How Much Power is Too Much?

[Shaun K]

Hi folks,

 

I am not so far from finishing the Tony Nijhuis 63" Spitfire plan build and being so close to getting airborne I'm working out the details like gear placement for the right CG. It seems common to need a bit of lead up front in the Spit which was designed around a .61 4 stroke glow engine.

 

I see ads in the mag for similar sized warbirds at a similar weight that recommend a .91 4 stroke. I see ads for 3D jobs that suggest a 1.20 4 stroke for an aircraft of a similar size.

 

So rather than add lead what's to say I shouldn't put in a bigger engine (other than cost) that makes up for the weight difference?

 

Can there be negative effects from upping the grunt from a .61 FS to 70/80/90 FS or is it all good if the CG works out right?

 

What about a bigger engine tamed a bit with a 3-blade prop?

 

Any thoughts would be appreciated.

 

Shaun K.

[Steve Hargreaves - Moderator]

Shaun.....its an interesting thought & one that I have had too.....the TN Spit is on my to do list but like you I am a bit sceptical about using a 60 FS in what seems a pretty large scale model........that said who am I to question what Mr Nijhuis recommends....his skills are so far ahead of mine in every department I feel I should accept everything he says...and yet...and yet.....!!

 

Certainly I'd rather have too much power than not enough & after all we do have a throttle if it all gets too exciting.

 

The other thing to consider is how well you can cowl the bigger motor....a 90 might be good but if you have to compromise the cowl & leave half the motor hanging in the breeze & then run the 90 a half throttle because you have too much power then I would suggest you'd just shot yourself in the foot......twice!!!!!

 

My own view for what it is worth would be to go for a bigger motor & ensure that the front firewall is strong enough to support the engine (both in thickness & fitting to the fuselage).

 

One final thought....from memory I think TN used an ASP61 in his spit......this motor is actually a big bore 52 so it is actually a pretty small case for a 60. the ASP 70 is a much larger beast (though still smaller than a 90) A great motor though....I have one & it really is superb....well worth considering in the TN Lysander in place of the 52 if you're a heavy builder...it should just drop in!!!

[Shaun K]

Thanks Steve.

 

As you say there's always room to drop the throttle, but so far my other planes have had the recommended engine size, so I've still a lot to learn.

 

Part of my justification for building the Spit is that I have an ASP .61FS in the trainer, but as I get closer to completing the build I'm thinking that it would be great to keep the trainer flying so I'll need to buy another engine. The question is do I use the .61 in the Spit as planned or do I up the ante a little bit and keep the .61in the trainer with a little upgrade in the Spit.

 

As far as cowling a bigger engine, the FS 61 sticks out of the Spitfire cowl anyway, it's just a matter of how much sticks out. With a grey painted underside and grey engine it's not going to be too nasty looking as a stand-off scale sport flyer. I can highly recommend removing the TN Spit from your to-do list and placing it on your 'doing' list! Tony has done a superb job of designing it. I'm now built, covered, ready to paint on some camo and I can't wait to fly it.

 

Shaun.

[Steve Hargreaves - Moderator]

Sadly my "doing" list already has 4 things on it & I only have limited space, time & money!!!!!!

 

There is a video of TNs Spit being put through its paces by the esteemed editor of RCM&E, Mr Ashby himself...it certainly doesn't look underpowered....might be worth dropping Graham a quick note asking for his opinion!!!

 

I can understand your reluctance to buy another engine exactly the same but you could always look towards building a twin Mosquito with two ASP61s in it!!!!!!

 

Another thought......keep the 61FS for the Spit & change the trainer motor for a 52 2 stroke......much cheaper than a FS & a bit more powerful than the existing engine so it would take your trainer to another dimension.....that way you keep the trainer in use & still learn from it without spending too much cash & you can keep the 60 for the Spit.

 

The list of ARTFs that will take a 52 & fly really well is huge also.

 

Final thought...if you are planning on moving from the trainer straight to the Spit I would council that this is probably not a good idea......buy a cheap ARTF low winger (BH Speed Air or Travel Air maybe) & cut your low wing teeth on that........

[Shaun K]

Thanks again Steve,

 

Soooo many choices. It's part of what keeps us interested I suppose isn't it...

 

To ease your mind I won't be going straight from the trainer to the Spit. My second plane and first plan build was Peter Miller's Miss Lizzy, a great little .32 size low-wing retro racer. It's quite a heavy little beasty for it's size so I've learned a lot flying that one. It's the red & white plane in my little photo icon. Trust me, I'll be practising my backside off on the Miss Lizzy for a few good sessions before committing the Spit to the air!

 

I was tempted by some club mates to put a .40 or .46 2 stroke in that one, but it's already nose-heavy with a .32 so I resisted the temptation. I reckon it would really scoot along though, if the little devil on my shoulder makes me do it. 

 

I've just watched the video of Graham Ashby again and noted that a caption actually informs us that it is actually being flown on an SC .52 4 stroke. So it seems that perhaps the 80/90FS options are perhaps overkill, but if Tony or Graham are reading this I'd certainly love some feedback from you.

 

Shaun.

 

 

 

Shaun.

 

 

[Aero120]

Shaun,

 

I had a similar dilema when I built a hangar nine Mustang last year. The recommended engine was a 60 two stroke and or equivalent four stroke. I fitted an Os 91 wth a custom exhaust. The Os91 is dimensionally the same as their 60. The result is a plane that flies very similarly to the real plane. Take off is at half throttle, with wheels up, open the throttle and the sound and performance is amazing. Throttled back it flies very quietly. The only downside is the landing as even on tickover the engine keeps the plane flying quite quickly so landing approaches are long and flat, starting at the downwind leg with wheels already down to create drag!

 

A video of it is here

 

http://www.youtube.com/watch?v=IUGNkA0ArXA

[Former Member]

[This posting has been removed]

[David Turner]

My own view would be that the .61 is adequate. It is good to have lots of power so that you can emulate those large, open, scale manoeuvres...the downside is landing, as has been pointed out.

 

I'm sure that you will find that an idling .91 will develop an embarrassing amount of power; enough to make a landing incident quite likely. And any little accident will likely see you having to repair those retracts.

 

You can counter the unwanted thrust with the provision of good flaps. Does your model have these and are you confident in their use?

 

In recent years, in the UK, there has been a trend towards the fitting of larger motors to quite modest airframes. I'm not quite sure how that has come about... a "more is better" mentality, I guess.

 

And don't be scared of flying a low-winger. After 10 minutes, you'll wonder what the fuss was about.

 

Anyway, that's my ha'porth.

 

And here is a video of a full-sized P51, to give you some idea of its actual performance. That's without  full fuel and armament loads.

 

http://www.youtube.com/watch?v=_7obOIOzqrQ

 

 

Oops! You're building a Spitfire, not a Mustang. Whatever. You get the idea. 

Edited By David Turner on 03/11/2009 09:57:54

Edited By David Turner on 03/11/2009 10:16:42

[Shaun K]

Thanks David,

 

The TN Spit was designed with no flaps and built as per design, so slowing down will most likely be an issue if it's over-powered. It's not my first low-wing so I'm not worried about any of the flight other than the first 30 seconds and last 30 seconds of the maiden....

 

Loved the P51 video. I had the very rare pleasure of seeing one fly overhead in the flesh on Sunday! Matt Hall of Red Bull Air Race fame is a local from my area and owns a P51 which he has bought 'home' for a limited run of joy flights. The Merlin sound is pure delight....

 

Thanks again to all - this is interesting stuff.

 

I will pose one more question - has anyone tinkered with a slight increase in engine matched with a 3-blae prop to dampen the power? Does a 3-blade prop offer better air braking at idle than a 2-blade?

 

Shaun.

[David Turner]

I've never tinkered with 3-blade props. I believe that they are generally less efficient than 2-blade props, though, so you might expect a bit more braking. Someone will be along shortly, I imagine.

 

I can say that you will get the best "braking" from a large diameter, fine-pitch prop. Upside is good initial acceleration and climb, maybe a little less engine noise, owing to reduced rpm.

 

 Downside is reduced ground clearance, increased precessive effects and reduced top speed of the model...ie, decrements to the ease of handling.

 

As ever, it's a compromise.