aerodynamics of multiple engines and efficiency

[unabob]

Hi All,

I'm working on an aircraft design and I've come to a dilema about the electric motor configuration. I can't really decide on whether to go with two smaller motors or one large motor from an efficiency standpoint (duration, not distance). Also, as far as motor placement aerodynamically speaking, is it better to have a motor pulling or pushing the aircraft? With two motors is it better to have them on pods on the wings or inline on the fueselage? Any thoughts and suggestions would be greatly appreciated.

[JetPlaneFlyer]

One larger motor would be more 'efficient' mainly because it will save weight compared with a twin especially when you consider the weight of all the associated structure motor mountings, wiring ESC's etc that would be needed for a twin. Large props are also more efficient than small ones so that's in favour of a single larger motor too.

Pushing/pulling in itself makes no difference, the prop works exactly the same. Location is usually down to practical issues like achieving the correct ballance and ground clearance for take off and landing.

Steve

[eflightray]

It depends.

It depends to a large degree on what sort of plane. If it's for duration I would assume it's basically a powered glider, as you would also want the most efficient airframe as well as power system. That way you can probably shut the motor(s) off for periods of time.

As for single or twin. I would assume, (I have no technical background just 'gut feeling'), that two props might be more efficient than one, (my logic says, if bigger is better, then two giving a bigger area, and drawing less current than one should work, remember that enormous flying wing with multiple motors that flew for days).

So my choice would be, (well, maybe I'd consider), two small inrunners with gearboxes driving fairly large folding props in pusher configuration on the wing T.E..

It all 'depends' on what you're actually trying to achieve. If it's a heavily loaded quadraplane flyingboat then I may wont to re-think, (and keep away from this thread :D ).

Depends ;)

[eflightray]

Here's an interesting twin, though a little on the large size.

Zephyr - Solar powered (http://www.telegraph.co.uk/news/2608943/Solar-powered-spy-plane-breaks-flight-record.html)

[unabob]

Thanks for the replies. I was leaning towards two motors in an inline configuration, one pulling and one pushing, that way I could use both for climbing and possibly shut one off and use one to sustain level flight. The one thing that was making me think otherwise is clearance on the back prop when taking off. I was thinking about possibly using a folding prop in the rear but not sure how it would work. Does anyone have experience with this? Also, what do you guys think about prop efficiency on an inline configuration? I don't know if a prop needs clean air to work well or if swirling vortexs heading into the rear prop will decrease the thrust.

[JetPlaneFlyer]

Two props inline is possibly the least efficient of all. You would have better performance with a single larger tractor prop in the nose. As you have already identified a pusher also gives you ground clearance problems. Pusher folding props can also be difficult because they have nothing to fold against and in any case the take off is where you would need most power so whats the point of having the blades folded just when you need the thrust most?.. Plus you would need a seperate channel for each motor.

As I said earlier;larger props are more efficient than smaller ones so the single motor with a large prop will be more efficient (motor efficiency's excluded) that two motors with smaller props eflights 'gut-feeling' is wrong in this instance. I'm not an expert in electric motors but I'd also suspect that that larger motors tend to have slightly higher efficiency than small ones?

The rear prop in a tandem configuration will be effected by prop wash from the leading prop. Generally more pitch is required for the rear prop to compensate for the fact that the airflow into the prop is already moving backward.

Steve

[vintage1]

One thing to note on multis is that you need odd pitched props.

Let's say yo have a single 10x7

Now, to get the same performance from a twin, with smaller props, you want essentially the same blade area and same pitch speed. If RPM is similar, you still want 7" pitch, but the diameter has come down..if you want to keep the area constant, you assume that area is proportional to diameter squared..so all other things being equal, two 7x7s = one 10x7 roughly..

In general its easier to get what you want out of one big engine and a big prop.

The reason for multis in real life is more about ability to survive engine failure and to actually get enough power from small engines, than to achieve a greater efficiency.

[eflightray]

Just to add to my 'thoughts'.

My reasoning was mainly that small inrunners are supposed to be more efficient than large out-runners, (or at least that's the impression I have from many posts on RCGroups).

So my simple mind said that two geared inrunners could be more efficient at turning the same or greater swept area. But it looks like I'm wrong.

[nmasters]

If duration is important the prop really should be on the front because unless you have a lot of wind tunnel time you're probably not going to get the aft fuselage airflow clean enough to avoid wasting at least 10% of your power. Here's a discussion of prop noise that points out some of the ways a pusher loses power: http://www.rcgroups.com/forums/showthread.php?t=990745&highlight=noise

--Norm

[unabob]

Thanks Norm, great info. on that link.

[Texas Buzzard]

This can be a very interesting question.

Way back in the 1970's a free flighter published a design for free flight in Class B. As you know the maximum displacement for Class B is 0.30 cu in.

He used two sized 0.15 engines, one pulling and one pushing.
He was legal in Class B. He reasoned that the two propellers
on the two 0.15's would push more air than one prop on one
engine. At least he didn't have a handicap for he placed second at the Nats.

Using electric power I always am thinking about the weight of the battery. At first approximation the battery for one elect. motor would have less weight that two batteries would for two elec. motors. Weight is a real factor.

Remember that a 10 x 3.8 prop is more efficient than an 8 x 6 at least I have found that to be true. Conclusion: Go with one motor and just do it.

[fnev]

The entire above is true… BUT: the request was done with endurance as a design driver.

This in itself changes the way to look at things: once the take off and climb are over you should have your propulsion designed to maintain the plane at its most efficient cruise speed (best L/D).

As it is rather difficult to have a very good compromise to get the best at cruise and still have enough power for take off and climb, it might be wise to use two different propulsion systems: one optimized for the best cruise and the other one to supply the extra thrust required for take off and climb.

The simplest solution is a push/pull configuration with the optimized propulsion system in front. It is not as simple as it sounds as you will have to do all the calculations to figure out if a single power source (we are in an electric configuration right) is the best way to go. A trade off study for mass is needed as well… and so on, but we are talking of getting the best endurance…

[DT56]

The entire above is true… BUT: the request was done with endurance as a design driver.

This in itself changes the way to look at things: once the take off and climb are over you should have your propulsion designed to maintain the plane at its most efficient cruise speed (best L/D).

As it is rather difficult to have a very good compromise to get the best at cruise and still have enough power for take off and climb, it might be wise to use two different propulsion systems: one optimized for the best cruise and the other one to supply the extra thrust required for take off and climb.

The simplest solution is a push/pull configuration with the optimized propulsion system in front. It is not as simple as it sounds as you will have to do all the calculations to figure out if a single power source (we are in an electric configuration right) is the best way to go. A trade off study for mass is needed as well… and so on, but we are talking of getting the best endurance…

That's what the designer did on this endurance airplane!

[Airboatflyingshp]

Adding to Rays observations a bit of Empirical evidence/observation and thread cross pollination - if you are looking for a endurance flyer then the motor glider is the way to go ................as for a single push or pull- each has advantages and drawbacks depending on what your trying to achieve.
Take a look at the AP ships and the UAVs also take a look at the Catalina Island project parts One two and Three.

Pre domination of out runners or inrunners Nigel Haws of RCM&E electric flight took an old Cambrian 72" slope soarer glider and added 4X simple DD S400 motors using small gunther props mounted on dowels stuck into the LE, I will check :confused: my memory isnt what it should be. I have it saved somewhere, but he was very pleased to discover that it performed far better than with a single large heavy motor and cells to match, winning several first up last down club competitions against more sophisticated models. You might also look at the success of geared multies by the likes of Ivan Pettigrew, dont go Ray :D thats as close as Im going to go to flying boats :rolleyes: here.

His viewpoint was one of prop disc area presented, put simply in drive mode many props make lighter work, as per Hepdogs experience, here http://www.rcgroups.com/forums/showthread.php?t=1027444 and can way less, you can also offset torque reaction, not on a big single unless you start getting into contra rotating motors! And that rules out folders.. ;) .

In line push pull is tricky, disturbed air from one prop hitting the other mostly ends up robbing the following prop of some of its efficiency , however it is a streamlined glide layout, and in a one motor out/ off situation it maintains the symmetrical thrust line ...eg as in the Cessna 336/7 Skymaster and its modern equvalents.

If you want to see a possible good starting point for your experiments look up a French designed twin boom pusher glider called Big Brother.. http://www.rcgroups.com/forums/showthread.php?t=644209&highlight=Big+Brother+twin+boom+glider and this one http://www.rcgroups.com/forums/showthread.php?t=1026525 . it works very well from the reports and it could form an ideal test bed airframe for your ideas, a small point but wing mounted folding props in traction mode have to be far enough away from the wing LE to fold and spin up without striking the LE. of the wing....if you try a larger prop the stand off has to be there to start with or also addable.
Think about the wing form is it going to taper? Is it to have a swept LE or Trailing edge..if your going to mount motors on it somehere.

If you are going to physically test your ideas ..... you might like to consider the concept of using geared brushed S480 motors...using swappable gearing such as in these http://robotbirds.com/catalog/product_info.php?cPath=36&products_id=381 Look at Ivans models :)
You will also be able to get away with one or two speed controller's rather than individual controller's for each more advanced type of motor, and experiment more with the prop sizes, the ideas are there, your the one who has to see if it will work for your actual craft.

Its an interesting Idea and could work well, big props need big UC if you want to ROG that adds wt and drag what you could do is use the smaller props to get you up and away then enter economy cruise mode on your single large pusher or tractor prop........ this should work if you can use folding props and seperate switching and/or controlers.

Interesting, if complex :) :cool:

[eflightray]

That's what the designer did on this endurance airplane!

A single ?, is that all you've got ? :D
I''l raise you a 14 motor, the Helios (the one on the right) ;) (Sorry about the photo size, but it's to a link).


http://upload.wikimedia.org/wikipedia/commons/6/67/EC99-45210-1.jpg

[builder]

There used to be a time when it was a simple answer, going with a multi electric was supposed to be more efficient because you could use higher voltage (less loss) and then split it at the motors using a series hookup. But this was for DC motors and we don't use them much anymore. I think it would come down to current draw vs thrust, it would be easy enough to test this in a static setup with one motor vs two. I think it would be valid if you kept all the motors at the same KV rating and the props the same pitch. Diameter would need to less for the smaller motors of course, perhaps choosing 2 props to equal the same swept area as the large one for the single might be a good place to start.

As for push-pull vrs seperate nacelles? History points to the push-pull. The advantage of less drag offsets the interference problem. A good example was the DO-335 of ww2, one book I have lists it's maximum speed at 477 mph, and it was not a small airplane. One account has a captured example easily losing it's pair of escorting p-51's, and the 51 was no slouch either. The big disadvatage is that the push-pull gobbles up a lot of fuselage space that could be used for something else.

And again, it looks like history gives a single motor airplane in the tractor configuration a slight edge, if you use "pushy cat" vs "nemisis" as an example.

[vintage1]

I think the success of multi-motored brushed setups was down to one simple fact. A typical DD setup is rubbish on these motors. The diameter is too small, and the pitch speed is too high.

By using more bigger plane the effective diameter goes up, and the pitch speed doesn't.

I.e. 4 speed 400s driving 5x5 props is about the same as one bigger motor driving a 9x5 prop at the same RPM and 4 times the power..hwever a single DD brushed motor would probably be barely able to swing an 8x6.


If you like, multis are a sort of implicit gearbox.

Once yiu use a real gearbox, all the advantages vanish.

Real life multis are really there either for safety, or because one engine isn't big enough.

Or in te case of ultra low power slow plane, because you can get the efficiency of a really ginormous prop moving very slowly, without the downside of it being too big or heavy to be practicable.

[MCarlton]

Is there also a theoretical advantage for low speed flight/landing/STOL and the like in using multi engines due to propwash over the wing/flaps etc?

I suppose there might also be an advantage where counter rotating props can be employed, in as much as torque effects can be cancelled out.

[Brandano]

Not to mention the height and weight of the landing gear.

[Work in Progress]

Both of those can be good advantages of the multi-engined design. Having a lot of wing 'blown' by the props can do wonders for your take-off performance, especially with big slotted or Fowler props in the slipstream.

However, you have to be very careful with this in full size aviation. It's bad enough losing an engine at any time, but losing an engine that is directly providing a lot of lift, just after take-off, leaving an asymmetric stall with a lot of yaw - pretty much no way out of that one!

[eflightray]

One way to reduce the worry of an engine out on a twin is to use series connection, as 'builder' mentioned in post #16. Unless one motor shorts out, both should quit if one fails.

My B25 has both battery packs and both motors connected in series through one ESC, (a big 'daisy chain'), double the volts, but current as for one motor. (Using brushed motors of course. One of the few advantages over brushless).

[builder]

Well, allways being curious about singles VS multis, I decided to do some testing with a couple of motors from my collection of brushless ones. I used an AXI 2826/10 with an APCe 10x7 for one test and a homemade small outrunner equivelant to about a 2208/34 with a M.A. 8x6 and 7x6. Both motors have a similar KV rating.
I have found this Axi to be pretty efficient from a draw vs work standpoint, so I used it as the test mule. The 10x7 APCe is one of my favorite props and was used for the same reason.
The test stand was pretty typical and used a digital fish scale to measure thrust and an ammeter to measure draw.
My results using the same 3 cell 2100 LIPO were:

AXI- 18 amps, 8700 RPM, 3.5 lbs thrust.
Homemade outrunner- 9 amps, 8200 RPM 1.9 lbs thrust with 8x6; 6 amps 9400 RPM 1.4 lbs thrust with 7x6.

The motor mount had a fairly large plate for mounting, so this would be very similar to the loss encountered by an engine nacelle. The smaller props would suffer the worst loss because of the larger percentage of interference.

The homemade single used some cheap magnets I found in a hand crank battery charger, no doubt a commercial motor would be more efficient.

With at least these two disadvantages going for it, the small motor still managed to give more static thrust than the big one if you figure in the multiples. Even figuring in the drag of extra nacelles it would still probably come out ahead, And the weight argument of extra batteries and speed controllers doesn't wash because you CAN run two ac motors from the same speed controller and battery.

At least in this test of static thrust the smaller motor had an advantage. If I were going to design an airplane and had the option of using single or double motors, I would use whatever was the most convenient for that design and not worry about efficiency because it looks like it is not worth worrying about.

[vintage1]

However the AXI has 24% more pitch speed. (8700x7/8200x6)

And so is actually delivering MORE power.

In your case and figures the twin is about 13% less efficient than the single, multiplying pitch speed by static thrust to get an 'output power' figure.

Apples and oranges..

[builder]

Pitch speed has nothing to do with it. The only thing that counts in this case is current draw vs thrust. The smaller motors will produce more total usable thrust before the battery expires as per the numbers.

[vintage1]

Pitch speed has nothing to do with it. The only thing that counts in this case is current draw vs thrust. The smaller motors will produce more total usable thrust before the battery expires as per the numbers.
If all that counts is thrust, I suggest a 10:1 gearbox and a 25" x 2" prop. The big motor will beat the pants off the two little uns that way;)

If I want to lift a ton with a 50W motor I can easily do it.

grams per watt is meaningless with different propellors.

[Airboatflyingshp]

Im a big fan of multies, but vint is right in this area, Its Horses for courses as we say and the prop and the airframe and the flight envelope, availability and the budget all have to be ballanced, tweaked and tested......... thats not to say you cannot turn up a few surprizingly effective solutions by empirical means.
Mr Low Tech -Ivan Petigrew is my bench mark in this respect... read his technical notes it may inspire you further.

[builder]

You both miss the point. As I stated before, I was testing for static thrust and in this case the smaller motor had a slight edge. I DID NOT use low pitched props to bias the outcome. The data is a valid starting point for further experimentation. I didn't bother to state the obvious that the only way to tell the results in real flight would be to be able to get readings of power consumption at a number of different airspeeds and make the comparison.

I always get a laugh when someone says "your test data must be wrong because it does not match my theoretical predictions".

Talk is cheap, and in the end results are the only thing that counts. I welcome anyone to do comparatve testing in real flight of different systems in an unbiased scientific way so we call all add the results to our collective store of knowledge. After all, that is the point of these discussions.

[BMatthews]

In terms of push-pull I would not be too quick to haul out historical examples. The Cessna has been shown in a few readings I've seen to be a compromise that allowed twin engine safety to be enjoyed by single engine certified pilots. So it was a compromise to achieve a goal. The Dornier DO-335 was also a case of adding some power and speed in a package that was flyable by single engine trained pilots. So it was a compromise to achieve a goal. And if you look at Rutan's world circling plane I suspect you'll find a set of compromises that made it wiser to use the inline setup rather than two outward spaced engines. Likely the idea that they could shut down one and feather the prop and cruise on the other or some such factor.

Any time I've seen info on prop efficiency it has always favoured the tractor prop due to it cleaving clean air.

Oops, almost forgot.

Since this is for duration the implication is that it'll be slower flight speeds where static ground thrust testing will have some validity. At that point testing options for thrust/amp ratios comes into play. There's no magic to this or any sort of obvious up front answer other than to test some options. With the wide variety of super efficient brushless motors available these days only testing combos will provide the answer to the question. But you can at least start with some of the published figures on some of the motors as to thrust/amps with various voltages and motor-prop combos. That should at least get you in the ball park in terms of single versus multi for which will be more thrust/amp efficient overall.

The other key is to know how much power you need to fly level and how much in reserve you need to climb at an acceptable rate. And along with this what the motor thrust efficiency will be at the cruise setting vs the climb setting.

[builder]

Bruce, I suppose there is not a large enough body of evidence to support the answer of push-pull vs wing mounted engines from an efficiency standpoint. Just going by the data the Germans collected on their airplanes in ww2 (they were quite meticulous about this) and comparing twins with similar power, wing area and loaded weight, the 335 came out ahead, but it WAS a pretty clean design.

The Cessna is shaped like a brick with an engine on either end, I would hate to use this as an example for anything except pure functionallity - something which earned it duty in the Viet war as an OV unit.

Voyager actually ran on the rear engine for the entire flight and used only the front one for takeoff and climb. But this cannot be used as evidence one way or another as the rear motor was liquid cooled vs air cooled in the nose and was probably used for fuel efficiency and other reasons,

Are these compromises? Absolutely! By their very nature all airplanes ever made are compromises. The best we can do is hope to design something which fills a specific task we had in mind.

You mentioned cruise efficiency, I had been thinking along the same lines earlier today and was planning on doing some part throttle tests to see if the small motor at full throttle can match the big one at part throttle from an identical thrust/draw viewpoint. I think I will limit testing to props of 60-80 percent ratio, that is, pitch to diameter in all cases. This is usually a good average for most flying. It excludes the low pitch props which are use for vertical performance, and which are a poor second choice for most other types of flying.

[Airboatflyingshp]

Go for it .............. Im trying to figure out if I can get a mud Duck to fly on a cheap geared Multi brushed motor set up. Im only able to work by comparison and empirical experimentation http://www.rcgroups.com/forums/showthread.php?t=1035197 :D Im also into Canards, lots of pushers and flyingboats ditto http://www.rcgroups.com/forums/showthread.php?t=448080 and a whole raft of push pull engine combos Do26 and Short Sarafand, many smaller props is very often the only the only way to go with flyingboats, my favourite twin is the Piaggio Royal Gull :cool: .. a gull winged pusher.
Ive also been watching this puppy grow http://www.rcgroups.com/forums/showthread.php?t=933242 :eek: . So Im not knocking just interested in what you find, it may have a direct baring on my own plans :) besides Im trying to get someone to build a Saro Princess and that is a challenge to do on a budget ;) theres also the little scale model challenges Ive set on this page http://www.rcgroups.com/forums/showthread.php?t=448080&page=5 take a look at the Westland wing.

[eflightray]

Just a thought, (my twisted mind often turns questions round the other way to try and find an answer).

But couldn't you ignore the motor(s) initially and determine what is the best, (most efficient prop), for the flight configuration. Based on diameter, pitch, pitch speed, and required rpm to achieve it.
Then determine what to drive it with for the lowest current draw.

Would that determine if one or multiple props was the best way forward ?.

[Airboatflyingshp]

I love twin boom pushers so I found this take a look http://www.rcgroups.com/forums/showthread.php?t=736589&page=2

[Airboatflyingshp]

Just a thought, (my twisted mind often turns questions round the other way to try and find an answer).

But couldn't you ignore the motor(s) initially and determine what is the best, (most efficient prop), for the flight configuration. Based on diameter, pitch, pitch speed, and required rpm to achieve it.
Then determine what to drive it with for the lowest current draw.

Would that determine if one or multiple props was the best way forward ?.

Ray :rolleyes: :D :D I got to dig out the pic of the old C.G. cambrian just for that :p

[vintage1]

You both miss the point. As I stated before, I was testing for static thrust and in this case the smaller motor had a slight edge. I DID NOT use low pitched props to bias the outcome.

No you used different pitch speeds which biased the outcome.

An 11x4 prop would probably have resulted in nearly identical figures..

[builder]

Well Airboatflyingshp, here is some more data for you. Remember the fairly recent article of the PBY in (I believe) RCM and E? The author of the article related his best static thrust from a number of props tested came with the regular Master Airscrew props. I just happened to have one on hand in the identical pitch and diameter (10x7) as the APC-e, so I threw it on the Axi. Well it gained a half pound of thrust with only half an amp more draw.

Another test just for the heck of it was to put one of the small props the small outrunner liked on the big Axi. The thrust to draw results were almost identical between the small and large motor.

A third test was to compare part throttle with the big motor/prop to full throttle with the small motor/prop combo. The large motor pulled ahead slightly in the thrust/draw category here, less prop slippage probably offset the lower motor efficiency at this lower throttle setting.

The first test was a bit of an eye opener, it makes me wonder what types of theory and testing the manufactures of the different props used to determine their idea of an optimum shape.
The second test was pretty much expected, motor manufactures list the small to medium size motors in their linup as having only a percentage point or two difference between them in efficiency.
The third test, again no surprise here.

I suppose the moral is the use the largest motor practical, but we allready do that right? ;)

It would be nice if someone could come up with a dataloger that could measure amperage and airspeed, it would go a long way in telling us e-flyers what the best prop for a particular airplane and flying style would be.

In the meantime it looks like I am going to have to dust off those old MA props I have lying around and try them on my twin flying boat.......

[Airboatflyingshp]

I don't have any out-runners just cans and gearboxes, if you've looked at what I'm contemplating you can see that I can go twin easy, matched pair, a S600/650 triple I can do but I'm wondering if I use a pair and a near to geared s700 on the center line pulling a larger dia same pitch prop. or do I have to stay with a matched set motors and prop. With a 4 its a matched set, a 5 possibly...but an unusual set up and excessive ditto 6........ and wt becomes an issue.
There's a chap who might be able to help ...but Ive got to find his site first..give me some time.

[vintage1]

If they are old MA electric props, put them in the wastebin without dusting.

On various models I've tried hand carved woods, custom made woods, APC thins E's ,GWS slowfly, GWS HD and APC sloflys. Even the odd glo prop. And MA electrics. One prop stands head and shoulders above the crowd as drawing the current but not delivering the results. The MA electric.


For what its worth, which is pretty subjective, here is roughly how they performed.

Hand carved thin blades, GWS slofly and APC slowfly..all pretty similar with maybe the edge on the hand made ones. Wood is stiffer than plastic, so less flexure?

APC thin E's and hand carved of similar blade style..very close. about 20% less draw and thrust than the slowflies.

generic glo props of including bought in wood ones with thickish blade sections, and GWS HD. Less draw and less thrust, but still very good. I suspect GWS write larger numbers on teh pitch than reality, Others write smaller.

MA electric. Same draw as above, but way lower thrust. Unusable.

[Airboatflyingshp]

Vint the S700 Neo with 2.7 box I can match the motor for £20 or less but if I was to double the gearing as I considered could a single work for the SportDuck?

Failing that what about the triple hybrid idea?
How much thrust could they deliver .......... I can imagine a computer going ..you want me to do what? with one of those :eek: .........sorry do not compute. :p :D :D
I'll go and harrass Ray again ;)

[builder]

Airboatflyingshp, I started out with the old can 05/550 style motors, tried a number of different rare earth and cheap can motors of various sizes through the years. They were good in their day but I have moved on to the brushless motors because the performance improvement is so huge. I have an old 550 motor sitting here and it weighs three times as much and uses twice the amperage to swing a prop the same rpm as the little brushless. I have somewhere between 80-100 brushed motors lying around and I can't even give them away.

As for gearboxes, they were a bit of a crutch to let smaller motors swing a big prop for more thrust at the expense of speed. They are a bit redundant now that outrunners offer so much torque in a small package.

Do yourself a favor and check out Hobby city.
http://www.hobbycity.com/hobbycity/store/uh_index.asp

I had a chance to test out the Turnigy TR 35-42C 1100kv Brushless Outrunner (Eq: 2820) and it did pretty well for itself. Not quite as efficient as the Axi's or Hackers I have tried, but decent workmanship and you can't argue with the price.

Anyway, some more data with smallish props from my early days, recently retested on the brushless. Figures are thrust VS amps.

Old M.A. 7x6- 1.4 pounds at 6 amps
New M.A. 7x6- 1.2 pounds at 6.5 amps
Old M.A. 8x6- 1.9 pounds at 9 amps
APCE 8x6- 1.8 pounds at 11 amps
APCE 7x5- 1.4 pounds at 6 amps
APC 7x5- 1.2 pounds at 6.5 amps
Grish 8x4- 1.8 pounds at 8 amps
T.F. wood 7x4 1.2 pounds at 5.5 amps
Cox 7x3.5 1.1 pounds at 5 amps
Cox 3 blade .7 pounds at 4.5 amps


Props from left to right-
apc 7x5
apce 7x5
apce 8x6
old ma 8x6
old ma 7x6
new ma 7x6
cox 7x3.5
top flight 7x4
grish 8x4
cox 3 blade pusher

I threw in the old 3 blade as it bit of a joke, and that was about the size of it.
I don't know why the 8x6 APCe was such a power gobbler, I think it may have been overproping the motor- a lot of blade area on that one.

[eflightray]

Now it just so happens that I have quite a few Keil Craft nylon props..................

and unless I take Vintages suggestion and bin them, I will probably still have them in the years to come :D Well I can't throw something so beautiful away, perhaps I'll frame them ?
I bought a 'bag of bits' from a stall at one of the shows years ago, and yes it had loads of Keil Craft props in it, amongst other junk. But it was a bargain.
As for MA electric props, they are great, for stirring paint.


Airboat, bite the bullet and go brushless, as I said before, Speed 700 motors make great boat anchors, (for full size boats in a gale).

My brushed powered models are only brushed because they are ancient, and I'm a cheapskate, (I only buy cheap brushless now). But I wouldn't use a brushed ever again, even with the slight advantage of only one ESC for multies.

[builder]

Airboat, bite the bullet and go brushless, as I said before, Speed 700 motors make great boat anchors, (for full size boats in a gale).


Thank you eflightray! I needed a laugh after fighting with the carb on the $#!!%#!&*#* lawnmower all afternoon.

Are the M.A. electric props really that bad? I only have tried out the glow versions.

[Airboatflyingshp]

I dont have the financial teeth to bite a peeled grape at the moment Ray and builder ..... and Im not likely to for quite some time :p .
So its geared brushed or Slimer/diesel for a solo duck and any multi I would only ever consider electric, I just need to replicate Ivans Bristol frieghter. That will loop and do silly things and has done for years.

Builder if you have that many motors and gear boxes going begging then theres a nice old guy out in BC who will put an awfull lot of them to very good use ;) . Were I closer I would be round like a shot :D Unfortunately the tax man vias the PO tends to get twitchy about people buying stuff elswhere :censored:

Pitty im going to need some gear boxes, Ray how much thrust did your S600/650 olympus set ups deliver, and how much with 3:1 and 3.5:1 masterflights? I'll go back and have a look 2 morrow. :)

[Airboatflyingshp]

DIY technical logging equipment he migh be able to help builder http://www.stefanv.com/index.html#rcstuff

useful s700 links dredged up http://www.rcgroups.com/forums/showthread.php?t=1678

http://www.stefanv.com/rcstuff/qf200112.html

Motor cals on line http://brantuas.com/ezcalc/dma1.asp http://www.csd.net/~cgadd/eflight/calcs_motortest.htm http://www.multiplexusa.com/

[vintage1]

700 geared is a fine motor. About 250-300W possible. But DO buy a decent prop!

[macboffin]

My two pennorth a bit late in the day!A pusher has lower drag than a pusher.No turbulent slipstream taking a spiral path the length of the fuselage, and interacting with root parts of wing, and the tailplane,fin. The push-pull Cessna twin is 30 mph faster on rear engine only than front only ; max altitude attainable on front only is approx 12,000 ft, i7,000 on rear only.This is not high falutin' theory, this is fact.Check it out.The Helios is no kind of application here ; it's designed for very high altitude operation in thin air, and with a straight "Plank" wing with reflex section the motors have to be well forward for CG reasons. Multiple motors for two reasons ; one is spar loading relief, the other is that it enables modularity of configuration, ie add on more sections span-wise to increase aspect ratio and efficiency.

[nmasters]

The push-pull Cessna twin is 30 mph faster on rear engine only than front only

That's because the aft fuselage shape is extremely poor. The rear prop creates a favorable pressure gradient which prevents separation. When the rear engine stops the aft fuselage pressure gradient goes unfavorable and you get massive separation and drag. Neat trick but if the fuselage weren't so blunt the engine out performance would favor the front.

--Norm