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Bringing things up to date - 30/06/05

Hornets

I realise, with some surprise, that it is almost exactly three years since I last added to my Hornet pages. Both models are still flown regularly, The fixed pitch version is still basically standard, although it has been periodically tried in different forms. The flybarless collective pitch version is basically the same but has had different types of blade fitted. The only change that I have planned for the future is to revert to the lighter wooden paddles on the FP version that I first used back in 2001. Therefore, this should bring things fully up to date.

My last update closed with the comment that dual bearing tail pitch sliders were not the answer to any tail problems. This is really fairly obvious since the tail is always pushing and any slop is taken up. Even on a collective pitch model, this still applies. I haven't changed my views on this. Both of my Hornets have received comments from onlookers about how stable the tails are. The FP model, in particular, has a lot of slop, yet is fine. The answer is lighter tail blades. More later.

FP Hornet - Flybarless CP Hornet - Standard CP Hornet - Motors - Gears - Batteries - Canopies - Flying

FP Hornet

For the record, this model now has 715 flights and is on its fifth motor (This motor has just died at 730 flights). I've completely lost track of how many tail pinions have been fitted. After some 360 flights, I fitted a brass motor pinion and this is still in use. The model is on its second main gear and is currently running with one tooth missing from the tail drive contrate.

Wooden paddles Wooden paddles

During its life, the model has been flown with lighter wooden paddles and without a flybar. These were both quite common mods during the early days of fixed pitch IC powered helis when it was found that the weight of the blades was quite critical, when removing the flybar. This proved to be the case with the Hornet.. I eventually managed to get around 7 grammes of extra weight into each blade, but this was not enough for comfort and the model was very twitchy. It could be flown with less than this but was very nervous. It was actually quite easy to fly in a restricted sapce in this form, but you simply couldn't relax. I had replaced the flybar with a short piece of carbon and this, plus the weight of the blades and high head speed led to rapid wear of the head mouldings, so I gave up for the time being.

The modification is very easy. I was reluctant to give details because I never did get it right. However, I've had so many requests for details that it seems like a good idea to save people the trouble of asking.

You replace the flybar with a short piece of wire (or carbon rod) and fit a ball to each end. You will need to experiment with the spacing of these away from the head, you will need some packing so that the links don't foul the head. I used some suitable sized balls from a Whisper. You may be able to drill out some plastic Hornet balls or you can get the right size metal balls from several sources.

Then you use your existing links to connect those balls to the swashplate. Its worth pointing out that you need a decent dual antirotation link to keep everything in line. The result now is that the swashplate rocks the head from side to side directly, instead of going through the flybar. The result will be a very responsive helicopter and you will need to add weight to the blades to restore some stability. I started with 3 grams and this was not enough. I increased this to 5 grams and still had a very responsive - but flyable - helicopter.

Flybarless FPFlybarless FP head

I doubt whether it is possible to add much more weight to the standard blades and you will need good servos. I originally used GWS Picos and these are not good enough. I replaced them with HS-50s, which made a difference, but still left me with a heli that was not really comfortable..

I played around with the teeter, using the standard set-up, my reduced teeter and no teeter at all and the results were similar. I gave up on it because the standard plastic head mouldings began to show signs of rapid wear due to the weighted blades and higher head speeds.

The mod is so simple and it is easy to change back, but you must add weight to the blades. If you have a tatty old set of blades you have nothing to lose. Please let me know if you try it and make it work.

The wooden paddles were simply cut from 3/16" balsa and sanded to a symmetrical section.

The model is currently flying with the following mods:
Titanium main shaft.
Dual capturing anti-rotation link.
Chris Rigoleth swashplate ball mod.
Limited head teeter.
Wooden tail blades.
Aluminium tube skids (5/32" dia).
Plastic card tailplane (mainly cosmetic).

The dual anti-rotation link was made from a wheel collar as shown on several sites. The other mods are detailed in the previous updates. I recently fitted a commercial shaft collar (stopper) but this is not essential. The most important mod in my opinion is the dual anti-rotation link.

I had tried the collective model with lighter (balsa) tail blades due to the high head speed that I was running at that time. The hornet tail blade holders pivot on the thread of a 2mm screw and need to be greased for reliable operation. The idea was to reduce the loading on the thread by reducing the blade weight. The standard blades are relatively massive. There is a side benefit here in that the blades break first in the case of impact and save the rest of the tail drive from damage. The wooden blades cost virtually nothing and a new pair can be made very quickly. I simply cut them from 1/16" balsa, reinforce the hub eith this acetate sheet and sand the blades to a lifting section. I then cover them with lightweight tissue and give them several coats of clear dope before painting to taste. My digital scale (1g resolution) simply refuses to acknowledge their existence!

Wooden tail blades

Having made some for the CP model, I decided to try them on the fixed pitch version and was amazed at the difference they made. No vibrations, no tail wagging and much srisper control.

Flybarless CP Hornet

This is still basically as previously described. My home-made auto unit (one-way) failed after a couple of bad crashes and the main gear was replaced with the new type with a bevel gear (conical gear to our American friends) tail drive. This seems to run more smoothly than the original type. There was no decernable change in the flying characteristics, or the handling, but I rather miss the sight of the blades spooling down at the end of the flight. I never did get to try an autorotation.

One other thing that was used with the original gear was a version of the idea used on X-Cell helis and referred to as a 'heavy duty tail drive'. This consisted of fitting an extra bearing on the tail shaft in front of the tail pinion to stop it lifting off the main gear. About 3mm of packing is needed between the bearing and the chassis to give the correct clearance.

Heavy duty tail druve

The model is currently flying with the following mods:
Titanium main shaft.
Dual capturing anti-rotation link.
Chris Rigoleth swashplate ball mod.
Main rotor hub made from 'Whisper' tail rotor.
MS collective blade holders.
Extra tail bearing as above.
New type main gear with bevel tail drive.
Home-made wooden main blades,
Wooden tail blades.
Aluminium tube skids (5/32" dia).
Plastic card tailplane (mainly cosmetic).

The model has been flown with several sets of blades. These were made from a mix of spruce and balsa as shown on the old John Kallas website. The first set were symmetrical section with 5g of lead in each blade. These were too stable for my taste and ran at a high head speed which didn't help the tail response - which is why I originally fitted the wooden tail blades. The second set had a bi-convex section and 3g of lead. These worked better. Finally I made a set with a flat-bottomed lifting section and 4g of lead. Thses are the best set to date. I've painted them in a stripy colour scheme which works well in some lighting conditions.

Stripy blades

As detailed on another page, I originally used the complete tail rotor from a Kalt 'Whisper' helicopter. The blade holders had a single bearing and the blades could flap up and doewn quite a lot. I later changed these to the MS blade holders from the CP Hornet which have two bearings. After many flights, these have just as much flap, if not more, than the Kalt units.

Standard CP Hornet

I built two of these for Dave Wilshere and found it difficult to get good performance, or flying times, due to the extra weight. Dave didn't like the lack of cyclic response and tried balsa paddles. This didn't give the improvement he wanted and he sold both models.

I got involved in sorting another model which had been built by a third party. This had HS-55 servoa and a Futaba 240 gyro plus an MS brushless motor and controller. It weighed around 350g. I was supplied with HS-50 servos (much better) but was forbidden to modify (lighten) the gyro. I removed as much surplus wiring as possible and got the weight down to 330g. Also supplied was a 7 cell Nicad pack which was far too heavy. I test flew the model on my own batteries and found that there was a very narrow window on the pitch throttle relationship where the model would fly.

Too high a head speed meant that the tail was very twitchy and would lock up due to the high centrifugal force on the standard blades. Too low a speed and the tail simply didn't have enough power to oppose the torque. I managed one flight of around 6 minutes. Things might have been better if I had instructions for the brushless controller, which I believe was stuck in governer mode.

I returned it to the shop that gave me the job. The store owner charged up the nicad pack and prepared to fly the model in the store. I walked out before the inevitable. I later learned that he wrecked it comprehensively. I did eventually get paid...

Brushless CP

Motors

All of the motors that I have used have been given some sort of break-in. Unfortunately, I didn't keep detailed records. It does seem that running-in under water works best, but it is very easy to overdo things. The shortest life was on a motor that was run under water for around 15 minutes. The best life (still in use) was a motor which was run under water for about two minutes.

All these were standard Graupner Speed 300 motors.

Flights on each motor at 15/6/08:
FP Hornet
Motor NoFlight NosTotalComment
11 - 118, 175 -250, 353 - 362203
2120 -174, 343 - 35264
3251 - 34291
5363 - 478115Brass pinion fitted
6478 - 730252
8731 - 917186
69181
CP Hornet
41 - 188188Brass pinion fitted at 41 flights
7189 - 313 125
9314 - 423109
Click
here for the latest figures.

Gears

I've already admitted that I've lost count of the number of tail pinions that I have used. Very few failures were due to actual wear, most were due to crashes.of some kind. these were dramaticall reduced when I switched to the wooden tail blades. Not necessarily because the tail blades were damaged instead, but because of the reduction in tail inertia.

The change to a brass motor pinion (from NHP) was very worthwhile. No more pinion failures and reduced noise. There is no discernable wear on the main gear after many flights.

Batteries

I had 3 of the original 7 cell 720 mAh Twicell (NiMH) packs, one of which is still in use (at 93 cycles). When the other two began to fail, I split them up and produced a 6 cell pack for use in aircraft and an 8 cell pack for the Hornets. Both of these have since failed.

There was a lot of traffic on the Hornet forum at one time relating to the use of 8 cell packs and I bought one pack of Twicells which is still in use at 213 cycles. At times this has given poor performance and has been put aside. After a few months rest it seems to recover most of its power. At the moment it while not lift either model out of ground effect until it has 'warmed up', when it gives good power and still reasonably long flights.

The Maplin electronics company has had several special offers on NiMH cells. The first of these was for 700 mAh Kodak cells and I bought a set. These only lasted a rond 30 cycles and appeared to have died. Some months later, I discovered that, like the Twicells, they had recovered. They still lack power and flight time but are still in use at 82 cycles.

The next Maplin offereing were 750mAh cells by Vanson. These are still available, from time to time, currently at 7.99 for a pack of 12 cells. I bought a large quantity of these a couple of years back. Thses were given a slow charge and set aside till needed. When used recently, they were not as good as the first ones used, so I don't advise this - particularly as the price continues to drop!

They don't have quite the same capacity as the Twicells, but are otherwise good. I've never seen more than 700 mAh on my Graupner charger, with 660 - 680 being more common. they give around 7 minutes on the FP and 6 - 6.5 minutes on the CP Hornets. The latest cells that I have appear to be vented. I have recently seen these cells packaged with a suitable charger and badged as 800 mAh. Until I get some to try I can't tell whether they are different.

Having used several 7 and 8 cell packs, the 7 cell packs always give longer flights and I have now converted all my recent packs to 7 cell. Not only is there no real point in using 8 cells, but they can give problems on my CP model. This is because of the self-calibrating nature of the Jeti ESC. Unless you give a short burst of full power to set the ESC before flying, you can get astronomical RPM and a difficult to control model.

We are constantly being told that NiMH cells don't have any memory effects. This is clearly untrue. If you use a particular pack in a certain manner for a while and then treat it differently, you will get very poor performance. In the early days of the Hornet forum we were advised by many to cycle cells that were losing performance. This always resulted in much worse results for me and it took as many as 4 - 5 cycles of normal use before they gave normal results. As with nicads, don't cycle. I have recently been flying a Piccolo - around 1.5 ounces lighter than a Hornet and with a Speed 280 motor, rather than a Speed 300. My old hornet packs really don't work at all. There is a steady loss of power from the very start and the model is down into ground effect after 3 - 4 minutes. Significantly, the batteries are getting barely warm. They still fly the Hornets normally with good duration, good power to the end with a short 'dump'. A freshly made pack gives 10 minutes plus in the Piccolo, but still that steady reduction in power.

Originally, all charging was done with the MS (Jeti) charger supplied with my original Hornet package. This gave best results when the batteries were charged until quite hot and then flown immediately. The acquisition of a Graupner 'Ultra Duo Plus II' changed all of that. I can now charge batteries and fly them next day with only a slight loss of performance. They only get slightly warm and I know the charge capacity. Highly recommended.

I've now taken to flying with a 2 cell pack of 1200 mAh Kokam lithium-polymer batteries. Although giving a maximum of only 8.4 volts, the reduction in weight of some 1.5 ounces produces a vast.improvement in performance with around 14 - 15 minutes on the FP and 11 - 12 minutes on the CP.

Canopies

The standard Hornet is canopy is about the lightest available at 7g. There are various after market canopies availablle, but all are heavier. My first canopy (Red/orange) is still in accasional use despite several repairs. My secong (yellow) has withstood a lot of punishment and is still going with slight repairs. Another, similar, canopy made for the CP had a very short life.

I have made a number of balsa canopies, based on the shape of the early 'X-Cell' canopy, some of which have been passed on to others. If carefully painted, these can be kept down to around 11g.

The type and material used for the canopy can have a surprising effect on the noise produced, due to the fairly rigid maounting to the frame, my X-Cell canopies are quite noisy.

X-Cell type canopy

Flying

The two models have very different handling characteristics and 'feel'. It can be quite dodgey to switch directly from one to the other. Neither have any discernablle 'pitch up' in forward flight. I believe that this effect is pilot induced because of the fact that the elevator response in forward flight is quite different to that in the hover. Pulling the stick back in the hover makes the model move back, or stop moving forward. In forward flight, pulling the stick back to stop makes the model climb unless you reduce power at the same time.

A side issue here is if the model is heavy, has a flexible head and runs at low RPM. In this condition, the blades will adopt a large coning angle - rather like dihedral on a fixed wing. This will make the model more stable by opposing movement in any direction, which can result in pitch up in forward flight.

If this is causing you problems the best cure is to trim the model for forward flight and add stick offset, as required, to hover. Most experienced fixed wing flyers do this anyway, because they are happier with the model travelling forwards. It's also the way that full-sized helis are flown.

I have deliberately avoided adding external links on this update because so many of the older established websites are either disappearing of changing their URL/format. Quite why this is baffles me. If you don't want people to access your site, don't post anything.

Tails


Other Hornet pages on this site:

Update 1 Update 2 Update 3 FP kit
Hints B'less CP Resources CP kit

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