OK, so why doesn't this fly?! - RC Groups
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Oct 13, 2001, 11:41 AM
Registered User

OK, so why doesn't this fly?!

Here's a picture of a Hotcat mutant clone. The wing is cloned of the Hotcat airfoil, with carbon spars and all corroplast alierons.The fuse is copied from the basic shape of the Hotcat. It uses a 6v graupner setup. I've tried grish 6x3, 6x4, 4bladed 5.5x4 and a few motors, the latest one is a 6V that draws about 11amps on 8x600. Upon launch it flitters around and simply drops out of the sky. Not noses into the ground on a glide, but as if suddenly all lift suddenly stopped and it just falls down. Any ideas? But then again, I never got my Hotcat to fly either. On that account it always seems underpowered. And there, I've also tried several motors and prop combos and just shelved it to see if this pusher mutant might fly. It looks good, but that's about it!
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Oct 13, 2001, 11:48 AM
Senior Member
Sort of looks like you need more tail moment
Oct 13, 2001, 11:49 AM
Mike Swan
Hi,whats your wieght ready to fly?
Oct 13, 2001, 03:33 PM
Registered User
Hmm.. if I recall, the weight is like 24oz or so ready to fly. As a Hotcat mutant the length of the tail from TE to tail is about the same as the Hotcat.
Oct 13, 2001, 04:14 PM
RPV builder & operator
Pierre Audette's Avatar
It's almost a flying wing, so the CG needs to be way upfront. Try 20% of chord, maybe even less.

Have you got the prop facing the right way as a pusher? Orientation should be the same as when it's upfront. Only the motor direction changes.
Oct 13, 2001, 04:16 PM
Lifetime Beginner
jlk's Avatar
What happens when you do an unpowered test glide? Is it controlable? What is the wing area? Where is the CG? What is the thrust line of the motor?

No matter if it is underpowered or not it still should fly controled.
Oct 13, 2001, 04:26 PM
Registered User
here is a quick formula to claculate tail valume, the thing that says if you have enough stabilizer.

St/Sw *L/Mac~1

St- surface of tail
Sw-surface of wing
L-distance between CP of wing and CP of tail (CP~25% of chord from LE)
Mac- maen aerodynamic chord.don't remember the formula. to find, draw your wing on a piece of paper(one half). measure the root and tip chord. now draw the length of the root chord as an addition to the tip chord (parallel to it) both front and aft. now do the same with the tip chord adding to the root.
I know it ain't very clear , but what you should get would look like the letter H with the wing as the center bar. now draw two lines from opposing corners of the H, as if placing this X over it. where the two lines meet, measure your chord. that's the Mac...
for the number you should get, 1 is quite stable. you can test other models you have for an idea of how much.

hope it is clear


p.s: if it simply hovered down, is it possible the propeler was rotating in the opposite direction ?
Oct 13, 2001, 09:24 PM
Fish & Fly
DaJudge's Avatar
Try a 2-blade 5.5x4.5

Sounds like your CG is too far back.

My Manx (very similar design) flew great with the 6V speed 400 on 8 cells.
Oct 13, 2001, 11:38 PM
iankraus's Avatar
mrneutron Hmm.. if I recall, the weight is like 24oz or so ready to fly.
i'm not sure at what weight a manx or hotcat will fly at, 24 oz may be on the upper side of what a 400dd can push.
Also try moving the cg forward, this will help ofset the tailplane size issue.
try some test glides over long grass

The plane has a cool look to it. I'm sure you'll get her trimmed out!!
Oct 14, 2001, 02:14 AM
Registered User
I think I've accounted for all the obvious reason related to it being a pusher, ie rotation, prop facing right way etc. I've played with the thrust line so it has a bit of the prop pointing down. For a while it tended to balloon and stall. I have not tried to point the prop up and add weight to move the cg forward yet. I've searched the threads for some mention of the weight of a hotcat and looking in the manual has not told me what the weight should be. The CG is pretty much up front, and I've experimented with it. I've also played with the reflex of the ailerons. It also does not behave well even in an unpowered glide. It glides erraticly, sometimes it rolls and just dives into the ground on an unpowered toss. I've also played with the AOA of the wing. From my search of the threads, the Hotcat flies with a grish 6x3 or a MA 5.5x4.5. This may be the case of junking this iteration and back to the drawing board.
Oct 14, 2001, 02:34 AM
iankraus's Avatar
you may need to build a 1/8 " balsa 1/4 scale model of your model to get the cg incidence etc worked out, it only takes 1/2 an hour to make one of these tester gliders
Also do you have any incidence on the wing.
The flight pattern that you described is consistent with a tail heavy model.
you may need some more lateral fuse area at the back of the plane. There must be more side area aft of the cg than forward of it - otherwise the model will have a tendency to want to fly backwards.
Dont give up - just chop and change till it flys right. Its worth the effort when you get it there.
Oct 14, 2001, 05:45 AM
Registered User
swami's Avatar
It might help if you didn't run your antenna over your wing. It kind of destroys the airflow.
You say:"It glides erraticly, sometimes it rolls and just dives into the ground on an unpowered toss." To the left i'm guessing.
Are your ailerons taped on well enough?
It looks as if they are letting go (right wing tip).

Oct 14, 2001, 07:39 AM
Registered User
Neil Stainton's Avatar
I think the C of G is too far back, and there is too little fin area.

If you can't move the C of G forward without adding weight then make up a new 3X orig size tailplane (and a bigger fin).

Oct 15, 2001, 02:42 AM
Registered User
The aileron looks detached becasue when the pic was taken the servos were disconnected. It doesn't seem to be an airflow problem it does the dive/roll/weird flop regardless of where the wire is. So far in listening to all your suggestions, I think I will lengthen the fuse behind the cg and do the calculation for how much tail plane it needs. As Iankraus suggested it seems to want to fly backwards.as for the cg it's pretty much about 20%, note the weight taped to the nose.
Oct 15, 2001, 08:59 AM
Registered User
Darwin's Avatar
Hi! Darwin from Garrison Aerodrome here:

I first want to say that I really like the looks of your concept. It is very cool looking in a FanTrainer sort of way.

Your description of the way it kind of flops out of the air is consistent with the early prototype of the Manx. It also had a problem with a similar failure mode, but only when I'd try to turn it.

In my case, I traced the failure to a lack of torsional stiffness in the wing. My solution was multi-pronged: first, the wing is cut from 1.9 lb density EPP. Second, I used an agressive taping schedule (completely cover the wing with fiber strapping tape and then go back and spar cap with another layer). Third, I used Ultracote instead of tape. This was especially critical to making sure that the wing could be "de-warped" after covering.

Also, I would suggest strongly against using coroplast for the ailerons. The stuff lacks torsional rigidity. I would suggest Ultrcote covered balsa.

In a conventional layout aircraft, the wing planform of the Manx/HotCat wants to balance on the spar which is 30% MAC. For flying wing duty, it should be 15% MAC, or halfway between the spar and the LE at the root.

I would suggest using a bungee launcher with this aircraft since its pusher arrangement doesn't allow the prop blast to increase the tail effectiveness at low airspeeds.

Regarding power: on 8 cells with a Master Airscrew 5.5x4.5 prop and a 6V S400 properly timed, the current draw from the pack is around 13 Amps at full charge. I've always had good success with this setup, but I haven't experimented with many props, so I have a lot to learn as well. I guess that isn't really a suggestion or anything, it's just a data point.

Good luck!

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