Tri-Mower Design

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Vigilant1

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Thanks for sharing the calculations.
What about a 24” diameter prop at 32 mph?
I'm using a rough method provided by Raymer in Aircraft design: a Conceptual Approach.
To simplify it further (for 2 blade fixed-pitch props with an aspect ratio of 8), we end up with :
D/q = 0.2 x (Prop dia in feet)
So, a 48" diameter prop would have a D/q of 0.8

To find the drag in pounds we multiply the D/q by the dynamic pressure (q) at our airspeed and air density. If we are at 60 knots and at standard sea level, q = 12.18 So, drag in pounds would be 9.7 lbs

For your case of a 24" dia prop at 32 MPH, drag would be about 2.61 x 0.4 = 1 lb

I suspect that this approach may underestimate the drag from props with very high aspect ratio. Raymer's underlying approach starts with the prop blade area and then applies corrections to it. So, a short "fat" prop gets the same drag estimate as a very long, skinny prop with the same area. My >guess< is that the long, skinny prop probably would have more actual drag, much as a thin wire normal to the airflow has more drag than a flat plate of equivalent frontal area. But, if we're just going for a ballpark drag estimate for stopped props of typical dimensions, it's what I use because it's all I've seen.
 

BBerson

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I think the drag concern of multi motors is in midrange speeds. At Ultralight speeds the nacelle and dead prop drag hardly matters. But the 1/3 scale, four nacelle B-17 didn't fly well even as a single seater with all four running and engine out was not attempted.
 

Vigilant1

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I think the drag concern of multi motors is in midrange speeds. At Ultralight speeds the nacelle and dead prop drag hardly matters. But the 1/3 scale, four nacelle B-17 didn't fly well even as a single seater with all four running and engine out was not attempted.
+1. For a multiengine design that has marginal power with one engine out, the ability to fly (and climb) at low airspeeds will be important. Flying slow reduces the drag from the stopped prop and (with an appropriate prop) lets the "good" engine(s) produce more thrust. Since induced drag predominates at low airspeeds, a long-ish wingspan can be of use to a design like this in its most critical flight regime: safe performance with an engine out.
 

harrisonaero

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Does anyone know of a published study of drag build-ups comparing single, twin, tri, and quad recip engines for a given airframe and total power output?
 

Vigilant1

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Does anyone know of a published study of drag build-ups comparing single, twin, tri, and quad recip engines for a given airframe and total power output?
I've never seen one. It would get very complicated since there could be multiple permutations of twin and tri layouts, you'd need to look at all engines running and with various engine out scenarios for each configuration, etc.
In very general terms, if we are talking about IC engines with fixed pitch props, a single will give the best performance (airspeed, ROC, payload) compared to a multiengine acft of similar installed HP. The single will have less engine weight and the prop can be selected for cruise efficiency (since there's no need to compromise with a lower-pitch prop needed to provide thrust for an engine out situation). The multi-engine configurations offer other advantages.
 
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BBerson

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Does anyone know of a published study of drag build-ups comparing single, twin, tri, and quad recip engines for a given airframe and total power output
I asked Mark Moore (then at NASA) about the nacelle drag on his 14 motor experiment. I think his reply was he had not done a study.
 

BBerson

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The "Carat" has automatic prop blade fold upon engine failure. A good safety feature. (if not total engine fold, as I suggested earlier)
The idea of engine fold or retract is the option to cruise on two engines efficiently instead of three or more.
 

blane.c

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Engines don't always completely fail. One of the nice things about multiple engines and one starts misbehaving is that provided you don't need whatever power it is giving you, you can shut it down. This option allows you to among other things "save what's left" or "prevent further damage".
 

Vigilant1

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The "Carat" has automatic prop blade fold upon engine failure. A good safety feature. (if not total engine fold, as I suggested earlier)
The idea of engine fold or retract is the option to cruise on two engines efficiently instead of three or more.
I suppose a lot depends on the specifics of the application. If the plane has just enough power to safely fly and climb with one engine shut down and the others at full power, I suspect in most cases it would be preferable from a fuel economy, engine longevity, and in-flight reliability perspective just to run all the available engines at reduced power in normal cruise ops rather than stow an engine or fold a prop and run the remaining engines harder. Things might be different for turbine engines which are often more efficient at high power settings.
 
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BBerson

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No, the Voyager cruised on one engine for best range, after a period of running both to burn off some of the excessive fuel weight. I think two engines running at near max power (but not so hot to need enrichment) is best economy if you want to cruise about 55-60% of total installed power.
If you cruise above 75% power, then yes use all three.
 
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Sockmonkey

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No, the Voyager cruised on one engine for best range, after a period of running both to burn off some of the excessive fuel weight. I think two engines running at near max power (but not so hot to need enrichment) is best economy if you want to cruise about 55-60% of total installed power.
If you cruise above 75% power, then yes use all three.
I would hesitate to use Voyager for comparison. It was a highly specialized design that didn't need to worry about having an excessively long takeoff run.
 

blane.c

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The voyager carried a second engine as a spare because of the trans oceanic crossings, no doubt they took advantage of the extra power to get to altitude.
 

lr27

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The Voyager carried so much fuel that the weight changed radically over the course of the flight, No need for two engines once the weight gets down to half what it was at takeoff.
 

Vigilant1

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No, the Voyager cruised on one engine for best range, after a period of running both to burn off some of the excessive fuel weight. I think two engines running at near max power (but not so hot to need enrichment) is best economy if you want to cruise about 55-60% of total installed power.
If you cruise above 75% power, then yes use all three.
I think I'm not following you. Assuming our 3 engines are the same size, if we want to cruise at anything above 66% of total installed power then we need to use 3 of them, right?

I don't know about the Voyager engines, but most spark engines have their best BSFC at right around their peak torque RPM. For the Honda GX 690, that's at about 75% power.
upload_2020-2-8_19-11-4.png
So, if we want to cruise with anything more than about 50% of installed power (that's .66 (2 engines of three) x .75 (power from each running engine) = 50%), then we'll be in an area of declining net torque and declining fuel economy if running on just 2 of our 3 engines.

But, more fundamentally, why run 'em hard if we don't have to? They'll be a lot more reliable and last a lot longer if loafing along at 60%-75% power each. And when they are running they tend to stay running. We'll need them all running during critical phases of flight anyway, so it seems to me best to just keep them happily turning.

Obviously, things are different if a person enjoys silent flight and wants to sniff around for lift, etc.
 
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BBerson

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Yeah, I guessed 55-60% as the crossover point of all three, but your 50% is probably closer. It depends on what the owner wants. I want to eliminate 1/3 of the nacelle parasite and cooling drag by retracting it or something.
I (in 50 years) have only seen one piston tri-motor in person. (Tri-lander)
Must be a reason.
 

lr27

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You just picked the wrong 50 years, that's all. There were quite a few at one time. Maybe it's because very few airplanes are constrained by the maximum available motor size these days.
 

blane.c

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I think I'm not following you. Assuming our 3 engines are the same size, if we want to cruise at anything above 66% of total installed power then we need to use 3 of them, right?

I don't know about the Voyager engines, but most spark engines have their best BSFC at right around their peak torque RPM. For the Honda GX 690, that's at about 75% power.
View attachment 93065
So, if we want to cruise with anything more than about 50% of installed power (that's .66 (2 engines of three) x .75 (power from each running engine) = 50%), then we'll be in an area of declining net torque and declining fuel economy if running on just 2 of our 3 engines.

But, more fundamentally, why run 'em hard if we don't have to? They'll be a lot more reliable and last a lot longer if loafing along at 60%-75% power each. And when they are running they tend to stay running. We'll need them all running during critical phases of flight anyway, so it seems to me best to just keep them happily turning.

Obviously, things are different if a person enjoys silent flight and wants to sniff around for lift, etc.
So fixed pitch propeller dictates power curve, so at 63% RPM 25% power


Jan Carlson about RPM vs POWER.png
 

Vigilant1

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So fixed pitch propeller dictates power curve, so at 63% RPM 25% power


View attachment 93066
?? Where does the 63% RPM come from? The Honda dyno chart shows peak torque at 2500 RPM (75% of max RPM), and at that point the engine >can< make up to 17 HP (of 22 max, or 77% power). Of course, when hooked to a prop it will just turn at whatever RPM the engine can make at the throttle setting we choose.
 
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