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Greetings from Paul.

Propeller change query.

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Paul Bowden 106/08/2019 22:47:45
3 forum posts

Hi All,
Greetings to everyone. I hope that you can help me with this query.

I have a small tricycle undercarriage trainer with 2" wheels, an OS 10 LA
up front with a 7" x 4" prop, and on a solid surface a ground clearance
for the prop of about 1.1", or 30mm.

I am striving for my "A" certificate, so will have to take off from the ground
for the test. At the moment the 'plane thinks it's the patch lawn-mower,
and often cuts grass before the motor stalls, rather than the desired R.O.G.

As a quick fix I thought I might try using a 3-bladed prop of 6" diameter,
to improve the ground clearance - albeit only by 1/2", but that should
probably be just enough to improve the situation. Some "very" basic sums
suggested a pitch of about 3" for this prop - based on swept volume,
but I was wondering if there is a more precise way to work this out.
I am keen to keep the engine loaded to much the same revs as the
current 7" x 4" prop does at the moment.

I realize that some efficiency will be lost due to the extra blade, but I have
have ample power from the engine, so can open the throttle a bit as required.

I have considered fitting larger wheels, but the plane would look really odd with 3" wheels, and anyway I don't really want to change the look of her - the old girl is just fine as she is ! ( I built her 55 years ago and am rather sentimental about her !! )

Is there a formula, look-up table or something to determine this change more scientifically ?

With best regards, Paul.

kc09/08/2019 17:24:57
6206 forum posts
169 photos

3 days and nobody has come up with advice for Paul! We normally get replies to a first forum post quicker than that.........

Paul if you mention the name of the trainer it might help get some interest in the problem. Posting during the daytime usually gets more attention too.

My first thought was bend the main undercarriage legs down so they get a bit more prop clearance and also check that they are correct in relation to the CG. No doubt others with experience of 3 blade props will comment on that aspect.

john stones 109/08/2019 17:29:34
10825 forum posts
1482 photos

Your motor n model are happy with the set up, I would put some larger wheels on for my test, then go back to keeping the grass cut.

Denis Watkins09/08/2019 17:37:21
4054 forum posts
75 photos

Just seen this Paul, as KC was good enough to comment

As your power plant is fine by your description, then do as KC suggests

It is often very easy to grind a new flat further up the noseleg to get extra clearance, moving this down

If she mows grass, then double check there is not too much forward weight achieving the C of G

Also, with banded on wings, it is easy to alter the wing incidence slightly, if it is that holding the nose down

Also just a few clicks up elevator to help the ground run

And John was typing good advice while I was typing

Edited By Denis Watkins on 09/08/2019 17:38:48

kc09/08/2019 18:09:38
6206 forum posts
169 photos

Several replies within 15 minutes is much more like our regular service to new forum members! Obviously the time of day matters......

My second thought was to question whether this would be the most suitable model to take the A Cert. Nowadays there is a weight limit - must be over 1 kilo - and examiners seem to want manouvres carried out to a high standard. It could be much easier to demonstrate control of the model with a model of 52 to 60 inch span which is easier to see as well. Maybe the 55 year old model should be kept for " best " and saved the flogging around doing circuits and landing practice?

Peter Miller09/08/2019 18:25:21
10483 forum posts
1246 photos
10 articles

A few ideas, increasing the wheels to 2 1/2" would not look too bad but give you the exra clearance.

Even adding a slightly larger nose wheel would help.

Even just spreading the main legs would change the angle of the model.

alex nicol09/08/2019 18:59:21
338 forum posts
15 photos

Hi Paul,

We had similar issues with a club members "jumper 25" , the root cause was the grass was too long and Unfortunately our grass cutter was otherwise engaged.

We put a bigger nose wheel on and changed the prop to a smaller diameter with greater pitch ( 3 blades are less efficient than 2 ) and spread the rear undercarriage legs to increase the prop to grass gap. Prior to take off the model was pushed forward a few feet to create a couple of tracks. The final step was to restrain the model until full power was reached and let it go with a little up elevator held in.

At first there was a few hairy take offs but we soon had the knack.

Hope this is of use to you



Don Fry09/08/2019 19:41:04
4382 forum posts
52 photos

HobbyKing do some really light wheels, look like motorbike wheels, ugly as sin, but big.

Keeps props out of the grass, and big enough to allow heels to roll. But Alex is on the money, a trike equipped plane needs the nose wheel to present the wing at the right angle, but not with the prop cutting grass

PatMc09/08/2019 21:27:05
4289 forum posts
524 photos
Posted by Paul Bowden 1 on 06/08/2019 22:47:45:


Is there a formula, look-up table or something to determine this change more scientifically ?

With best regards, Paul.

Click this link then on the left under "Other Topics" > "Propellers" > "Equivalent Multiblade Propellers" will give the info you want.

From the link :

A 3 blade should be 0.904 x the 2 blade’s dia.

Note the pitch should remain unchanged as the engine will be turning at around the same rpm.
I.e. - the pitch x rpm should be the same as before for the model to fly at it’s original speed.

However you ain't going to find a commercially made 6.33" x 4" prop in any blade count so sticking with the existing prop & following the advice already posted seems the best option.

Edited By PatMc on 09/08/2019 21:28:59

Paul Bowden 118/08/2019 18:21:50
3 forum posts

Hi All,

Many thanks to everyone for their comments and suggestions.

To KC's comment about the aircraft - it is a Graupner "Amateur" from late 1960's - a "Cessna" style model, having a high wing over a "cabin" and conventional tail. The rear 2 wheels were spread to give a positive wing incidence after first trial take-offs all those years ago showed no inclination to take-off at all !

Thanks to John Stones 1, Peter Miller and Don Fry about fitting larger wheels. I have bought some 3" wheels with that very thought. The present ones are held on with soldered washers - not collars, so less easy to change. What also surprised me is that the u/c wire is metric - 3mm - but this is a German kit. I would need to sleeve the ones I have to avoid the "supermarket trolley" syndrome.

Thanks to Denis Watkins for your 2 suggestions - the front leg is bent from a single piece of wire, not amenable to adjustment. I use max UP at take-off to lighten the front ground pressure, released quickly after the plane unsticks, so I know what Alex Nicol is referring to !

Thanks PatMC for the link - I didn't realise the extreme dependence of diameter on the power requirement of a prop. I have bookmarked the site for future reference.

My initial desire by changing the prop was to attempt a simple solution not involving much work - lazy I know - just to see if that would work sufficiently well. I suspect a 6" x 4" 3-bladed prop will be my next move.

With thanks to all again, Paul.

Engine Doctor18/08/2019 18:28:44
2382 forum posts
29 photos

Hi Paul . You could of course ask whoever mows the club patch to cut it shorter ?

Peter Miller18/08/2019 18:40:25
10483 forum posts
1246 photos
10 articles
Posted by Engine Doctor on 18/08/2019 18:28:44:

Hi Paul . You could of course ask whoever mows the club patch to cut it shorter ?

Or even cut it yourself. Oh dear, Silly me! Only the few suckers cut the grass, the rest just moan. How do I know??

Paul Bowden 118/08/2019 23:37:51
3 forum posts

Hi Engine Doctor and Peter,

When I was a member of a previous club quite some years ago, I was one of those volunteers who drove the mower on a rota basis. I think that the patch is as short as it's reasonably likely to be, given it's in a field with sheep. There is an electric fence around the patch, but this doesn't deter the lambs who slip under the wires, or badgers/foxes or whatever that dig small holes in it. It's a case of balancing length verses scalping I guess !

For an odd size prop, I guess you could cut a slightly larger one down, and then carefully balance it ?

With regards, Paul.

Edited By Paul Bowden 1 on 18/08/2019 23:42:09

Peter Miller19/08/2019 08:21:26
10483 forum posts
1246 photos
10 articles

One thng that I have found in the past is that very narrow wheels tend to cut through longer grass better

Paul: We have a rota of three of us, well two because one is working but does it occasionally which is fair enough as he services the mower etc. Of course with only 14 members.....

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