Is MotoCalc accurate with 6" hotliner type props?

[ShamelessMale]

Please tell me MotoCalc 7 does a poor job with thin folding props in psuedo-hotliner type applications! I give you a comparison between a free Electric Motor Calculator posted at http://brantuas.com/ezcalc/dma1.asp and MotoCalc. The Electric Motor Calculator likely doesn’t take into account the prop is stalled and seems way optimistic at 61.5 ounces of thrust from a 6 x 7.5” prop at 18,747 RPM. But come on, 26 ounces from MotoCalc at 414 Watts seems pretty harsh.

I must have better than a 1:1 thrust ratio to make my project worthwhile (estimated at 30-32 ounces) and a top speed of less than 80 MPH would be very disappointing. So I need about 10% of the extra thrust the Electric Motor Calculator predicts. Is there any hope?

[ShamelessMale]

Sorry for the blurro-vision.

For what it is worth battery power will be 4S Thunder Power, exact choice depends on how much I believe MotoCalc 7. The hoped for choice was 4S2P TP1320s, it is looking more like 4S2P 2100s...

[vintage1]

I woldn't have thought static thrust was that interesting really. What is more imporatnt is what happens at around 80mph.

You may be able to simulate this by using a much finer pitch. (i.e fine pitch ststoc is same thrust as coarse pitch at 80mph...)

ou are probably operatuiing outside the region that wither programs algorithms have any hope of accuracy.

[ShamelessMale]

Truthfully, I was hoping for a plane that would cruise for at least 10 minutes in the 60 to 80 MPH range, with a few bursts approaching 100 MPH.

The original plan was to utilize 3S Li-Poly with an aero-naut carbon 7X6 folder and a +5° yoke, but it doesn't seem to have a hope of hitting the high speeds. So now it is looking like 4S with whatever the motor can tolerate in 30 second bursts for prop.

The application is a lightweight hollow foam pattern parkflyer that is being stuffed full of carbon fiber to get the strength for the high speed stuff. So I really hate to go to a big battery and motor combo, because vertical on demand is nice in the park.

Not asking for much am I! :D

[vintage1]

Mmm. So you want to say around 25oz AUW, get over 100mph, and have vertical capability yes?

Going to need about 200W per lb I'd say. So about 300W input. And all the efficiency you can get.

Mmm. I am looking at motocalc now, and I suspect something like an 8x8 prop may provide the speed, but seems to need about 15k RPM to do it.

It will have the thrust allright. I am looking at 3s2p LIPO and a mega 16/25/3 in direct drive...pulling about 50A :)..mmm maybe too much for the motor. 4s2p and the mega 16/25/4 is more like it.

Or 16/25/3 on 6x5.5E prop. And 4s LIPO. THAT starts to look promising. 30oz thrust dropping to 25oz at 50mph, pitch speed over 115mph. Pulling just under 30A...nearly 400W input. Looks like you need nearer 300W/lb actually. Motocalc reckons teminal speed of 30mph straight up, and 80mph on the level for the airframe I keyed in - a 43 inch foamie aerobat. Should be able to beat that dragwise..

I think you have to shoot for 300W/lb, and whatever it takes pack wise to get it. 4s2p looks good..


...and maybe a different motor. That one seems small for what you want to achieve.

To get to this sort of power to weight and duration, I would guess at least half the weight - probably more is going to need to be pack and motor.

Putting the equivalent of a .30 2 stroke in a 30 oz plane eh?
Ho hum.

[ShamelessMale]

....
Putting the equivalent of a .30 2 stroke in a 30 oz plane eh?
Ho hum.

At least progress marches forward. I had a similar setup with a Strontium 600 and CP2300 cells about three years ago , and it got a lot of "That thing would fly great with an .18 in the nose!" comments. ;)

Thats kind of what is driving the speed requirement, along with the need to to be able to fly at normal field "pattern" speeds for a respectable time.

Its really a double whammy, the equipment with the power to go fast gets heavy. And the power required to accelerate vertically with a prop that is quite inneficiant at slow speeds while toting this heavy stuff around is equally insane. It seems that before long, it all might get so heavy that the parkflyer ability will be gone!

So the choice may be optimize for TP1320 3S2P and forget the speed, or optimize for TP2100 4S2P and forget the park :(

[vintage1]

I don't know.

Even full size planes have a problem with flight speeds much more than 4:1 from flat out to stall speed. Look at a 747, flies at 500 or so, takes off at 180? And that bird comes in with extensive wing sections, flaps and lord what knows else.

The real secret here is a fair amount of wing area, a very slippery shape, and low weight, of which at least 50% will be the power train. I am flying something - not a peed model - just a speed 400 fun thing - with LIPOS and a 480 can up front, and whilts it is possible o fly it in a park, frankly its too fast for me anyway. At 30" span and at least 60mph, its a dot in under 2 seconds and I find it very hard to tell which way its pointing. Mostly I fly it around at 30mph on half throttle or less, and only use the power to pull huge loops etc. Its very wearing to fly too. Blink and its gone - somwehere. I daren't fly it low.

As a technical exercise I believe that what you are attempting is do-able. Whether it will result in a pleasurable model is a moot point.

I think you need to shoot for 300W/lb and accept that its going to be mostly motor and pack, with a little bit of carbion fibre thrown in to make a structure. If its light enough to hover and do 3d, its almost certainly going to be too fragile to survive a high G corner. Maybe not tho. On the motor you have on 3s2P I reckon you should try and get the weight under 20oz, then you have a chance.

[Extreme Bob]

I've been feeding details of some of my existing planes into Motocalc, and it claims that none of them will fly. Ah well, I must have dreamed all those aerobatics then.....

So, I don't think Motocalc is particularly accurate at all. You're better off sticking a motor in and seeing what happens, or looking at what others are using successfully in similar planes.

[Bill Glover]

With a small-ish plane capable of 100+ mph you don't really need a high thrust to weight ratio. From a full throttle pass you are easily able to "zoom climb" vertically until you reach the point where orientation & control become difficult. To all intents and purposes that's the "unlimited verticle" ;) that so many people post about.

For example, my Projeti is not far off your specs (although it's hardly a "pattern plane"). It runs a CAM 4.7x4.7 at 22,500 rpm via a Kontronik brushless on 8 cells (1700 AUP nimh). Static pitch speed is 101 mph, I get 9-10 mins (timed) duration. From a high speed pass it will easily go straight up until I can't see it properly any more, yet the static thrust (according to MotoCalc) is only 12.5 oz (for 24 oz AUW).

Obviously it won't climb vertically straight from launch ... but I can live with that.

I'm in the process of converting the Projeti to 3S Kokam 2000 (15C) LiPos. That will up the static pitch speed to about 120 mph, increase duration a little, and drop the AUW by 3-4 oz.

Incidentally, 100 mph is a pretty tall order for any foamie unless very sleek and with a smooth surface finish (like the Projeti ;)). Most claimed "100 mph" models are more like 70-80 mph tops. Of course small planes always look like they're going faster than big planes :)

PS I guess your definition of "park" must be different to mine - I wouldn't dream of flying a 100 mph 300W model in what we'd call a park over here.

[Sail 'n Soar]

An interesting discussion, but here is a different perspective. At 100 mph with your ~ 300 sq in design and a hard control deflection (wing CL ~ 1), Your wing is going to produce ~ 53 pounds of lift, or over 26 g's. Design it well!

[Sail 'n Soar]

Note that I've used an average CLmax of 1, while your MotoCalc run shows .64. I took your 301 sq in and assumed a 40 inch span and came up with numbers similar to those you show. For that case, at 100 mph your average Reynolds number is approaching 600,000. At that with any decent airfoil should expect something closer to 1 than .64, but even with .64 you are speaking of over 34# and 17 - 18+ g's.

[Bill Glover]

Yup that's a lot for "lightweight hollow foam" to handle, even with carbon reinforcement.

I used to race pylon, and did a spreadsheet to estimate G in a different way ... based on the acceleration towards the centre of a circle. Flying a 50' radius (not a particularly tight turn) at 100 mph works out at 13.4 g.

A racer doing say 130 mph at the end of the straight and pulling a 40' radius turn (in a vertical bank) around the no 1 pylon is pulling over 28 g. 30' radius puts it up to 37 g!

[ShamelessMale]

The fuselage will be strong, with the foam acting mainly as an unstressed skin.

The wing is going to require some more work...

[ShamelessMale]

An interesting discussion, but here is a different perspective. At 100 mph with your ~ 300 sq in design and a hard control deflection (wing CL ~ 1), Your wing is going to produce ~ 53 pounds of lift, or over 26 g's. Design it well!

I can’t believe how prescient some of you guys were. This is a “Big Stick” slimer plane I have been benchmarking in anticipation of the hot-rodded electric plane.

The first four flights don’t really count as the OS .91 was running like cr@p due to quality control issues they have had with the needle valve assemblies, and the next four flights were for exploring the flight envelope, setting the maximum throw rates, and identifying the need for mixing. The wing held on during WOT loops utilizing 45° of elevator throw with a 12 X 12” prop that was spinning 11,500 RPM static on the ground. Call it a 130 MPH pitch speed in the air, although a very thick wing holds the real speeds down.

On the ninth flight, I enabled mixing to fix some nasty roll coupling during knife-edge flight and more importantly turned on the flaperon mixing. It didn’t survive the next loop…

[ShamelessMale]

While I am thinking about it, here is/WAS the cure for fixing an OS .91 that just won't run correctly. The plane and engine had sat in my garage for two or three years without being flown once because it had a severe problem with "air bubbles" between the needle valve and the carb. The O-ring never did seal properly even on a replacement needle valve holder, and the high crankcase temperatures and vibration probably foam the fuel at the needle valve even if it does seal. An OS .46 remote mounted needle valve assembly cured all, but won't help much with the dirt ingestion.

I am not complaining, because this POS is what started my adventures in electric. Although I must concede it eventually ran pretty well, unlimited vertical as soon as you could pull the nose up on takeoff. Would slowly climb vertically at about 3/4 throttle stick.

[ShamelessMale]

A close up of the snapped off wing bolts.

Its too bad you can't see it in the picture, but each of the nylon bolts has a void in the center right where they snapped. I suppose the 10% or 20% reduction in cross section probably wasn't the largest contributing factor though. :rolleyes:

Next plane is getting steel or if I can find them aluminum bolts.

[ShamelessMale]

Here is a picture from near my runway of the general vicinity where she went down. That is my brothers house that is really almost 1/4 mile away.

I took off and experimented with the rudder a bit to check on the mixing and then flew down a bit closer to the house and did a few circles so my "niecies" would hear the plane and come out to watch. Since I was flying in sort of the same vicinity as a house (within a hundred yards or so), I was at about 10 mistakes high.

Turned the plane back towards myself, dropped to about 7/8 throttle, and pulled what I thought was a moderately sedate loop. Perhaps it was actually starting in on the second loop, and then the wing popped off. It flipped its way down to a soft landing by the third electric pole. Without the drag and weight of the wing, the fuselage was doing torq rolls while accelerating nearly veritically like a sky-rocket :D

My first instinct was to chop the throttle in order to limit the impact speeds, and to see if I had any control to steer away from the deep woods and the general area of the house (where by now the kids could have been outside!) And of course to follow the second "golden rule" of crashing, that is in keeping an eye on where the wing was fluttering off to. My under-stress thinking at the time was perhaps I would get some "prop braking" in this lawn dart configuration. (Yeah, I here the joke coming:o )

Like I said, it was nearly a 1/4 mile away so it was very hard to keep good orientation on the tail while it was spinning. If it would have been closer, I would have attempted "the worlds greatest save" by trying to hover the beast down from a good 300 feet; per the first "golden rule" of crashing "Never give up flying 'till it hits!" As it was I figured I would just end up augering it in faster, as you can imagine how fast the torq rolling would get without a wing to dampen the spin rate. :rolleyes:

It hit about 2/3 of the way across to the house only on the extreme right edge of the picture, the impact site isn't visible due to the rise about a 150 yards out that makes it look like the bottom portion of the house is missing.

[ShamelessMale]

Another picture that is just unabashed carnage ;)

[ShamelessMale]

Note the top of the wing looks like new.

On the bottom, there is just a little scuffed balsa where one of the bolts dug in a little on the way out, and a scratch near one of the wingtips.

I never liked the Great Planes strategy of not supporting the bottom of the wing. Since it really loads this area in tension, I originally epoxied on some carbon fiber tape to stregthen it up without wrapping fiberglass cloth around the center section of the pre-covered wing (low-cost ARF).

Should have been thinking about the bolts...

[ShamelessMale]

So after all of that, here is the killer part. Since I thought the wing landed in the road, I was in kind of a hurry to go retrieve it. Still in a bit of shock, I ran over my flight box!

One picture is of me with pieces of my box strung back together to pump the fuel out of the pieces of the plane!

I figured I got what I deserved for buying an ARF and not taking the time to really determine if the design was safe with my power level when the plane crashed, so there wasn't much anger there. Running over the flight box wasn't even too bad, because I almost never fly nitro planes. On the other hand, remembering the radio was on the edge of the flight box was MOST IRRITATING :mad: :mad: :mad:

So what do you guys think about the Royal EVO 9, now that I need an upgrade and we are off topic?

[ShamelessMale]

:) :) :)

[vintage1]

Frankly I think you are mad, but heck? So am I!

When I was 16 I made a rocket out of aluminum tube, some washers epoxied on the end, packed it with sodium chlorate and sugar, and mounted it on a couple of wire loops on me mums washing line. A jetex fuse poked in the end was duly lit...and there was an almighty explosion.

I found the ripped up piece of aluminium tube the following summer in the hedge. The thought of what that piece of shrapnel could have done to me if it had decided to fly in my direction still gives me goose bumps.

[ShamelessMale]

Frankly I think you are mad, but heck? So am I!

.


Yeah, I seem to have turned into a raving mad lunatic. :(
Perhaps I will get better when I am finished being angry at myself.

Except for the very strong yaw-roll coupling (typical sport plane I guess), I was really impressed by how this plane flew. For $32, I think I may give it another go :)