Hanno Prettner Magic

[Bob Cotsford]

the tapping on my Genesis quiet pipes is on the front cone just in front of the maximum diameter, though I've seen people tapping into the manifold as an alternative.

[Martyn K]

Thanks Bob

I don't suppose you know where I can buy pressure nipples from?

Martyn

[TinPot Pilot]

Martyn,

I got some pressure nipples from Just Engines, if that helps. I don't know if they are listed on their site, I sent them an e-mail and they were very helpful.

[Martyn K]

Thanks, I'll try them.

Martyn

[Martyn K]

Well I have been buying bits:

I have got one of these:

Its an OS91 DF Round Head - I am not sure when the engine went out of production, but this has never been run. The price on the box was £320, I am quite pleased I got it for £42.50 off fleabay . The rubber seals all seem OK which is a good sign.

It was actually listed incorrectly in Model Marine engines and had only 2 bidders. However, I have just found out that DF = Ducted Fan, its says in the running in guide to run it in at 18000 rpm...

It will have to do.

If you look at the picture, you can see the prop driver is in front of a taper. Closer inspection indicates that this could be removed and the prop driver fitted with the correct woodruffe key. Neither task was particularly easy. I suspect that the previous owner had fitted a flywheel at one point although there is no evidence that the prop nut had even seen a spanner.

Not sure if you can see this, but the piston is totally unmarked.

I also got one of these:

Another brand new unused item from fleabay and even better, the seller lived only 3 miles from my house so no postage costs. This is a period Graupner 60-90 Tuned Pipe, but less of a bargain than the engine. It should be OK though.. (he says hopefully).

The engine had the needle and a different valve assembly in a sealed polythene bag. However, I can't see what it is for. The only thing missing off the engine was the instruction manual.

It may be a remote needle valve, perhaps the collected geniuses here can help. There is a blanking bolt just above the assembly which I removed before screwing the needle valve and sleeve in..

Next on the shopping list is the undercarriage, but that may need to wait until next pay day. I think I'll start work on the tailplane in the meantime.

Please don't tell me that it won't do... 

BW

Martyn

 

Edited By Martyn K on 29/04/2013 22:45:13

[Bob Cotsford]

That's a big pair of 'oles in the back Martin. Do you remember DF engines - 20k rpm and incredibly loud. It'll be interesting to see how it throttles. You did do a good job of securing the firewall didn't you?

I seem to remember seeing serious power figures for this sort of motor, the .46 Df engine was rated over 3hp so what the 91 would put out, God alone knows.

[Martyn K]

Hi Bob,

That are a bit big aren't they

Some tech info for the engine from the warranrt sheet I found at Hobbico:

Practical RPM range 2500-25000 rpm

Power 4.8PS at 22000 rpm

Weight 662g (actually lighter than the .61 sized variant).

Is it worthwhile continuing with this or should I look for something tamer? I'm tempted to bench run it first to find out just how useable that rev range really is. I don't have to run it at 25000 rpm, I am slightly tempted to over-prop it and try and keep the revs down to mid teenK.

The design also has provison for flaps, I am intending to use them.

BW

Martyn

[Bob Cotsford]

I run my MVVS 90 on a 14*8 at around 10k or so rpm with a quiet pipe. When I had it in a Joker (60 size) it was impressive but not over the top really. If I were you I'd set the motor up in a test stand and see how it runs and throttles on a bigger prop. I'd imagine it would still be quite poky even on 13-14" props.

The problem I find running 14" props (or even 12" on the Tornado) is ground clearance with these older designs which would originally have run 11" props as a rule.

[Bill Michie]

Hi Martyn, some comments from me if that's OK:

This is a RACING engine, ie designed to turn a small dia fan at silly speeds, it will probably have little useable throttling and torque at 10-12000rpm as it's ported for very high speed operation and will DRINK fuel................. LOL. And the noise.................

IMHO, you'd be better off with a modern (or classic) side exh sport engine, or if you can find one, a "period" rear exh .61 eg Webra (speed or LS), ST X61, OS .61 VF or RF, or Rossi/OPS etc.

Great purchase but not suited to a Pattern airfrme IMO.............

Best regards, Bill

[Martyn K]

Hi Bill

Yes - I am inclined to agree with you. I am still on the lookout for a rear exhaust 61 or 75 - I really want to keep the pipe hidden and maintain the clean lines although I have considered side mounting and using a swan neck to get from the exhaust outlet to the pipe.. Not as nice as a fully enclosed engine though.

Martyn

[Bob Cotsford]

for the now extinct Joker I JE made me a custom swan neck manifold for the inverted side exhaust MVVS91 with the exit on the engines centreline to match up with the underside pipe tunnel. Could you do something similar so that just a small portion of the swan neck would be outside of the cowl?

[Martyn K]

Hi Bob

That is exactly what I am thinking about doing. The problem with these exotic rear exhause engines is that they are likely to cost lots of (in my house) exotic pound coins. A cheap and cheerful side exhaust 75 may be a better and more affordable bet.

BW

Martyn

[Bob Cotsford]

I was sure I that I'd seen a rear exhaust Irvine 61 with pipe on ebay the other day but can't find it now. As you say, side exhaust engines are at least reasonable if not exactly cheap as chips.

[Martyn K]

Hi Bob

I have (hopefully) just managed to acquire a pumped RE Webra from a fellow forumite .

Problem solved- he says in anticipation.

Martyn

[Bob Cotsford]

pumped too - perfect, no worries about where to site the tank.

[Martyn K]

Hi Bob

You have no idea how chuffed I am . Now what the 'eck do I do with that OS91DF?

Martyn

[Martyn K]

The official start date for this build was the 1st June 2013. That was when I started cutting parts out.

I have actually made a start on the construction. While I am still collecting parts for the wing and fuselage I have started on the tailplane. Not very exciting, but it is a start...

Both the leading and rear spar/elevator junction have false LE and TE attached. I also want to use this model to trial the use of Kevlar sailcloth sheet as a full length hinge for all the moving surfaces. To achieve this, the rear spar is laminated and a piece of kevlar glued into the sandwich. The Kevlar takes cyano very nicely, it is far more flexible than 3 of thick mylar hinges and far tougher. The moving surfaces will also be laminated and sandwiched up with the hinge when I am ready to add them.

The tail ribs were marked using good old fashioned carbon paper drawn through the plan and cut from some very nice 1/16" quarter grain sheet balsa.

After a delay (I have been away for nearly a week and also disrupted by building microscope boxes for a Natural History Group that I am a member of), I have managed to get the first half of the tailplane under way.

At this rate, this will be the longest construction project I have taken on! To be fair though, I have also cut out the Firewall, the false firewall and drilled the engine mount..

More to come

Martyn

[Martyn K]

Nearly two months since I last updated this blog. Always a bad time to start building at the start of a flying (crashing) season ...

A bit of progress:

The tailplane is now completed and ready for a final rub down then painting:

Tailplane tips

Adding the laminated elevators to the full length Kevlar hinges

Covered with Fibreglass cloth (attached using watered down PVA) then 3 coats of water based varnish, sanding down with fine wet and dry inbetween coats (used dry).

Started work on the fuselage. Three splice joints to get the front , rear and tailplane mount plus 1/16" ply doublers at the front and 1/32" ply doubler under the tailplane.

Then ran into problem no.1. The front former carries the engine mount. The curve for the upper decking is correct but does not line up with the fuselage sides.

and confirmed by colocating the former over the template for the 1/16"ply neat front former.

I have cut slots in the fuselage sides to get them to fit but this will mean that the fuselage will be a bit thin as it flows into the upper deck. Should be OK though.

Finally, the first two formers are glued in adding the 1deg downthrust. You can just see the datum line drawn down the fuselage. I have also added the 1/2" triangle supports. Next to go in will be the front retract mounting plate and the rear formers but that will have to wait until I get back from my holidays.

More to come - hopefully things will speed up a bit now as the evenings are getting longer

Best wishes

Martyn

[Steve Hargreaves - Moderator]

Just adding a post so I can follow along Martyn.....

If your Webra turns out to be no good I'm pretty sure I'm right in saying you can rotate the whole cylinder on the MVVS motors through 90 degress so you can have a (r/h) side exhaust, rear exhaust (l/h) side exhaust & even that well known favourite the front exhaust ....

[Martyn K]

Cheers Steve

Front Exhaust - a very short pipe and used for pushers! - the mind boggles but nice to know info anyway.

I have already got the next 2 models lined up so I need to get on with this. The engine does look (and feels) pretty good, I really must get round to bench running it first - help me get the pipe length (about) right before I really start worrying about getting it to fit.

BW

Martyn

[Bill Michie]

Hi Steve & Martyn, remember, you can (or SHOULD!) only rotate the cylinder on the MVVS BEFORE it's been run, to avoid differing wear patterns/ovalling the bore,etc.............

ie choose your option from new, then stick with it............

Bill

PS I've acquired a Webra Speed .61 (side exh) that'll need stripping, cleaning, new bearings etc, for sometime in the future. AND it's got a Dynamix carb! (took me ages, a long time ago, to learn how to set it up; see if I can remember now!)

[Martyn K]

Hi Bill

Yes - very good point.. I am not aware of the internal architecture of the motor so cant really comment but it could be a good reason for a totally worn out engine in a very short space of time

Martyn

[Martyn K]

Back from my holidays. 3 weeks with no internet access. Once the withdrawal symptoms had worn off it felt quite liberating.

Last night was the first night in the shed for over a month (although I did do some building in Italy - a peanut scale FF rubber Fairchild PT-19).

I cut the next 2 formers out and got them glued into place:

I have added the triangle stock - this is not shown on the plan.

Those old UPS batteries are wonderful for keeping things square and true...

And then this morning I attached the other side..

The UPS batteries are now my datum to ensure that the fuselage sides are parallel laterally..

Finally, a shot with the tank in place. The plan suggests a slim14oz tank. I could only find a slim 16oz tank

More to come.

Martyn

[Martyn K]

A bit more progress in the past couple of days.

Got the fuselage in the jig and carefully aligned it so that there are no bananas:

Added the 1/8" ply upper decking. Took quite a while to cut this out including the hole at the front for the tuned pipe. The pipe will sit over this deck and will be covered by a GlassFibre cowl.

Added the fin post. Like the elevators, it will use kevlar cloth for the hinge material. The post is laminated from 2 pieces of 1/4" sheet balsa. One additional benefit for the cloth is that it adds a certain amount of strength to the supporting structure.

I have used the top decking as the lateral reference - all flying surfaces will be aligned laterally and vertically from this.

However. the post isn't vertical when the longitudal datum is horizontal. It has a degree or so of rake. The exact measurement was taken from the plan, the fuselage levelled and the rake carefully sanded.

The fin outline is laminated from 3 pieces of 3/16" sheet balsa, tapering from 50mm at the fuselage to 12mm at the top. The swoopy curves were roughly cut before the fin was glued into place.

Here is the first pass at fitting the fin outline. Most of this weekend subsequently has been spent carefully sanding it to the correct shape (or at least what I imagine the correct shape should be - there is no profile info shown on the plan).

The next task will be to get the tailplane mounted plus the fin subframe in place. The tailplane kas +0.75deg incidence. It will be fun working out how to get that measured accurately.

More to come..

Martyn

[AlexJ]

Hi Martyn,

I have just looked at the Trenton site, seeing the article with the picture of Mr P, it reminded me of a signed picture I had from a Sandown exhibition. I was quite young at the time I think and just asked for the autograph because everyone else was, little did I know I was in the presence of modelling royalty! This has taken me right back to standing behind the railings at the flight line!

Regards

Alex