2004 7GCAA Prop / Cruise Speed Increase Question

[GWC]

I have an Adventurer, with the Lycoming 0-320-B2B and a SENSENICH 74DM6S8-1-56 prop.

Does anyone have any experience with re-pitching the propeller for higher cruise speed?

Any real world numbers would be helpful.

I’ve checked with MT and it doesn’t seem like they offer a propeller for this airplane that has been STC’d.

I’m trying to figure out if repitch is worth pursuing and what my other options are for better cruise performance.

Thanks in advance.

 

[Bob Turner]

Dunno, but I brought one likevthat from Denver last year - nicest Champ I have ever flown, and that includes my Decathlon.
It was 105 mph all day long at 6.3 gph. Stunning!

I can probably find out what prop.

Yeah - here you go:
Prop – item 20(a) – 74DM 6S8-1 74/56 - 2450-2550 static

 

[GWC]

Thank you. Yes, those are the numbers I’m getting with that identical, stock/factory prop.

Just hoping for a propellor change or mod to get a faster cruise out of her.

 

[Bob Turner]

You might be up against "hull speed.". Anything over 100 indicated is pretty good for a Champ.

 

[GWC]

When you say “Champ” are you referring to the 7AC? Because that’s not what I’m flying.

I don’t think it’s a hull drag issue.

The 8GCBC Scout has the same frontal cross-sectional dimensions as the 7GCAA Adventurer, and yet can obtain up to 130 knots with an adjustable prop/constant speed, hence my question about a re-pitch on my existing prop.

 

[Bob Turner]

When I say Champ I mean everything not Cub. I call my Decathlon a Champ. It has a 180HP C/S with shorter and less draggy wings than a Scout, but 130 K is about ten knots faster than it will comfortably cruise (ground speed, 7500' no wind). The Champ/Citabria airplanes are not known for speed. That is due to hull, not power. I think you are looking for a Mooney. A good B or C model will give you close to 130 K.

 

[Tangogawd]

As long as you stay with in static limits, I guess you could repitch a touch. It would help, but not sure how much.

The Scout is 180hp. And I have flown several scouts, one with 200hp, another 180 HP with CS MT prop and non of them did 130kts.... 120ish for the MT, the others were 115mph.

A 7GCBC like mine is closer to your plane. I get 110mph out of it. I burn 8.5gph

 

[Bob Turner]

That sounds about right. When the fuel burn starts going up out of proportion to the speed, you are asymptotically approaching hull speed.
105 mph for 6.3 gph vs 110 for 8.5 - 4.8% speed increase for a 35% increase in fuel burn? We could extrapolate, but it is non-linear.

 

[Bartman]

I’ve never heard the term’ hull speed’ applied to a plane. A boat, yes, but not a plane.

 

[Bob Turner]

I am up for a more aeronautical term for the concept.

 

[Slipinonfinal]

Max efficient aerodynamic drag vs "hull speed" unless it's the Spruce Goose. I guess we could just invent a word how about Eco-drag?

 

[Slipinonfinal]

Or if your are a aeronautical engineer the "Eco-drag coefficient"

 

[Bartman]

Reading about hull speed is headache inducing and surely has the effect of displacing cherished childhood memories so use caution! It's a reflection of efficiency though for displacement hulls and represents a maximum before efficiency drops way off so I'd compare it to best range speed where going faster helps the schedule but reduces overall efficiency. Read about best range at your own risk as that too will displace more cherished memories.

Understanding Vessel's Hull Speed And Its Determination

Marine Insight - The maritime industry guide.

www.marineinsight.com

Why You Rarely Fly At Best Range Speed In A Prop, But You're Close To It In A Jet

Your "Best Range" or "Maximum Range" condition is a term you've probably heard. But, in a propeller-driven airplane, it's a setting you will probably never use.

www.boldmethod.com

 

[Slipinonfinal]

Bart,
I don't want you to loose anymore childhood memories, however the author in the above bold-method piece under "the difference between thrust and power" got the thrust equation wrong and yes I'm splitting hairs here but in my first introduction to thrust and power in the military a long time ago we were told repeatedly a jet engine pulls (not pushes) an aircraft forward through reactionary force. This is more understandable when on a wet (near the dew point) day you magically see this vortex spinning wildly in front of the jet engine intake and the training diagram of the danger area in front of these engines is burned into your brain. (thinking my earliest days and small operating area aboard an aircraft carrier. We watched the front end a lot more than the back end.) I digress! and yes this is an endless conversation, none the less a good one.

 

[Bob Turner]

Go back to my post #8.

There we see an efficient cruise at 105 mph. Add power and you go faster, of course, but with the data we have it takes 35% more fuel to get an additional (less than) 5% increase in speed.

We don't have the numbers for 115 mph, but you can bet that additional 4 1/2% increase (from 110 to 115) is going to cost you dearly, if it is achievable at all.

Sure sounds like hull speed for the GCAA is 105 mph indicated. Remember, hull speed is simply a shape being forced through a fluid. The difference for a boat is that it is only dependent on the part in the water. I am sure aerodynamicists have a name for it - I have a retired UCSD professor in fluid dynamics on speed dial -I will check.

Edit: I haven't checked yet, but maybe the hull speed idea is not the best analogy. The formula for hull speed is simply related to length.
Still, there is a point where a given shape will only go a little bit faster if you add a lot of power. The Cubs are a good example - a 65 hp Cub will cruise at 75 or so. You can get that to 80 with very little extra fuel. If you want, say, 105, you need to streamline the cowling and windshield, and double the horsepower. With a more efficient 160 Lyc you can get 105 mph at about 7 1/2 gph. Hang a 180 hp engine on the same airframe and you can get 110 mph for an additional 2 1/2 gph.