SC52 Peak Revs.

[denco 1]

I have a SC52fs on the front of the Seagull PC9, Roulettes version 63" wingspan. Although a tad lapsy on take off the engine has handled the plane pretty good, until last weekend Sat. 28/7/12 when i had to abort two take off attempts due to lack of ground speed enabling rotation, we have a grass strip, never a problem before. I removed the engine to check it and adjusted the valve clearances, all okay on that front, however, on starting and running the engine, the maximum revs. shown were 8900/9000 on a tacho. The specs. give 9800 so as you can see i am down about 900. Am i right to assume that what i am achieving at the moment should be sufficient to fly the plane? Can anyone suggest what might be the problem. The engine is pretty new with not much flying time, also i have changed the plug, before someone suggests doing so. 12X6 prop fitted. Cheers Den.

[Stephen Grigg]

Doesnt souynd to bad for a 4 stroke what size prop do they recommend ,if yo go to an 11x7 you may get more revs.

[Steve Hargreaves - Moderator]

Well if it was OK before & its not now then something has changed, the question is what....from your description I'm guessing that it was a bit marginal before so maybe not a big change required to make the difference.....

Different fuel? Is the grass longer or wet perhaps? What did you set the valve clearances to? 0.1mm is usual... Are the cylinder head screw tight?

If it worked before we should be able to make it work again...

[denco 1]

Steve, the fuel is the same, grass no significant change, cylinder head screws are all tight. The valves have been set as per the makers spec. What i will do is try flying it again as soon as poss. weather permitting, and see what happens.

[Bob Cotsford]

ok, the specs say 9800, but on what size prop and what did your engine give before?

What was the wind like on that day? What was the air temerature? I know that when we had that mini heatwave a couple of weeks back my ic engines were down on power, and with light to no wind take offs were a bit iffy - quite a bit longer than usual.

[denco 1]

Bob, we had a slight crosswind nothing much and it was quite warm but not excessive. When i first flew it i had an APC 11x7 on, but was advised to change to a fourstroke prop ie.Master aiscrew 12x6. Thinking about it , may well be that the rev. output might have reduced slightly, but hardly noticeable. Unfortuantly i did'nt take any tacho. readings prior to this, probably flown about half a dozen flights on both props.

[Erfolg]

I think Bob has it!

So many times with IC motors, the claimed bhp and revs are achieved using a propeller size that is not often used.

For many manufacturers these figures are produced to impress and make sales.

I had a car, a Triumph GT6 with a claimed 9.6s to 60 mph. I think it could probably do it, if reved to near death and dropping the clutch. I had another car years later that the manufacturer claimed about 9.6s to 60 mph. The difference was that my later car had another 1 lt capacity, 4 valves per cylinder, 250 bhp. Magazines obtained 0-60 times up to half those claimed. It all depends how much you need to impress. Reality can be very different.

[Lee Smalley]

Is it me or are we missing something blindingly obvious this is a 63inch span model with an sc53 four stroke in it the asp and sc engines are great but struggle to pull the skin off a rice pudding. You are faffing.around looking for a few hundred rpm that the manufacturer claims (yeah right) my harmon rocket has an asp 70 in it and it is just adequate, in my opinion you are at the bottom end of the power range and rather waste your time looking for those elusive rpm you would be better off getting a bigger engine

[Ian Rayne]

Well said Lee, I thought straight away that this SC was to small but then I'm a newbie what do I know. I've just bought a Seagull Edge540 120 size and my min size engine I can use is a 120 but then again I have to take my altitude into consideration .

[Lee Smalley]

Ian no problem mate ihave checked the model out and it is listed for a 46 engine like my harmon to be honest seagull always seem to be undestating the power requirements always remember fourstrokes are far less powefull than 2 strokes and asp/sc engines are much lower on power than the likes of saito and os but they are great reliable engines, that model really needs a 70 fs, just suprised no one else mentioned it

[Ian Jones]

Hmm, 52 4 Stroke is definitely at the bottom of the manufacturers recommendation for this aeroplane and I don't actually disagree with Lee & Ian however... I have always flown these PC9s on a 46 or 53 2 stroke and would not have gone lower than the 46 but earlier this year I flew one with a 40 fitted and it flew amazingly well. All of these engines were Irvines by a happy coincidence so it may well be a fair comparison. I just can't remember if the one with a 40 in was one of the newer (and I think lighter) version that can be built IC or electric.

Even though the 40 could possibly have been belting out 11500 revs that would have been on a 10x6 so all in all I would have thought that even if it was not exciting that it would at least fly on an SC52 (which I have in a heavier more draggy biplane and it's just right. Of course the wing loading is lighter though)

With my first PC9 I had similar problems and there were two reasons for it. The least major was that the noseleg worked fine but had slipped upwards in its mount by 1/4" - effectively shortening it. That's not much perhaps but I'm certain it contributed to the problem. The real big issue though was CG balance, it was nose heavy and just would not leave the ground if there was the slightest excuse not too. (Short noseleg, wet grass, long grass, no wind etc). I progressively added weight to the tail, (about 20 grammes in all) which shifted the balance point back 15mm and I had years of happy flying with it after that (on an Irvine 46). Take care if you do this of course, however I haven't found the change detrimental to good handling.

Edited By Ian Jones on 03/08/2012 22:36:48

[Martin Harris - Moderator]

I don't know about yours but our grass has been unusually lush this year and most people have experienced extra drag. With a tricycle undercarriage to heave through the green stuff it's quite conceivable that this is affecting performance on the ground significantly.

Also, did you check the wheels for free rotation?

A lot depends on the model, of course, but I habitually take off using around half throttle with my OS52 FS (12 x 6 APC) equipped 8 1/2 lb 1/6 scale Chipmunk - otherwise it's flying far too soon for any semblence of a scale take-off.

Edited By Martin Harris on 03/08/2012 22:42:41

[Bob Cotsford]

Lee, you're not wrong that it's marginal on power (but then I'm quite happy with my SC52FS in an old AcroWot which is apparently to small to be fun), but the OP has what he has. It flew before, so it should fly now - unless changing conditions have degraded performance a little.

Higher air temperature, longer or thicker grass, a change of fuel, all will be enough to make the difference with marginal power.

Every ARTF I've had seemed to need 30% more power than the manufacturer/distributor claim, a CMPro Cessna for a 46 struggled on a 65 FS, a YT Katana 46 only really came alive with a 60 2stroke, my BH Extra is just nice with a 90FS etc..

For reference my SC52FS Acrowot has an APS 12*7 fitted.

[Bob Cotsford]

ref Martin's post, my Acrowot has big boots, 3" wheels I think, and it gets off reasonable grass pretty quickly. My trike models with 2 1/4 or 2 1/2" wheels can struggle to accelerate unless the grass is really short. These are 60 and 90 2 str powered aerobatic models, not short on power! I think the smaller nosewheels are the main culprits.

[Erfolg]

I do agree that the engine may not have the power necessary for the application.

The original point that was made the engine seemed down on manufacturers claimed figures, with respect to revs. Bobs point was that quoted figures are often obtained by using a small propeller to get the revs.

As we all know power = 2*pi*n*t, so going for those revs helps the cause.

Again as we all know that a bigger prop can convert power into useful thrust more effectively, the ^2 relationship.

So I think Bob is correct and you are all correct in suspecting that it is a lack of cubic inches that is the issue.

Yep a different more powerful motor is the solution, as stated.