3D and speed.

[vintage1]

In an idle moment someone said 'what I want is a plane that is really fast and will do large accurate pattern aerobatics, and still hover and do 3D, and yet still be able to fly like a pylon racer'

Which was a challenge I could not ignore.

Now 3D seems to mean little more than a large wing area and plenty of control surfaces, and lots of thrust. Ive seen models with fairly high aspect ratio wings hover as well, so that means that stumpy wings are not de rigeur.

Speed models generally have highish aspect ratio tapered or elliptical wings of thinnish section. That doesn't seem incompatible with 3D either.

Pattern models generally have a very symmetrical appearance, and higher chord depth, presumably to give a high drag-to speed relationship to make them able to fly at constant speed better: likewise they often use low pitch props to give great thrust but not a high top speed.

The only inherent inconsistencies I can see between combining all these are the wing thickness, which must be thin for speed, and the prop, which needs to be coarser pitch for speed. which means my putative design would have to be much more actively throttled to do smooth low speed aerobatics.

I've already sketched something out which looks a bit like a spitfire with a big tail, but with far more fuselage side area to make it knife edge better. One of the absolute necessities for a successful design is to make it look pretty and different from an ARTF.

At which point I thought 'ah, why not ask everybody and get feedback: you've probably forgotten something'.

So here I am. Initial Motocalc simulations suggest that a 250W lightly built 36" plane should do about 70mph flat, 40mph vertically, and be able to hover (power wise).

[Brandano]

I think the elliptical wing will cause trouble when transitioning to hover, not to mention the fact that you can't add washout if you want to do any serious amount of inverted flight, especially at low speeds. A tapered wing, perhaps with rounded wingtips, seems a better compromise. Also, sharpening the leading edge on the inboard portion of the wing will reduce the chance of tip stalls by ensuring the central portion stalls first, and that will work for both upright and inverted flight. A couple of small, sharp leading edge root extensions should do the trick.

[biber]

You will have a majour issue with controll throws.
If you have enough servo travel to namage 3D and hovering,
even dual will hardly bring the needed precision for high speeds,
as servos and linkages will limit that.

Big controll surfaces are likely to raise some flutter issues, aswell, depending on what your idea of speed is.

biber

[vintage1]

I think the elliptical wing will cause trouble when transitioning to hover, not to mention the fact that you can't add washout if you want to do any serious amount of inverted flight, especially at low speeds. A tapered wing, perhaps with rounded wingtips, seems a better compromise. Also, sharpening the leading edge on the inboard portion of the wing will reduce the chance of tip stalls by ensuring the central portion stalls first, and that will work for both upright and inverted flight. A couple of small, sharp leading edge root extensions should do the trick.


Good points. Not sure that tip stalls are a big issue with 3D though..I was thinking about a leading edge root fairing anyway..

[vintage1]

You will have a majour issue with controll throws.
If you have enough servo travel to namage 3D and hovering,
even dual will hardly bring the needed precision for high speeds,
as servos and linkages will limit that.

Big controll surfaces are likely to raise some flutter issues, aswell, depending on what your idea of speed is.

biber

Yes, that had worried me as well. Flutter should NOT be a problem..realistically it won't do more than 65mph flat, and maybe a 100mph or so in a dive. I've had models that fast running without trouble, albeit not with quite such large surfaces. As long as the linkages are slop free and the natural graininess of the servos is not too much, I think I should be able to find a decent compromise with rates and expo.

I am not expecting anything too unrealistic - merely taking a sport design and making it a bit more aerobatic and a bit faster and hoping to get some 3D capability. I suppose the idea is an advanced intermediate style plane that a Sunday flier could take, and would still be presenting challenges after a few months of flying.

Especially in the speed RAN+GE I want something that lands sensibly, and flies fairly sanely at 1/2 throttle, but is pretty manic when opened right up. 1:3= stall:flat out, seems a realistic target

[JetPlaneFlyer]

Vintage,
Prop selection may be a real headache... 3D demands low pitch but high speed needs high pitch. Realistically variable pitch is out so I'm not sure how you could resolve this one? I think if you tried to hover with a prop pitched for speed you would get blade stall.

Changing props between flights is not a big deal I guess if necassary

[biber]

!:3 is indeed moderate and with that anticipation you'll be in the ballpark quite safely.
I'd try a clean setup with an SD8020.

Regarding the prop, you could always install more power to get away with a higher pitched prop and still have the thrust to hover.
Don't seek for maximum prop efficiency though, no free lunch there, anyway.
If you can do with a bad prop efficiency it's much easier to have it working over a wide speed range.
It will be crucial to have not to much prop diameter.

Good luck.

biber

[ZAGNUT]

looks like you have something like the hotselliptic in mind:http://static.rcgroups.com/forums/attachments/1/4/1/9/9/a667753-153-Hots.jpg

this one belongs to pmackenzie but i used to fly them on glow. at three pounds with a piped YS .45 and high pitched prop they were extremely fast yet could also fly backwards in just a light headwind. with a little more control surface area and ailerons extended inboard it would be hoverable.



dave

[vintage1]

Yep. That sporstelliptic looks very close, doesn't it!

I've done the prop stuff.. a 7x5 with about 250W into it seems to have enough to go straight up, and a top speed of about 65mph. Not sure how slow I can get it, as the wing wont be hugely efficient at slower speeds and the wing loading will be fairly high unless I make the wing grossly oversize. All I can do in that area is to simply build as light as possible with great attention to using structure efficiently.

I won't have the luxury of a 36 tuned pipe style power to weight. But I wont have the vibration either.

So I think I can certainly get full vertical, less than 20mph stall and more than 60mph top speeds. What is less sure is the controllability in hover, and at very high speed.

[Brandano]

I'd fit canard surfaces right behind the prop just for hover control. They'd have to be well in front of both the CG and center of drag to be effective, though, and control might be a bit extreme. That would depart a bit from the sport model concept...
Btw, controlling differential tailerons mixed with the ailerons might be sufficient to get roll control in hover while keeping the conventional aileron setup

[JetPlaneFlyer]

I was not thinking so much about having enough thrust to climb vertically, it's having a prop that will work in the hover and at speed that would be tough, I think? Adding more power is not necassarily the solution to this issue.

[vintage1]

I was not thinking so much about having enough thrust to climb vertically, it's having a prop that will work in the hover and at speed that would be tough, I think? Adding more power is not necassarily the solution to this issue.

Well adding more power always results in better vertical and top speed, so it does work, up to the point at which the model is essentially too heavy for its wing area to be pleasant.

The power and prop I ended up with seem to be the best compromise for top speed and vertical..whether the model will be CONTROLLABLE in a hover, is a very moot point.However sufficient in-the-propwash surfaces should make it possible.


I tested out (Motocalc) 6x8 7x5 and 8x4 sport of props..6x8 had more speed but no vertical. 8x4 had plenty of low thrust but no top speed. 7x5 7x6 or 7x7 seemed the right sort of ballpark. Going much beyond a 1:1 Pitch:diameter ratio is not very good for static thrust per unit power.

This is not a hugely serious exercise by the way, merely pushing the boundaries to try and come up with something a bit different, that can give interesting performance on a cheap motor and pack.

[sairaghi]

It is perhaps a stupid idea, but why don't you use two different motors. One in the front of the plane with retract propeller for 3D and one pusher for high speed with small propeller and high pitch.



Well adding more power always results in better vertical and top speed, so it does work, up to the point at which the model is essentially too heavy for its wing area to be pleasant.

The power and prop I ended up with seem to be the best compromise for top speed and vertical..whether the model will be CONTROLLABLE in a hover, is a very moot point.However sufficient in-the-propwash surfaces should make it possible.


I tested out (Motocalc) 6x8 7x5 and 8x4 sport of props..6x8 had more speed but no vertical. 8x4 had plenty of low thrust but no top speed. 7x5 7x6 or 7x7 seemed the right sort of ballpark. Going much beyond a 1:1 Pitch:diameter ratio is not very good for static thrust per unit power.

This is not a hugely serious exercise by the way, merely pushing the boundaries to try and come up with something a bit different, that can give interesting performance on a cheap motor and pack.

[vintage1]

What a novel idea! and some sort of switch or mixer..I think if I wanted to go the whole hog I might think about it.But right now the challenge is to get high speed and slow speed together out of one aircraft, whilst not going the 'high alpha delta' route. Which wont really hover anyway.

[BMatthews]

Easier with the new hollow shaft motors to go with variable pitch. Probably use the landing gear switch to just jump from one pitch to the other. Because it would suck a lot of current in the high speed mode you'll want to click into it only when you get up to max low pitch speed and maybe even then only from a dive. Sort of like run it to the redline in first before shifting to second.... :D

The real issue will be to keep it light enough and stiff enough to avoid high speed flutter from the big surfaces needed for good low speed and 3D modes.

You've managed to bite a big chew out of the cookie this time Vintage. But sometimes it's fun to stretch the brain cells. Certainly this is better than trying to do a Mensa crossword.... :D

[Work in Progress]

To eliminate flutter, adopt full-size practices. Hinge the control surfaces well back from their leading edges (not more than about 20% of control surface chord, to avoid aerodynamic overbalance), and mass-balance them too. Hinge very freely and ansure you have no free play or slop anywhere in the linkages.

Dial in lots of expo, and off you go.

[Texas Buzzard]

Vintage, when I see you have posted - I read it. Your posts are always welcome for you say the "right" thing 99.9% of the time. I love to read your "stuff" - - - - ----------> maybe because they agree with me! ha ha.

You seem to be one of those very experienced builders who learned empirically and from books too. Empirical data cannot be passed off as garbage as can some of the high-powered mathmatically inclined poo pee doo stuff that the non-fliers pass off.

Now to your question or proposal.
Your choice of a big tailed Spifire with a "tall" fusalage at first approximation seems to fill the ticket. Yes, a larger ( 50" to 65" w.s. with maybe 725 sq. in. + wing area.....maybe more) plane built to minimum weight with excess power just might be a good starting point.......might be the end point too ha ha.

BAck 20 years ago some of us who fly at our field started to have "SLOW RACES"...i.e., see who could fly the slowest in a straight line for about 100 yards. Two of us added big flaps. It helped. but that is for upright flight only! These were high-winged trainer type planes.

Vintage, your English and you thinking is so like someone who was educated in a U.S. University that I wonder if you are an American working in Germany.

Tell me about the winds in Germany. I was ther for a short time in the USAF and in the mornings (Summer) the winds were light. Good Luck to you Vintage.

[vintage1]

Texas, no I am English through and through and live in England. If you check my profile you can even see my house on Google earth using the lat/long. I'll wave as you fly over ;)

I got my degree in engineering in Cambridge..no not Massachusetts, England ;)

Theory is just a way to make things work. The proof is always in the pudding. Theories can help answer questions, but they never tell you what is the right question to ask. Theory can help tell you when a wing will break, but if you don't consider whether the wing WILL, you wont use the theory, and the wing WILL break!

And sometimes theories are a very slow way to solve problems. Sometimes just making the thing will be faster.

[Texas Buzzard]

Texas, no I am English through and through and live in England. If you check my profile you can even see my house on Google earth using the lat/long. I'll wave as you fly over ;)

I got my degree in engineering in Cambridge..no not Massachusetts, England ;)

Theory is just a way to make things work. The proof is always in the pudding. Theories can help answer questions, but they never tell you what is the right question to ask. Theory can help tell you when a wing will break, but if you don't consider whether the wing WILL, you wont use the theory, and the wing WILL break!

And sometimes theories are a very slow way to solve problems. Sometimes just making the thing will be faster.
>>>>>>>>>>>>>>>>>>>>>>>>000<<<<<<<<<<<<<<<<<<<<<
RIGHTO OL' chap. I AGREE WITH YOU 100%!

For a time IN THE USAF I was at BRIZE-NORTON RAF BASE. I was with a B-47 Bomber Group. Brize-Norton was about 45 miles West of London, so they say. I was there 4 months TDY and they never let us get off the Base but one time on a 12 hour pass. I went to Cheltenham and ate a meal and had some warm ale. As I remember that summer it rained almost every day.
Where are you located? If Cambridge is close to Cheltenham I was there too on my pass. I saw college men on the side walk shaking hands with some odd sign language....I am a graduate of the Univ. of Texas- Chem Major with a Masters degree. I talked to the college men and was surprised to find that the colleges are spread out all over town , unlike universities here in the US. Yes I know a Univ. is made up of several or more colleges. I was surprised.
Keep up the good work, Vintage and KEEP THINKING.

[Texas Buzzard]

VINTAGE,
I PUT UP SOME PHOTOS OF MY NEWEST ONE ON FUEL PLANE TALK.
gO TO ---------> FUEL PLANE TALK
THEN LOOK for MY ENTRY....GO TO--------> "CHASING IT'S TAIL."

a SHORT MESSAGE FOR YOU AND 6 OR 7 PHOTOS.

[eflightray]

Does it have to be a conversational wing and tail model?

What about the deltas? Lowish drag at high speed, plenty of drag at slow.

I don't know if it's possible to do a hover with a pusher delta, but there has been some very 3D type flights with vectored thrust pushers.

The 'accurate pattern aerobatics' may be a problem with a delta, though I've never tried it.

Swing-wing?