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  #1  
Old 07-01-2020, 11:13 AM
Ed_Wischmeyer's Avatar
Ed_Wischmeyer Ed_Wischmeyer is offline
 
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Location: Savannah, GA
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Default Fixed Pitch vs Constant Speed Prop

Quote:
Originally Posted by dbaflyer View Post
This is the first time I've flown at WOT at altitude. The first time I was going over 2700RPM and was unsure about staying over for a little bit of time. This time I wanted to measure top speed so I stayed at WOT long enough to do that.

So at 2750 RPM, 21.4 MP, 73% power, 9.7GPH, 138 IAS, 162 TAS, 8670 DALT

When I compare this to Vans web site 75% power and 8,000 solo weight they state 187MPH or 162.5 KT. This matches up pretty well and I'm happy with that.

So the questions from a newbie here is... If I'm at full throttle producing 73% power, I presume with a fixed pitch prop there is no more in the tank so to speak. This is going to be my maximum speed. So is the only way to get to Van's top speed number with a constant speed propeller?

This is with an O-320 carburetor running one Mag and one EFII ignition. The prop is a 3-blade Catto 67x70 carbon composite.
Fixed pitch props can, in general, match the pitch range of a constant speed prop from most coarse pitch (cruise) to most fine pitch (climb). So, in general, a constant speed prop won't gain you any extra speed over a fixed pitch prop. The advantages of a constant speed prop are uniformly good performance over the speed range of the airplane, more drag on descent and landing, and, if your plane is tail heavy, more weight up front.

However... the Sensenich metal fixed pitch props have a 2600 RPM redline, so the engine won't be able to achieve max RPM, hence, won't achieve max power.

When I flew over West Texas a few summers ago with the autopilot holding altitude, I had to retard the throttle in the updrafts to avoid overspeeding the prop, and then add power in the downdrafts. Presumably would not have to do this with a constant speed prop.”
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  #2  
Old 07-01-2020, 12:15 PM
PilotjohnS PilotjohnS is offline
 
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Default Question

So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?
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  #3  
Old 07-01-2020, 01:29 PM
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Carl Froehlich Carl Froehlich is offline
 
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Default

Quote:
Originally Posted by PilotjohnS View Post
So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?
My first RV (RV-8A) started off life with a FP prop. I replaced it at 300 hours with a Hartzell BA CS prop. It was a totally different airplane from climb, to cruise to top end.

I tell builders that the one big mistake I made on my first RV was that FP prop. I did not repeat that mistake on the next two RVs.

But - factor in your mission. If you want a joy ride once a week for a hamburger get whatever you want - any prop will still make your RV climb better than most Cessnas. If you want high efficiency cross country cruise, get the very nice Hartzell CS BA prop from Van?s. Flavor of RV (other than the 12) makes no difference.

Keep in mind the efficiency of a prop is more than FP vs CS. I?ve flown one RV with a custom composite CS prop that put the load on the engine but at RV speeds it was not as efficient at translating that engine power to thrust as the Hartzell (less speed for the same fuel flow). I suspect trying to match a FP prop to peak at whatever flight condition you are aiming for may have a similar issue.

Carl
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  #4  
Old 07-01-2020, 01:45 PM
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mike newall mike newall is offline
 
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Default

Unless you blow the engine, you will not get more than that power at that altitude.

You have one of the best FP props on the market.

You are happy with the speed you are achieving in the cruise, so fly on and enjoy !

We sit at 23/23 at 8,000' in our RV7 behind an IO-360 and MT 3 blade CS prop. Guess what, the power is mid 70% range, so we are similar. I see around 140 IAS - maybe 160 TAS ish. Quite frankly, I am not bothered, I know I am cracking on in an awesome aeroplane

My buddy has an RV6 with an O-360 and a Prince P-Tip prop. He sits beside me at 2400 rpm in the cruise at the same speed as me at 23/23.

Sit back, relax, enjoy the view and look forward to the destination arrival at 200kts downwind........
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  #5  
Old 07-01-2020, 01:59 PM
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RV6_flyer RV6_flyer is offline
 
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Default

Quote:
Originally Posted by PilotjohnS View Post
So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?
You only get what you pay for.

I have always had a CS prop on my RV and really want a CS prop on any RV that I own. Of the RV series of aircraft, only the RV-12 would be acceptable to me with a fixed pitch prop.

Check in with Tonya and Scott Card. They have a constant speed prop on their RV-9A and they also fly formation. They can report first hand how they do with only 160 HP and constant speed prop with other RVs having larger engines and fixed pitch props.
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Last edited by RV6_flyer : 07-02-2020 at 05:09 AM. Reason: spelling
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  #6  
Old 07-01-2020, 02:32 PM
Scott Hersha Scott Hersha is offline
 
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Location: Cincinnati, OH
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Not all Sensenich aluminum props have a 2600 RPM limit. If you check the Sensenich spec sheets on these props, and page down the the ones designed for Vans aircraft, the O-360 powered airplanes, and some O-235 airplanes, including the RV9, dont have that RPM limit on the prop. Last page identifies details.

http://www.sensenich.com/wp-content/...1349891787.pdf

I don?t know why that is. The O-360?s usually have prop hub requiring 1/2? bolts, and I think most O-320?s require 7/16? bolts. That might not have anything to do with it, and what about the O-235? It sounds like your prop is set up pretty good for your airplane. 2600 RPM seems to be your only limitation.

I have a 72FM (71?) Sensenich with 85? pitch on my O-360 powered RV4, and my limit is Lycoming?s recommended limit of 2700. However, mine turns up 2900 at WOT level at 8000?, so I?m underpitched, and I only ran it that way to determine what I needed. Initial acceleration is slower than my CS RV formation leads, but once airborne I accelerate quickly and can climb usually much better than my counterparts, most likely due to my lighter weight.
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  #7  
Old 07-01-2020, 03:04 PM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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Default

Quote:
Originally Posted by PilotjohnS View Post
So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?
Hot summer-temps climb I get about 1400fpm at takeoff (2000msl), winter temps I get about 1800fpm. Catto cruise-pitched prop gives me about what Van's indicates my top speed should be. Happy camper, especially since I appreciate simplicity and value. YMMV.
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  #8  
Old 07-01-2020, 03:24 PM
PilotjohnS PilotjohnS is offline
 
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Default Really?

If is true you are getting anywhere near 1400 fpm and didnt make a mistake and add an extra zero, I think I will be thrilled with the performance of a Catto fixed pitch.
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WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.

Dues paid 2020, worth every penny

RV9A- Status:
Tail 98% done
Wings 98% done
Fuselage Kit 98% done
Finishing Kit 35% canopy done for now
Electrical 5% in work
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www.pilotjohnsrv9.blogspot.com
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  #9  
Old 07-01-2020, 03:37 PM
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snopercod snopercod is offline
 
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In my humble opinion, the only benefit of a CS prop is takeoff distance and climb performance. The downside is cost and weight up front.
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  #10  
Old 07-01-2020, 03:52 PM
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erich weaver erich weaver is offline
 
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Quote:
Originally Posted by snopercod View Post
In my humble opinion, the only benefit of a CS prop is takeoff distance and climb performance. The downside is cost and weight up front.
Cost yes, weight not necessarily. I am a bit on the tail heavy side of the CG envelope with my constant speed Whirlwind (composite) prop.

Erich
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