the speed. Thats 13 knots increased. Today I had an appointment at the prop shop. They found the prop was much closer to 79" then 78". they repitched to to 77". Also on one blade on one section was more then 1 degree off. During the static balance it was determined it was way off. They had to grind one side down to balance the prop. The trip home 2000 feet full throttle airspeed went into the yellow. At 8000 still got 160 knots true. I think the takeoff and climb was improved, however the prop still didn't unload very fast. We're gotting closer. Thanks for all the help. Next on my list is the elevator.

Mark
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  #27  
Old 09-12-2006, 09:57 PM
Bob Axsom Bob Axsom is offline
 
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Default Now you are in the right ball park

Now you are in the right ball park Mark. My guess is you should be able to get between 165 and 170 kts if everything is right but 160 is probably as good as most owners are getting with an O-320.

Bob Axsom
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  #28  
Old 09-13-2006, 04:11 AM
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Kevin Horton Kevin Horton is offline
 
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Location: Ottawa, Canada
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Van's perf data shows 187 mph TAS at 8000 ft for a 150 hp RV-6. 160 kt TAS = 184 mph TAS. You are starting to get into the range of normal aircraft to aircraft variation of performance.

You say you are using the GNS 430 to calculate the speed. I'm not sure what that means. Does the 430 have a function where you can input IAS, altitude and temperature and it tells you the TAS? If so, there may be errors in your indicated airspeed. Most airspeed systems have some error, and this could easily be 5 kt or more.

Or, are you taking the GNS 430 ground speed and somehow manipulating it to get TAS. You can't simply take average ground speed over various headings to get TAS. This will only provide an accurate TAS if there is zero wind. If there is wind, the amount of error is small if the wind is low, but the error may be quite large if the wind is strong.
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http://www.kilohotel.com/rv8/

Last edited by Kevin Horton : 09-13-2006 at 04:17 AM. Reason: Added comments on using GPS to get TAS
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  #29  
Old 09-13-2006, 01:48 PM
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gmcjetpilot gmcjetpilot is offline
 
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Posts: 4,285
Default Congrats Mark; Agreed Kevin

Quote:
Originally Posted by Kevin Horton
But, apparently he has the pitch that is recommended by Sensenich (78"), and Stein said that on his 150 hp O-320 powered aircraft, that it would turn up to almost 2600 rpm.

So, it must be either an engine that isn't making the expected power, or an airframe with much, much more drag than normal, or a bad tach, or bad airspeed info, or a prop that isn't at the pitch that is advertised (i.e. the actual pitch does not match the part number).Good luck. Keep us posted.
I totally agree, that was my original point of my post to Bob. I was just stating on first it does seem the prop is off pitch a little. However I think it could be a little of everything, engine power, drag. My first post was

Thrust - drag = speed. Maximize thrust, minimize drag.

Again I agree with you 100%. It still could be a little bit of the prop?

Did any one check to make sure it is pitched properly? Doha! It may be off?


PS: Hey Mark good on you for finding the fairing and checking the prop. Make sure all your other fairings are tight and not opening up in flight, gapping or shifting. Give that engine a good tune up: Clean the air filter, clean and gap the plugs (bomb test them), change the oil and filter, check the timing and compression of not done yet. If still down on power how old are the points? Sounds like you had real speed brakes with those saggy gear intersection fairings.
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George
Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767

2020 Dues Paid

Last edited by gmcjetpilot : 09-13-2006 at 02:16 PM.
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  #30  
Old 09-14-2006, 08:20 AM
Mark7986 Mark7986 is offline
 
Join Date: Jul 2006
Location: Fairmont MN
Posts: 41
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Garmin 430 is an IFR Cert. GPS Capable of Calulating TAS.
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