What is the performance hit of fixed pitch prop vs C/S?

[Kyle Boatright]

As I've previously posted, you can reduce diameter when you add more blades and still have the same or more mass-flow. That, plus the lower tip speed when you reduce diameter for the same rpm, results in a lower tip speed with less loss.

Paul,

If this is the case, why don't we see smaller diameter, 3 or more blade props in the F1, Biplane, and other "small engine" races at Reno or even on GA types?

KB

 

[David-aviator]

....Better safe than sorry, I always say! How about this; call Lycoming and ask them if their engines will blow up if operated beyond the red-line on the tach. Push them for a good answer on this and TBO. 'Bet all you'll get is obfuscation and CYA! You do know that the geared Lycomings operate in excess of 3000 rpm! Ask them about that and why those engines can take it but yours can't. Lycoming states for the GO-480 and GO-435. "In the climb configuration, we recommend full throttle throughout the climb...with RPM reductions initially to 3000 RPM and then 2750 RPM for prolonged climb." (Lycoming Flyer Key Reprints, top, P.43) This IS EXPERIMENTAL aviation!

Paul, I am not sure what motivated this post except maybe "full throttle and let it rip" (rip meaning rest in peace) and I must point out that the TBO of the GO435 and GO480 is 1200 and 1400 hours respectively. Prolonged high rpm does not equate to long life with a Lycoming engine (period).

Having been an advocate forever of low and slow for long engine life, I will say you can have it both ways even with a fixed pitch prop. Forget the mach one cruise stuff and go for good take off performance with a prop that turns up 2350-2400 on take off and pull the throttle back after take off. At least that was my mantra when I was married to the faithful beast and we got along fine.

 

[osxuser]

Wow... this is, at least in my experience, a completely flawed way to go about it. First off with the example. A customers RV-6, O-320, single EI, single Mag. Started out with 160Hp configuration, with a climb Performance Propellers 3-blade. Prop specs were 60", 68" Pitch. He then put on 10:1 pistons, prop turned 2650 on takeoff, 2750 in climb and cruise turn out a whoopin 2900rpm. He ran with this prop for 4 months. Decided he wanted more cruise pitch out of it, so he had a Catto 3-blade made up. This new prop is 66" 3-blade with a 72" pitch (I believe). Now static turns about 2500, climbout is about 2650, and WOT level seems to be touching on 2800. Results?

New prop climbs better, cruises faster, and turns slower, with theoretically less HP.

Why? It's simple, prop designer design blades mamimum efficiency around 2700 RPM, therefore, even if the tips aren't exceeding the speed of sound, prop efficiency usually plummets after the percieved engine redline. Personally, I don't have a problem busting redline for races, but seriously, why ask an engine to do something that it wasn't designed to do? For that matter, Hartzell CS props are good to 2800RPM, why not just push that TO RPM a little bit on those? If you want all out power and speed, get a bigger engine, if you want to do things cheap, stick with the O-320.

If you REALLY want to do this, talk to Craig Catto, he may be able to design you a prop that's efficiency range is up to 2900 or 3000, on the other hand he may not...

 

[gmcjetpilot]

It's MORE than performance

Since Sensenich has come along and made dedicate metal props for RV's the C/S prop advantage is smaller but still there. Wood has the disadvantage of being more maintenance and more subject to erosion (like rain). On the plus wood was cheaper and lighter. The lighter is a double edge sword. Also many new Wood/Fiberglass props cost as much as a Sensenich.

Performance wise for fixed pitch props, the Sensenich is a winner. Why? It is thinner as is all metal props. You just can't get as thin with wood. Remember the prop is near super sonic. How thick are wings on supersonic planes?

Wood has the advantage not being subject to harmonics and metal fatigue. However a properly tested and designed metal prop (Hartzell, Sensenich) this is not an issue. However if you have a radical engine, the chance is there is no tested metal prop (or wood/composite prop) for your engine. Each engine needs to be tested with each prop. However wood is more likely to tolerate a radical engine from a vibration stand point. However truth be told the manufactures of wood/composite props are not going to approve or tell you that your fire breathing engine and their prop is 100% safe. Most experimental props are NOT tested. Even MT has limited tolerance or KNOWN test on experimental engines with mods.


Performance is secondary in my opinion, it is cost, weight, complexity and maintenance.

I love the Hartzell Constant speed props, especially the BA prop which again like the Sensenich is designed for the RV's. However it's at least a $5,000 option. You can get a used prop, governor and so on but new for new constant speed cost a premium. You will get better overall performance and efficiency and economy with a constant speed prop.

Cheers George

 

[szicree]

I think that is what BOBM said. Piston speed in an O-360 matches that of a O-320 at a lower RPM. At same RPM, O-360 would have higher piston speed.

I'll remind my buddies of that when they are tweaking their governors.

This thread got me thinking about what the pistons endure. Here's what I figured out:

The pistons in an O320 turning at 3000 rpm will have a max speed of 53 feet/sec and will experience a whopping 643 g's (very briefly).

For an O360 turning 2700, we get max speed 54 feet/sec and 600 g's. Run it at 3000 rpm and it goes to 740 g

I was surprised at how low the speeds are and how insanely high the accelerations are.

 

[Kyle Boatright]

This thread got me thinking about what the pistons endure. Here's what I figured out:

The pistons in an O320 turning at 3000 rpm will have a max speed of 53 feet/sec and will experience a whopping 643 g's (very briefly).

For an O360 turning 2700, we get max speed 54 feet/sec and 600 g's. Run it at 3000 rpm and it goes to 740 g

I was surprised at how low the speeds are and how insanely high the accelerations are.

Scary ain't it? I try not to think about it when I'm over unfriendly territory.

That helps.

Some.

 

[BOBM]

I was surprised at how low the speeds are and how insanely high the accelerations are.

Steve,
I wonder if you can figure what these reciprocating loads are in psi, & how they compare with the 1000-1100psi created when the mixture fires?

 

[szicree]

Steve,
I wonder if you can figure what these reciprocating loads are in psi, & how they compare with the 1000-1100psi created when the mixture fires?

I'm afraid my math is rock solid, but my knowledge of engineering is non-existent. I'll let somebody else pick it up from here.

 

[jdmunzell]

Well, my plan is to put a 180hp Lyc (or clone) on the front of my RV-8 attached to a Sensenich fixed pitch cruise prop. I think a powerful yet economical combination ideally suited to this type of aircraft.

I have a hard time justifying an extra 5 or 6 grand for a few extra knots. Compared to my old Cessna, this will be wicked fast anyway!!

To each his own though....

 

[pierre smith]

Not

Jeff,
You don't necessarily have to take "a few knot hit" with a FP. Our 6A with the Catto three blade pitched for max cruise beats Van's numbers and will true 204 at 2700 RPM. The prop is allowed 2800 continous. Where we do take a little hit is in takeoff and climb. However, we still get off in around 600-700 feet and solo I can get close to 2000 FPM at 140 MPH and 1100 FPM at 160 MPH where the engine will turn around 2500 RPMs.

Remember, the Formula One guys turn engines 4000 RPM or more and I know of a bunch of Lycs in Pitts's and other acro airplanes going over 3000 RPM routinely.

Regards,

 

[tloof]

CS vs FP

Turning the cruise RPM way up to around 2900 RPM with a FP prop in order to get a FP to cruise reasonably equal in speed to a CS prop & still offer a decent climb rate is all fine & dandy, but it always ends up being very fuel hungry when doing so. The CS prop on the otherhand can be slowed way down to 2200-2300 RPM at the max manifold press available at whatever cruise altitude is desired and thus eat way LESS fuel for the same speed as the FP version that is running at a much higher RPM.

By the way, I regularly turn my CS McCauley 2900 RPM on takeoff and get an extremely short takeoff (about 250-300ft on a paved strip & 350 ft max on grass) and also get a phenominal climb rate in my 0-360 powered RV6 of over 3000 ft/min at sea level. I can then drop the RPM back to 2300 at a manifold press of 23" at 8000' altitude to get the fuel burn down to around 8.5-8.8 GPH at a GPS ground speed of 212 mph! I don't think ANY FP prop will even come close to matching those performance #s. All that and I can still get the great braking effect to slow the plane down in the pattern!

Why would anyone ever consider using a FP prop? If it's only because of the extra cost, then how can you possibly even justify the entire cost of the plane itself when the CS vs FP price difference is only about $3500? (this is less than 5%, which will pay out in about 500 hrs in fuel savings during cross country flights...then again in most local flying it will probably cost slightly more in fuel due to the constant use of the higher RPM in order to maintain that exilerating feeling during extreme climbouts both on take off and during aerobatic flying! Ha)

 

[OldAndBold]

OK then, how much fuel cost can I save...?

tloof makes an interesting point about fuel savings over 500 hours. I wonder if the others might fill me in. Given similar performance (and I know there are many variables) will a $10,000 CS prop and governor pay for itself in decreased fuel costs over a $2000 FP?

 

[BOBM]

Turning the cruise RPM way up to around 2900 RPM with a FP prop in order to get a FP to cruise reasonably equal in speed to a CS prop & still offer a decent climb rate is all fine & dandy, but it always ends up being very fuel hungry when doing so. The CS prop on the otherhand can be slowed way down to 2200-2300 RPM at the max manifold press available at whatever cruise altitude is desired and thus eat way LESS fuel for the same speed as the FP version that is running at a much higher RPM.

Well, this is not what the Cafe Foundation found in their RV9A test where they reported 11% better mpg at 2600rpm than at 2300rpm. This info is here on pages 6 &7:

http://cafefoundation.org/v2/pdf/rv-9a.pdf

There is just no way a cs prop can EVER be justified on a pure cost-saving basis. It is much more expensive to buy & to operate.

 

[David-aviator]

Well, this is not what the Cafe Foundation found in their RV9A test where they reported 11% better mpg at 2600rpm than at 2300rpm. This info is here on pages 6 &7:

http://cafefoundation.org/v2/pdf/rv-9a.pdf

There is just no way a cs prop can EVER be justified on a pure cost-saving basis. It is much more expensive to buy & to operate.

It probably depends somewhat on the prop design, but I do know when I was turning a 3 blade Performance wood prop at 2600 with the 0360 at 8500, it burned 8.5 gph per hour. I used that number for flight planning forever. Up at 13,500, it would back off to 6 gph.

I tried FP prop with the Subaru engine but could not get it to work because of the wide rpm range of the engine. It either lumbered down the runway like a heavy B-52 or at a finer pitch, over sped shortly after take off. I did this with a CS prop in a fixed pitch mode at different blade angles. Even with that data, I ordered and paid for a wood FP 3 blade and tried it with the H6. This prop was cut for an 0360, but the pitch was much to steep for the Suby, it turned up only 1900 on the runway. I sold that prop to a RV-4 0360 friend and it was perfect for him untit it got damaged on a nose low runway situation.

I like the FP wood prop and if I could use one with the Subby, I would. It was simple, cheap, you could have more than one prop, and best of all very light. The down side is maintenance and rain. But wood props have been used in aviation since dirt was invented, they are a valid economical choice for the RV and a Lycoming engine.

What the heck, the original ME-109 had a FP wood prop. You can always upgrade when circumstances necessitate it.

 

[jdmunzell]

Catto 3 bld FP

Pierre,

I don't know much about the Catto props but I intend to find out. They look way cool on an RV anyway!

I figure a good F/P cruise prop would give me pretty close to what a C/S would at cruise power. Sure, there is a bit of a hit on the initial takeoff and climb performance, but for my needs...still plenty of power!!

Now that PT-6-15 you have on that Air Tractor...I don't suppose there are any performance issues there!

 

[pierre smith]

Airtractor

Hi Jeff,
My Air tractor weighs 3800 lbs empty and I can fly it at 8400 lbs with 500 gallons of chemical and 126 gallons of Jet A. At that weight on a hot summer day, it takes close to 3000 feet to get off. So even with 680 SHP, it's not nearly as overpowered as it seems . Empty, whoooosh very short takeoff and homesick angel type climb!

Heavily loaded, I can't bank more than 25-30 degrees or she'll fall out of the sky.....pucker time.

Regards,

 

[elippse]

Hi, Kyle! I originally started this post the other day to answer your question, but I took a wrong turn somewhere and the post ended up as faint writing! The three and four blade props on the biplanes at Reno were 59" diameter vs the original 64d/70p two blade on "Phantom" and Jeff Lo's MacCauley metal prop, which was at least 64" but probably longer. Jeff said he got at least 10 mph more speed with the three-blade than with his best-of-two MacCauleys! "Phantom" qual'd at 221.3 mph in 2003 with the two-blade, and 241.05 mph in 2004 with the three-blade, at 7.6% less rpm/power. That's a 39% improvement in converting HP to thrust.(remember, speed change is based on hp/thrust change cubed!) But in 2006, he did 252 mph in the Gold race with the four-blade prop, at the same rpm as in 2003 with the two-blade. That's a 47.7% improvement. All documented and recorded! As far as why I posted about using higher rpm than going to the expense and trouble of getting and installing a replacement engine and prop, it's simple! This is EXPERIMENTAL aviation! I'm just trying to jog your minds out of the conventional, orthodox, and sometimes incorrect myths that abound in aviation. If you're happy with running less than red-line or using a CS prop, well, that's great! I'm not looking for converts. I try to be a spigot of info for those who want to try another way of doing things; I hope what comes out is true! Look at what I've done to the world of prop design and the mis-information about multi-blade props. I keep hearing that props are theoretically limited to a maximum of 88% efficiency, and yet my experimental results point to my obtaining at least 90% in cruise, and possibly 1% or 2% more. Either that or when my prop gives someone a 30% or 40% improvement, the prop that was tested against must have really been horrible. I'm glad that my posting at least stirred up a little hornet's nest of controversy abot rpms and CS vs FP; WE all learn something from good, scientific, well thought-out arguments, and WE all become winners in expanding our knowledge base!

 

[Kyle Boatright]

Interesting stuff, Paul. I'm not interested in turning my engine 2900 RPM, but I would love to try a prop that gives increased performance vs the Aymar - Demuth I have been flying behind for 6 years. My understanding is that the A/D props are among the best of the wood props.

I don't suppose you have any loaner props available? I'd gladly pay the shipping and report side by side test results...

 

[tin man]

Not trying to distact from this prop post but remember it was in that 2002-2004 era that they recalculated the course length and everyone was a couple of mph faster becuase of it.
Tom

 

[elippse]

Hi, Kyle! I was hoping to try out my original two-blade this weekend on an acquaintance's RV-9 to get some comparative data vs his Sensenich, but the CA weather has tossed us a curve; rain through Wednesday! Then I'm going to loan this prop to Jim Smith in Kansas to try on his RV-6; if that works out well, Fred Felix is going to make him a three-blade of my design based on Jim's test results! I'm trying to get him to let me design it for 2700 rpm at 14,500 dalt. I'll report the results on this site! This prop has about 100 hours on my friend's Lancair 235/320, for which it was designed for use as a race prop, and about 100 hours on my 235. On his Lancair, I tested it at three different dalts, 1000', 5500', and 10,000, at 10 mph TAS increments from 180 mph to 242 mph, and it met all estimates within 1-2 mph, usually faster! It would go 242 mph at 3200 rpm at 1000' dalt, at 5500' dalt it was 240 mph at 3150 rpm, and at 10,000' dalt it was down to 233 mph at 3090 rpm. My three blade, designed for 2800 rpm, 10,000 dalt and 200 mph TAS, gives me about 201 mph and at 1000' dalt it runs 213-214 mph at 2950 rpm. Usually cruising at 11.5 to 12.5 baro, 14 to 15 dalt, it runs about 197-198 mph TAS at 2760 rpm, 5.7 gph, for about 34-35 mpg. The only fly in the ointment when designing a prop for someone is what their actual INSTALLED hp is vs rated hp, which can be affected by induction and exhaust.

 

[westwinds]

Hey guys, one thing to think about when you spin your props up to higher rpms is that the centrifugal force loads on the hubs, blades and clamps do go up tremendously. I asked McCauley about bumping the rpms up a bit on my Comanche 250 so i could get better takeoff perfomance as i am based at GWS at 5900'. (RV8 not done yet!) They did give me some figures which i have since forgotten, but the forces do increase dramatically. Granted we are building/flying experimentals, but if you chose to use a Hartzell/McCauley CS prop with aluminum blades at some point you will be pushing past their known safety factors. You will then be an experimental prop test pilot. I also wonder how much the gyroscopic forces on the crankshaft go up with rpm increases. It would be interesting to know how high these forces on the crankshaft get in turbulence. I am not trying to scare anyone because this is not a big problem area right now. I think the fixed pitch prop builders/ flyers may have an advantage in this area because their props are much lighter and of course are one piece. I do not know how much over the published rpm limit i will be willing to push my prop until i have a conversation with the designer of the prop. In my case i am quite sure i will be selecting a CS prop for my 180hp RV8 as i think i will get better all around performance in the altitude range i intend to fly. Tony

 

[allensilberman]

What type prop and pitch?

Regarding the original post, you should check Van's website and construction manual for the designer's thoughts on it. My understanding is that this plane was originally designed with fixed pitch in mind. Furthermore, I've got a feeling that a lightly built 150hp, fixed pitch RV is an f18 compared to the spam cans that most of are stuck with. Sure the CS is better, but I'm sure I'll be thrilled to death when my simple wood prop'd bird
is humming along at 200 mph.

I have a 150hp engine in my rv-4 with a perfomance prop. I don't think I am getting optimum performance. I fly formation frequently with friends whose Rvs have constant speed props. They have to slow down for me and also consider that speed control coming down is more involved with a fixed pitch prop. Any prop suggestions for speed without too much sacrifice of climb.

Allen
RV-4

 

[osxuser]

Many certified airplanes have redlines over 2700 rpm for takeoff. Examples: Piper Seneca III's with a 2800RPM redline for 5 minutes (TO power) on McCauley 3-blades. Beechcraft Duke/Baron 56TC's which zip all the way up to 2900 on their Hartzell 3-blades.

The list goes on, but I won't. Bottom line the stresses are higher, but for TO power, it's designed into the props...

 

[elippse]

Not trying to distact from this prop post but remember it was in that 2002-2004 era that they recalculated the course length and everyone was a couple of mph faster becuase of it.
Tom

It was 2003, and the course length for biplanes, 3.1716 mi., is calculated on 3G turns at 200 mph. Since Tom Aberle was going over 250, he was at a disadvantage. The F1 course distance, 3.1875 mi., based on 3G turns at 250 mph, is slightly longer. Using the F1 distance for "Phantom", his speed would have been 252.862 rather than 251.958!

 

[elippse]

If you've seen a picture of the planforms of my props, one thing will be immediately obvious, there is very little area outboard toward the tip. What this means is that centrifugal loading and bending forces on the hub are considerably less than what is obtained with conventional planforms which have the majority of their thrust in the last 25% of the span and much more mass outboard, where V^2 predominates.

 

[szicree]

Picture of prop

Is this the kind of thing we're talking about? Unusual looking, but if it works, cool.

http://www.contactmagazine.com/Issue77/PaulsProp.JPG

 

[elippse]

Is this the kind of thing we're talking about? Unusual looking, but if it works, cool.

http://www.contactmagazine.com/Issue77/PaulsProp.JPG

Yes, that's it! But that was at Golden West about three years ago, and since then I've shaped the tips to a point they way they were designed to be!

 

[elippse]

Hey guys, one thing to think about when you spin your props up to higher rpms is that the centrifugal force loads on the hubs, blades and clamps do go up tremendously. I asked McCauley about bumping the rpms up a bit on my Comanche 250 so i could get better takeoff perfomance as i am based at GWS at 5900'. (RV8 not done yet!) They did give me some figures which i have since forgotten, but the forces do increase dramatically. Granted we are building/flying experimentals, but if you chose to use a Hartzell/McCauley CS prop with aluminum blades at some point you will be pushing past their known safety factors. You will then be an experimental prop test pilot. I also wonder how much the gyroscopic forces on the crankshaft go up with rpm increases. It would be interesting to know how high these forces on the crankshaft get in turbulence. I am not trying to scare anyone because this is not a big problem area right now. I think the fixed pitch prop builders/ flyers may have an advantage in this area because their props are much lighter and of course are one piece. I do not know how much over the published rpm limit i will be willing to push my prop until i have a conversation with the designer of the prop. In my case i am quite sure i will be selecting a CS prop for my 180hp RV8 as i think i will get better all around performance in the altitude range i intend to fly. Tony

I would never recommend that you take a metal prop past its design rpm. My original posting was for those flying FP props, which can be designed for the higher rpm. My Lancair-owner buddy and great philosopher Oscar Grassle, when we were discussing this red-line stuff this morning over coffee, pointed out to me that in the 172 handbook, the operating power vs altitude is based on getting full rated, 75%, WOT, red-line rpm at 8000'. This means that at altitudes below this you must throttle back to stay at or below red-line. The TBO is based on this operation. The Lycoming is tested for certification to fly around at 100% power all day. So, why not on your plane, get a prop that will extend the red-line to the highest density altitude you normally fly at, and, like the 172, reduce power to red-line below that altitude, and benefit by the higher take-off and climb power. Besides, why do all your cross-country at the bumpier, more crowded air where the 172s fly!

 

[osxuser]

Lycoming is tested for certification at 100% power (at least the ones we fly), BUT that doesn't mean ANYTHING about TBO. I don't think i'd expect a lycoming that was continuously run at 100% power to make 2000hr's. Lycoming itself says in it's care info to use 75% or less power for best engine life...

I would never recommend that you take a metal prop past its design rpm. My original posting was for those flying FP props, which can be designed for the higher rpm. My Lancair-owner buddy and great philosopher Oscar Grassle, when we were discussing this red-line stuff this morning over coffee, pointed out to me that in the 172 handbook, the operating power vs altitude is based on getting full rated, 75%, WOT, red-line rpm at 8000'. This means that at altitudes below this you must throttle back to stay at or below red-line. The TBO is based on this operation. The Lycoming is tested for certification to fly around at 100% power all day. So, why not on your plane, get a prop that will extend the red-line to the highest density altitude you normally fly at, and, like the 172, reduce power to red-line below that altitude, and benefit by the higher take-off and climb power. Besides, why do all your cross-country at the bumpier, more crowded air where the 172s fly!

 

[rvbldr3170]

Another Opinion

Guys,
All this chatter is great for pondering, but the final choice is not about performance, because the CS gives the best overall performance.
It comes down to 3 things, cost, complexity, and weight.
FP props are simpler, lighter and cheaper.
The decision has to be based on all the factors that are important to the builder, but ultimately if maximum performance is the goal, CS is it!!
Me, I am a simpler, cheaper kind of guy, and I am building light for performance, so FP is for me.

That's my story, and I'm stickin' to it.

 

[rtry9a]

I'm not particularly comfortable with the idea of over revving long-stroke big-bore reciprocating engines for long periods. High rpm adds a LOT of excessive internal stress on the crankshaft and oil-supported bearings, which has to add to wear rates. It also undoubtedly adds a lot of extra heat into an already marginal cooling (and lubrication) system.

I AM quite interested in Paul Epps' unique propeller design- I just wish it were available and tested a bit more in our "normal" aircraft. IMHO, it has the potential to be the next quantum leap in a very innovation-adverse market.

 

[Yukon]

Sensenich for Me

I was planning on an MT electric 3 blade for my 9 until I saw the 500 hr TBO. Now I'm thinking that any piece of equipment with a TBO that low
is developmental, and not anything I want on my airplane. Must be having blade or hub problems to have such a low life limit. I ordered a Sensenich FP.

 

[David-aviator]

Lycoming is tested for certification at 100% power (at least the ones we fly), BUT that doesn't mean ANYTHING about TBO. I don't think i'd expect a lycoming that was continuously run at 100% power to make 2000hr's. Lycoming itself says in it's care info to use 75% or less power for best engine life...

Negative on the 75%.
From Lycoming Flyer Key Reprints (in bold print, item 9, page 38)
...for maximum service life maintain 65% or less rated power.....cht 400 or less.....oil temp 165-220.
These engines will routinely go beyond TBO, I've seen an Aztec engine at 2600 hours running just fine, (before 135 mandated overhaul at TBO) but for sure not at 100% and probably not at 75% either.

 

[markpsmith]

$

So what is the cost difference? After reading this thread I have seen #'s thrown around from 2k to 10k...Could someone post some real world #'s on the costs? How much MORE am I going to spend by getting a CS?

Thanks,
Mark

 

[Harvey L. Sorensen]

C/S V FP

2005 prices. governor 1050.00, cable bracket, 13.20, cable 10.71, prop new Hartzell for 320 360 Lyc. 5220.00. total $6293.91. Fixed pitch Sensenich for same $1935.00. $4358.91 difference.
Prices have gone up a bit.
I fly a RV9-A with a C/S, don't know if it is worth it or not but I love it.

 

[alpinelakespilot2000]

So what is the cost difference? After reading this thread I have seen #'s thrown around from 2k to 10k...Could someone post some real world #'s on the costs? How much MORE am I going to spend by getting a CS?

Thanks,
Mark

All depends on which props you are comparing. If you buy from Van's your lowest cost options are Hartzell c.s. and Sensenich fixed pitch:

The basic Hartzell constant speed costs $6090, plus you will need a governor for $1100. Total constant speed = $7190. If you buy a Sensenich fixed pitch, that cost is only $2060. No governor needed. Difference between C.S. ($7190) and Fixed pitch ($2060) = $5130 more for the c.s. at a minimum.

A couple other things to think about:
1. for all of their advantages, the c.s. require more maintenance (= $$).
2. you can get a Catto 2-blade fixed pitch for $1200 which would increase the savings over c.s. even more.

 

[RV6_flyer]

Negative on the 75%.
From Lycoming Flyer Key Reprints (in bold print, item 9, page 38)
...for maximum service life maintain 65% or less rated power.....cht 400 or less.....oil temp 165-220.

A link to the article is at: http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/operation/tboTradeOffs.html

 

[David-aviator]

A link to the article is at: http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/operation/tboTradeOffs.html

Gary, the information I was referring to is located here:

http://www.lycoming.textron.com/mai...ns/keyReprints/operation/leaningEngines.html#

 

[kevinsky18]

CS vs Fixed

I'm surprised this debate hasn't been done yet.

Im building an RV-8

I've decided on an IO-360 and now I need to decide on the prop. I haven?t decided on the exact IO-360 engine yet but I will be buying new from Aero Sport. Probably the non-counter weight engine with high compression pistons and electronic ignition. I?ll pay the extra $300 and get a constant speed crank regardless of whether I go constant speed or fixed pitch.

I?ve heard and read that above 200hp you really should have a constant speed prop. But an IO-360 with high compression pistons will be just slightly below that boarder line. I think somewhere around 190hp.

What are the pros and cons on a constant speed verses fixed pitch for this size of engine?

What sort of performance gains verses weight loses and costs am I going to experience?

My main mission will probably be long x-countries.

 

[John Clark]

It has come up. Run a search, I got 153 hits.

The bottom line is more weight, cost, and performance (CS) vs less cost, weight and performance with the fixed prop. Before the debate starts anew, you can engineer a fixed prop to climb or cruise with a CS but not both.

John Clark
RV8 N18U "Sunshine"
IO360/CS
KSBA

 

[gvgoff99]

FP vs CS (both are good)

Your mission and mine are similar. I use my 160 HP Lycoming powered RV-6A for long cross countries. For me the cost/maintenance factor decided the question. The FP will cruise as fast or faster than the CS according to tests VANS has run. The CS weighs more which is not all bad in a 6 or 7 because that usually gives more luggage allowance. The 8's tend to run nose heavy, I have been told, so there the extra weight on the nose may be a minor factor. The FP is 1/3 to 1/4 the initial cost of a CS and there is no comparison for the overhaul and maintenance issues the FP is just a lot less to own all the way around.

That being said I have a Catto FP and it, like most wooden and composite props, cannot be run in the rain above about 2250 to 2300 RPM. This is a problem for IFR and sometimes for VFR work. There is a protective layer that can be applied to the leading edge that may or may not affect performance. (I have been told both). I tend to think that anything interfering with the air flow would affect something but that is pure conjecture and NOT scientific. It may speed up the plane for all I really know!

CS: better acceleration and climb and excellent cruise, most are metal and rain is no factor.
FP: as stated in an earlier post: set up for climb it will climb similar to a CS
FP: set up for cruise will cruise similar to a CS
It won?t do both (no surprise)
FP: wood and composite are not great in the rain. I suggest that you get a metal one if you get FP and will be flying in the rain on the long x-countries.

I would have gotten a metal FP prop except for the RPM restriction to not exceed 2600 RPM on the 320 Lycomings (150 or 160 HP). The restriction seems to not apply to the 360 Lycomings though it may be a good idea to check the CS RPM restrictions if you decide to go that route.

I did put in a Pictorial Pilot autopilot and altitude hold with the savings and those two things are "must haves" way more than the propeller issue for a cross country flyer IMO.

 

[gmcjetpilot]

Search

Use search in the drop down menu. Lots of good info. As usual it comes down to money.

The FP will cruise as fast or faster than the CS according to tests VANS has run.

This not true at my friend. You are getting cruise and top speed mixed up. I know what you are saying but you are miss stating the facts. The fixed pitch prop (Sensenich) was faster than the constant speed props by turning 2730 rpm, more than you would want to "cruise" at, while the c/s props where limited to 2,500 rpm. However in the test you refer to, the Hartzell BA prop was faster than the fixed Sensenich, hands down comparing similar RPM. Further the Hatzell BA would give higher top speed at low altitudes, better climb rate and shorter take off.

In the above fly off, the Sensenich corrected speed at 2,500 rpm was estimated by calculation and was slower than the Hartzell BA. If they would have actually flown it at 2,500 rpm, it would have actually been slower. When you run your engine with a partially closed throttle in cruise you lose engine efficiency. With constant speed you can run wide open throttle (more efficient) while controlling RPM independently of throttle.

However with that said the Sensenich fixed is pretty good value and good for all of the reasons you give, but a constant speed prop will be MORE EFFICENT in cruise, meaning less fuel burn, rpm, noise for a given speed.

 

[gvgoff99]

I stand corrected (sort of)

From what you are stating on the speed differential it stands to reason that in a controlled environment a FP and CS could be set up to be equal at a given RPM and therefore, in that perfect environment at that RPM they would be equal. Obviously our environment is not very well controlled and I agree that the CS is a more efficient propeller. I don't believe it is fair to take cost out of the equation it is a large consideration.

I am not a math whiz kid and nor have I ever seen the figures for how much more efficient the CS is over the FP. If you know the efficiency coefficient you can then calculate the overall "savings" said efficiency is going to get you. Using that you can then calculate how many hours you will need to fly to regain the extra $$ spent (don't forget to include OH, etc.). I don't think you will ever see a monitary savings from the difference in efficiency. You will see it in climb and a bit in cruise assuming you have a FP prop set up for cruise.

I agree with everything that you said. I agree in the title that both propellers are good viable options and most of the decision depends on how much one wants to spend in $. Money is the largest factor to many. To me the maintenance was as big a factor as the money. I like the KISS principle and FP does that for me. I imagine that if the initial cost and maintenance costs were equal there would be very few flying FP.

 

[gmcjetpilot]

U DA MAN

I like the KISS principle and FP does that for me. I imagine that if the initial cost and maintenance costs were equal there would be very few flying FP.

YOU DA MAN George G. You got it. We agree 100%, just correcting (nit picking) the wording.

 

[rv969wf]

Woosh

I'm surprised this debate hasn't been done yet.

I've decided on an IO-360 and now I need to decide on the prop. I haven?t decided on the exact IO-360 engine yet but I will be buying new from Aero Sport. Probably the non-counter weight engine with high compression pistons and electronic ignition. I?ll pay the extra $300 and get a constant speed crank regardless of whether I go constant speed or fixed pitch.

Kevin, this is only a suggestion OK. If your not sure about Fixed prop or C/S prop, go find a few buddies or pilots with RVs and go for a ride with them on a short trip. Fly in a few with a F/P and a few with C/S and you make the judgement call. The C/S will impress you!!!! After flying with a C/S prop will spoil you, just my opinion, feelings and thoughts. If you have the money go for it. In rough air a C/S prop will not surge the engine up and down through the RPM range. If you set the engine at 2,450RPM it will stay there at all times unless your in a dive. If you plan on flying in and out of very short fields with high density alltitudes at gross weight, the C/S will give you some safety margin.

Safety is one reason I'm running the C/S for the above reasons because I live in a 100F++ area with very short grass airstrips and I need all I can get on takeoff at times. $$$$$ will eat you up at prop rebuild time and the gov.. is not cheap to work on either. Weigh your costs either way and like I said go for a few rides in RV's with different props. Good luck and either way you go, your RV will bring pleasant smiles.